Unit 4 Chemistry notes

A2 Unit 4 Kinetics, Equilibria and Organic Chemistry

Mr Lund 03 March 2015

1

AQA A2-LEVEL

Student Guide to Unit 4

Kinetics, Equilibria and Organic Chemistry

See me in glorious at:

Shared Areas Chemistry Read Chemistry Mr Lunds Classes A2 Chemistry Unit 4 All programs



2

Equilibria

dynamic (not static - do you know the difference!) equilibrium is achievable in a closed

system (e.g. solutions in a test tube if there are no gaseous reactants/products).

rates of the forward and reverse reactions at equilibrium are identical

concentrations are unlikely to be 50 50

Le Chateliers Principle and Qualitative Aspects

can you unambiguously write down what LCP states (see 148 of the AS textbook)?

remember this is a predictive tool used to determine the effect on the position of equilibria when a change in concentration, temperature or pressure is made

it is NOT an explanation of WHY it happens so avoid statements such as because of LCP, LCP causes and learn to state LCP predicts that ..

LCP is not suggesting that a system completely reverses a temperature change when establishing a new equilibrium as the new equilibrium will be that for the changed

temperature

it implies that the shift in the position (in terms of reactants and products) of equilibria is in the direction that seems to minimize the effect of that change

a new position of equilibria in which the relative rates of the forward and backward reaction are once again in balance under the new set of conditions is eventually arrived at

the position of equilibria is changed by:

concentration (which can be easily understood using rates/collision theory)

temperature (EA will be larger for the endothermic process so it will be relatively more

favoured by a rise in temperature)

pressure (only applicable where there is an imbalance between the number of moles of

gaseous particles on either side of the equation)

catalysts - do not affect position of equilibrium, just the time to achieve it

Monitoring Equilibria

remote sensing this is non intrusive (e.g. level of absorbance of a given wavelength of light by a coloured solution) so will not effect the position of equilibrium

titrimetric analysis this will effect the position of equilibria (since the concentration of one of the reactants or products will be changed) so is only applicable to a system with a slow

response to a change in conditions or where quenching (dilution or cooling) is used to slow

down the rate of reaction and thus the effect of the investigative technique

A2 Chemistry (Nelson Thornes) AQA 20 and Chapter 9 in the AS Book

A2 Chemistry (Heinemann) AQA 16 17 Q 1 - 3 on page 16 17 Q 1 on page 32

Chemguide Chemical equilibrium

How Science

Works: A, B



3

Equilibrium Law (i.e. equation)

aA + bB cC + dD Kc =

eqm

b[B]

eqm

a[A]

eqm

d[D]

eqm

c[C]

only applies to systems at equilibrium so dont use initial concentrations

Kc is calculated from concentrations at equilibrium (note that the concentration of a solid is constant and so solids do not appear in the equilibrium expression)

equilibrium constant (Kc) is related to reaction stoichiometry (a, b, c etc) and is a constant at constant temperature

the value of Kc is an indicator of the position of equilibrium (reverse reaction = inverse value)

the value of Kc is not indicative of how fast the reaction proceeds

you must be able to calculate the numerical value of Kc (possibly using data from an experiment you will carry out yourself)

note that concentrations are used i.e. moles/volume not moles although quite often V will cancel down or cancel out completely (when the powers is the same on the top as the bottom) but show ALL working in exams

determination of the units of Kc you must show workings in the exam but .. check using (moldm-3)top powers - bottom powers

Summary Questions Page 21 1 - 3

Exam Style Questions Page 28 1

determination of the concentrations of reactants/products present at equilibrium, given appropriate data including the numerical value of Kc

this may involve the determination of an unknown value of x to solve an equation (but not a quadratic one on this syllabus)

you may be required to realise that the equation can be simplified by taking the square root of both the top and the bottom terms and the value of Kc(see the example on page 22 23)

you might also be asked to determine the amount of given reactant required in order to produce a given amount of product (see the example on page 23 24)

Summary Questions Page 24 1


A2 Chemistry (Nelson Thornes) AQA 20 24

A2 Chemistry (Heinemann) AQA 18 20, 22 - 27 Q 4 6, 8 - 13 on pages 18 27 Q 3 on page 33

Chemguide Equilibrium equation

This equation can vary and

simply illustrates the principle

behind its construction



4

Changing Conditions and Equilibria

Pressure or Concentration Change

the value of Kc does not change with variations in concentration or pressure

to improve yield wrt a more expensive reactant a cheaper reactant could be added.

removing the product as it is formed (assuming that the response of the system is fast enough) would also improve yield whist also allowing reactants to be recycled (see Haber)

pressure only has an effect IF gaseous particles are involved and in addition the stoichiometric ratio of gaseous particles is unequal on either side of the equation

in all cases a pressure increase will increase the rate of reaction involving gaseous reactants

Kc and Temperature Change

value of Kc increases as temperature increases for endothermic reactions i.e. the equilibrium shifts to the RHS i.e. more products

value of Kc decreases as temperature increases for exothermic reactions i.e. the equilibrium shifts to the LHS i.e. less products

Temperature Exothermic reaction Endothermic reaction

increases Kc decreases Kc increases

decreases Kc increases Kc decreases

Kc and Catalysts

value of Kc DOES NOT CHANGE when a catalyst is used

therefore catalysts do not change the position of equilibria i.e. change the yield

HOWEVER, the rate of the reaction will be faster (for both the forward and backward reaction) hence equilibrium will be achieved sooner

adding a catalyst to a system already at equilibrium will not change its position

How science works Page 26 Development of the Haber Process


Exam Style Questions Page 28-29 3 5

A2 Chemistry (Nelson Thornes) AQA 25 - 27

A2 Chemistry (Heinemann) AQA 18 27 Q 4 13 on pages 18 27 Q 3, 5 on page 33

Chemguide Equilibrium equation

How Science

Works: E, F



5

Kinetics

review basic ideas of collision theory and activation energy from AS

Rate Equations

the rate of reaction is the rate of change of either the reactants or products in a chemical reaction

the rate can be determined from concentration versus time graphs

the mathematical expression for the graph on page 4 of reaction A + 2B C:

Rate = dt

Bd

dt

Ad

dt

Cd ][

2

1][][

there are different ways to express rate

(i) average rate

(ii) instantaneous rate (this is what the equations above depict) which is the gradient of

the tangent of the curve at selected values of concentration or time

(iii) initial rate is the instantaneous rate right at the start of a reaction

initial rate (t0) the gradient is easiest to determine with confidence since it goes through zero

Measuring Reaction Rates

see AS Module 2 guide for general practical techniques for monitoring progress in a given reaction

generally the main methods comprise two different strategies

1 following a single reaction measuring colour change, change in conduction/pH of one product or reactant

then plotting a concentration time graph and determining the rates at given concentrations

2 clock techniques measuring the time to an observable event from known different initial conditions e.g. the thiosulphate cross (the one you did at GCSE) or the iodine clock this must be relatively early in the reaction so that the concentration of other reagents

(other than the one being varied) can be deemed unchanged.

initial rate 1

t

How science works Page 5 Damn fast reactions indeed Summary Questions Page 6 1 - 5

Dont panic !! this can be explained

easily in words ask your teacher



6

Rate Equations and Order of Reaction

each reactant may or may not affect reaction rate (so its not quite as clear cut as it seemed at GCSE/AS)

ALSO it is not necessarily directly proportional when it does we shall see why later

for a given reactant we can state

Rate [reactant]n

for two reactants A and B the general rate expression is:

Rate [A]m[B]n moldm-3s-1

m and n are the order of reaction for each reagent (values are limited to 0, 1 or 2 at A-Level)

overall order = individual orders)

unlike with equilibria the orders of reaction and thus the overall rate equation cannot be determined from reaction stoichiometry they can only by determined by experiments

e.g. 2H2(g) + 2NO(g) 2H2O(g) + N2(g)

doubling [NO] quadruples the rate i.e. double2 = 4

doubling [H2] doubles the rate

i.e. Rate [H2][NO]2

NOT as suggested by the ratio of reactants rate [H2]2[NO]2

k is the rate constant (units depend upon overall order)

rate = k[H2][NO]2 moldm-3s-1

units of k will vary depending upon the overall order of the reaction

determination of the units of k for 0, 1, 2 and 3rd order reactions overall should be attempted as they are likely in an exam and here is

another cheat for checking your answer:

units of k = (moldm-3)1 - overall order (s-1)

note that if the order is 0 this term will not appear in the rate equation e.g. A2BC0 = A2B x 1 = A2B

Maths tip:

Any number raised

to the power zero = 1

Note that rate always has these same

units.

[NO]2

The effect on

rate of the

change

here

..is raised

to this

power i.e.

its second

order wrt

to [NO]



7

Catalysts and Rate Equations

catalysts will be involved in the rate expression (this might only be a modification of the value of k itself for example a solid in heterogeneous catalysis)

e.g. CH3COCH3(aq) + I2(aq) CH2ICOCH3(aq) + H+(aq) + I-(aq)

by experimentation it was found that the reaction is:

first order wrt [H+] and [CH3CO CH3] zero order wrt [I2] i.e. second order overall

rate [CH3COCH3]1[ H+]1[I2]0

rate = k[CH3COCH3][ H+] moldm-3s-1

what are the units of k?

Rate Determining Step

most reactions occur in several steps (this is exemplified by organic reaction mechanisms)

each step will take place at a different rate

the slowest step will determine the overall rate of the reaction and is known as the rate determining step

the order of the reaction regarding each reagent can provide information regarding its involvement in the rate determining step

obviously a reactant with 0 order will not be involved in the rate determining step

study of reaction kinetics can yield important information regarding the mechanism of a multi-step reaction

in the reaction above iodine would not be involved in the rate determining (slow) step

at this point you might ask your teacher to explain why there are variations in the mechanism for the hydrolysis of a haloalkanes as discussed on pages 15 16 (or perhaps research SN1 and SN2 yourself)

Summary Questions Page 9 1 4 Page 16 1

A2 Chemistry (Nelson Thornes) AQA 4 9, 14 - 16

A2 Chemistry (Heinemann) AQA 4 6 Q1, 2, 3 on page 5-6

Chemguide Order of reaction, rate expression, SN2

H+



8

Concentration/Time and Rate/Concentration Graphs

rate/concentration graphs can show the order with respect to a given reagent

it is glossed over at A-Level that the other reagents will have to be present in xs so that their concentration can be deemed to be unchanged during the course of a reaction where a series

of measurements of the concentration of a given chemical are measured.

or in a clock technique the event measured in relatively early in the reaction so better reflects the known initial concentration of each reagent

zero order e.g. decomposition of ammonia (tungsten catalyst), rate is independent of concentration i.e. graphically it is a flat horizontal line

2NH3(g) 3H2(g) + N2(g)

first order e.g. thermal decomposition of dinitrogen monoxide to nitrogen and oxygen (gold catalyst), rate is directly proportional to concentration and will be a straight line gradient = k

(any points not on the straight line will be anomalies and require identification and

explanation (e.g. temperature variations))

second order e.g. thermal decomposition of ethanal to methane and carbon monoxide will produce a graph that curves upwards (rate against concentration2 is a straight line)

Initial Rates Method

initial reaction rates are determined by plotting the tangent to the time/concentration graph for different initial reagent concentrations at the start of the reaction (t = 0) when reaction

concentrations are accurately known (and at a fixed temperature/catalyst)

the gradient of this line is the initial rate

orders of reaction can be determined from initial rates data by inspection

the value of k can also be determined from this data

the best way to grasp this idea is to try examples


Exam Style Questions Page 17 - 19 1, 3, 5


A2 Chemistry (Heinemann) AQA 7 11 Q4 on page 8

Q1 on page 14

Chemguide rate

W Do you have an idea as to why it might be zero

order?



9

Variation of k with Temperature

Maxwell-Boltzmann Distribution Curve

distribution of energies amongst particles at different temperatures give rise to the Maxwell-Boltzmann distribution curves based on the Arrhenius expression

RTE A

Aek

A = Arrhenius constant (determined by collision frequency and orientation factor)

R = gas constant (8.31 JK-1mol-1)

T = absolute temperature (K)

the main significance of this equation (which you dont need to know unless you are trying to understand the subject) is that a small rise in temperature has an exponential (i.e. big)

effect on rate

ln(k) = ln(A)

ln(k) = - + ln(A)

y = m x + c

Plot ln(k) against 1/T gives gradient = - and intercept = ln(A)

Exam Style Questions Page 17 - 18 2,4

Page 158 1


A2 Chemistry (Heinemann) AQA 12 13 Q2-4 on page 14 - 15

Q1 on page 150

Chemguide Arrhenius expression

AE

R T

R

EA

R

EA

T

1

Extra info for

those who do

Maths how can we get

values for EA

and A



10

Acid-Base Equilibria

Arrhenius definition of an acid - hydrogen ions and oxonium ions (H3O+(aq))

soluble base = alkali

alkaline solutions have relatively high [OH-(aq)]

Brnsted-Lowry Theory of Acids and Bases

Arrhenius limited to aqueous solutions but acid-base concept is broader e.g.:

NH3(g) + HCl(g) NH4Cl(s)

acid-base equilibria involve proton transfer

acids are proton donors, bases are proton acceptors

water is amphoteric

it behaves as an acid with NH3

and as a base HCl

conjugate acid-base pairs undergo proton exchange

competition occurs for protons between bases on either side of the equilibria

relative strength of the base determines the equilibria bias

a relatively strong base has a relatively weak conjugate acid and visa-versa

note that in the protonation of nitric acid in the nitration of benzene nitric acid acts as a base

Ionic Product of Water (Kw)

water undergoes slight auto-ionisation (dissociation of the water molecule)

ionic product of water again the equation defines the concept and dont forget units

Kw = [H+(aq)][OH-(aq)] = 1 x 10-14 mol2dm-6 at 298K

variation of Kw its value increases as temperature increases - LCP - (hence [H+(aq)] increases and therefore the pH of a neutral solution decreases!)



A2 Chemistry (Heinemann) AQA 34 35, 36, 42 - 43 Q1 on page 35

Q6 on page 37

Chemguide Brnsted, ionic product

pH Calculations

You must make the

effort to learn the

definition of a Brnsted-

Lowry acid and base



11

define pH using an equation NOT in words as it is more certain to get full marks.

pH = -log10[H+(aq)] learn to use your calculator !

pH of monoprotic (release a single proton into aqueous solution) acids e.g. HCl (note: pHs lower than 1, including ve values, are possible)

you should be able to calculate pH after a strong acid of known volume and concentration (or pH) is diluted by a known volume of water

pH of diprotic acids NOTE: [H+(aq)] = 2 x [H2SO4(aq)]

calculating [H+(aq)] from pH values

[H+(aq)] = 10- pH learn to use your calculator !!!!!!

calculating the pH of alkalis using Kw to calculate [H+(aq)]

note for Ca(OH)2 that [OH-(aq)] = 2 x [Ca(OH)2(aq)]

but why might it give a weaker acid than sodium hydroxide?

[OH-(aq)] = ][H

K

(aq)

w

How science works Page 33 Measuring pH

Summary Questions Page 31 Questions 1 - 5

Exam Style Questions Page 50 Questions 3


A2 Chemistry (Heinemann) AQA 38 40, 42 - 45 Q7 - 10 on pages 39 - 40

Q13, 14 on pages 37 38 Q 3, 4 on page 55

Q 7 on page 56

Chemguide Strong acids pH



12

[HA(aq)]

(aq)](aq)][A[H-

]O

O

(l)2

-

3

[HA(aq)][H

(aq)](aq)][A[H

Weak Acids and Ka

weak acids are only partially ionised

HA(aq) + H2O(l) H3O+(aq) + A-(aq)

Kc =

[H2O] ~ constant

[A-(aq)] = [H3O+(aq)] = [H+(aq)]

given that it is a weak acid we can assume the degree of dissociation is minimal hence:

[acid]equilibrium ~ [acid]initial

Ka = Kc[H2O] =

Ka =

pKa = -log10Ka

Ka = 10- pKa

relatively higher Ka / relatively lower pKa = stronger acid (given the same concentration)

value is independent of concentration and therefore more useful

you should be able to calculate the pH of a weak acid of known concentration using Ka and

calculate Ka from the pH of a weak acid of known concentration




A2 Chemistry (Heinemann) AQA 35 36, 40 42 Q11 - 12 on pages 41 - 42

Q 1 on page 55

Chemguide Weak acid

[HA(aq)]

(aq)][H2



13

Titration Curves

as an alkali is added to an acid the pH increases and visa-versa

you should also be able to calculate the pH at any point in the addition of sodium hydroxide to a monoprotic acid (and visa-versa) including weak acids

typical errors to avoid are not converting to moles and getting the stoichiometric ratio wrong

most likely you will forget to use the total volume of the solution created and thus get the concentration wrong and therefore the pH

another likely error with a weak acid where xs alkali has not been added is assuming that the remaining acid is fully dissociated i.e. forgetting to use Ka to determine [H+(aq)]

equivalence point is where two solutions have reacted in stoichiometrically the correct molar ratio this will be the vertical point on the titration curve where the pH changes markedly

pHs at equivalence point and appropriate curves for:

http://www.avogadro.co.uk/chemeqm/acidbase/titration/phcurves.htm

pay particular attention to the position of the initial pH for strong and weak acids and look carefully at how it changes at the start

also carefully note the position of the equivalence point the mid point of the vertical section

finally ensure that a sensible final pH is shown to reflect the use of a strong or a weak base.

you should be able to calculate concentrations of an unknown acid or alkali from the results of a titration pretty much as was the case for AS level but there will be more likely hood of diprotic acids cropping up (e.g. sulphuric acid)



A2 Chemistry (Heinemann) AQA 44 48 Q15 on page 45

Q 16 on page 48

Q 2, 8 on pages 55 - 56

Chemguide Titration curves

SASB WASB SAWB WAWB



14

Ka of Weak Acids from Titration Curves

at the half equivalence point (half neutralisation point as stated in your text book) given that:

[HA] = [A]

Ka = [H+(aq)]

pKa = pH

the half equivalence point can be determined practically by determining the pH at half the equivalence point (half the volume) from a plotted titration curve

note that it is not half the pH value itself at the equivalence point that is used!

alternatively a titration can be repeated with half the volume of the already determined equivalence point and the pH then measured using a pH meter

End Point of an Indicator

indicators can only be used for a titration curves with a vertical section of >2 pH units

suitable indicators for acid-base titrations will have an end point and range that lie within that vertical section i.e. will thus exhibit a sharply defined colour change

pH meters can be used on coloured solutions

indicators are weak acids

HIn(aq) H+(aq) + In-(aq)

Ka (or Kin) = ][HIn

]][In[H

(a q )

(a q )-

(a q )

at end point [In-(aq)] = [HIn(aq)] Kin = [H+(aq)]

pKa = pKin = pH




A2 Chemistry (Heinemann) AQA 49 51 Q17 - 18 on pages 50 51 Q 5 on page 56

Chemguide Indicators



15

Buffer Solutions

the effect of pH changes e.g. lemon juice and the proteins in milk

buffers are designed to maintain pH stability

you must understand buffer solution in terms of the response of an equilibrium system to the addition of hydrogen or hydroxide ions

Acidic Buffer

HA H+ + A- reservoir of HA provided by acid

NaA Na+ + A- reservoir of A- provided by the sodium salt of the acid

Basic Buffer

NH3 + H+ NH4+ reservoir of NH3 provided by weak base

NH4Cl NH4+ + Cl- reservoir of NH4+ provided by the ammonium salt

Added H+(aq) is removed by A-(aq) provided by

the salt as the equilibrium shifts to the left

Added OH-(aq) is removed by

reacting with H+(aq) provided

by the acid as the equilibrium

shifts to the right

Added H+(aq) is removed by NH3 as the

equilibrium shifts to the right

Added OH-(aq) is removed by

reacting with NH4+(aq)

provided by the salt and the

equilibrium shifts to the left

An acidic buffer (for pH 7) consists of a

weak alkali and its soluble salt



16

Important Buffer Solutions

Carbonic acid-Bicarbonate Buffer in the Blood is by far the most important process for maintaining the acid-base balance (in our bodies there are also phosphate and protein

buffers).

a pH change of over 0.5 can be fatal

buffer solutions are also used in:

the food industry (acidity regulators) fabric dyeing where AZO dyes are used hair care products which are kept slightly acidic pH 5.5 as alkaline conditions

make hair look rough as microscopic scales on the surface of the hair are made to

stand up.

calibrating pH meters biochemical research (enzymes are denatured by pH extremes) and there are numerous other areas

Preparation of Buffer Solutions

a buffer solution can be made in two different ways:

1 adding a suitable soluble salt to an acid

2 partially neutralising a weak acid with a strong base up to the required pH

when [HA] = [A-] the buffer solution is equally able to deal with the addition of acid and base by equal sized reservoirs

for this reason a weak acid with a pKa relatively close to desired pH is selected for more effective buffering (see the calculations in the next section)

Added H+(aq) is removed as the

equilibrium shifts to the right

Added OH-(aq) is removed by reacting with

H+(aq) shitting the equilibrium to the left



17

[HA(aq)]

(aq)](aq)][A[H-

[ACID)]

](aq)][SALT[H

[SALT]

[ACID]

Calculating the pH of a Buffer Solution

HA H+ + A-

Ka =

but [HA] ~ [ACID]i and [A-] = [salt]

Ka =

[H+(aq)] = Ka

it is assumed that the volume of the weak acid solution is unchanged by the addition of a small quantity of its solid salt

since all particles are present in the same total volume of solvent we can make life easier by appreciating that the acid to salt:

moles ratio = concentration ratio

the best buffer will be that obtained at half the equivalence point:

[ACID] = [SALT]

and so [H+(aq)] = Ka

pH = pKa

equimolar amounts of acid and salt produces a buffer solution with a pH of the same numerical value as pKa

you should be able to determine the required combination of acid and salt to produce a buffer solution of a given pH

you should also be able to demonstrate by calculation that adding acid or alkali to a buffered solution changes the pH by less than for an un-buffered solution


Exam Style Questions Page 49 1, 2

Page 51 5

Page 160 5


A2 Chemistry (Heinemann) AQA 52 54 Q17 - 20 on pages 52 54 Q6, 9 on page 56

Chemguide Buffer

Will the dilution

of a buffer

solution change

the pH?



18

Organic Nomenclature

displayed formula every bond and every atom should be shown in examinations!!!

structural formula e.g. CH3CH2CH2OH

skeletal formula may be useful for prospective medical etc students

it is important that you understand the difference between 2-D displayed formula and the actual 3-D molecular shape as this will prove particularly relevant later

functional group one or more reactive sites on a hydrocarbon skeleton

homologous series same general formula + same functional group (hence same chemistry as the increase in chain length has little effect on their chemical reactivity)

nomenclature is based on four criteria:

root longest unbranched hydrocarbon chain (including main functional group)

suffix determines principal functional group (a pecking order exists)

prefix other changes to root molecule (e.g. side chains, functional group)

locant position of branch or substituent (e.g. double bond) on the main chain

look for the longest chain NOT the longest straight chain of carbons

start numbering at the end of the chain that results in the lowest numbers in the name or for the primary functional group position

alphabetical order is used where more than one type of functional group or branch is required in the prefix

mono, di, tri and tetra indicate multiple functional groups or branches of a given type (dont change alphabetical order)

commas and dashes are important - so learn how to do this correctly !!!!

An e is dropped if the next letter is a vowel: propan-2-ol, propane-1,2-diol

An a is added if inclusion of di, tri, etc., would put two consonants consecutively: buta-1,3-diene, not but-1,3-diene propanenitrile, not propannitrile or propanitrile.)

you will need to aware of nomenclature examples of the following (including cyclic variations):

alkanesalkeneshaloalkanes alcohols aldehydes and ketones

carboxylic acids esters acyl chlorides amines amides

amino acids benzene and its derivatives




Chemguide nomenclature

How Science

Works: H

How Science

Works: H How Science

Works: H



19

UNIT 2

Isomerism

isomers have the same molecular formula but different chemical and/or physical properties

remember that there are a number of ways in which isomerism exists that you have already met:

UNIT 1

isomers

structural isomers stereoisomers

position chain functional geometrical optical

group

Structural Isomerism

structural isomers have the same molecular formula but different structural formula

one form of structural isomerism is called chain isomerism unbranched chain and branched chain i.e. different hydrocarbon skeleton

e.g. How many versions of C4H8 can you find that represent structural isomers?

functional groups that are present at different positions are called positional isomers (there will be different numbers in the name)

e.g. propan-2-ol and propan-1-ol

functional group isomerism exists where the molecular formula is the same but different functional groups (and therefore chemical properties exist)

e.g. propane-l-ol and methoxy ethane, but-2-ene and cyclobutane

UNIT 4



20

How Science

Works: H

Stereoisomerism

Stereoisomers have the same molecular and structural formula but differ in the spatial

arrangement of their atoms.

Geometrical isomerism

this is consequential of the non-rotation of a double bond (unlike in alkanes) which is what you will state as the fundamental requirement in the exam

this lack of free rotation is consequential of the bond present in the alkene

a single carbon-carbon bond cannot give rise to this type of isomerism as (unless a chain is present) there is unrestricted rotation

Geometrical isomers have the same molecular formula, same structural formula but a different

spatial arrangement of the atoms due to the non rotation of the carbon-carbon double bond

whilst the minimum requirement is the presence of restricted rotation consideration should also be given to the substituents on each carbon

the two molecules on the left above are identical even though there is a carbon-carbon double bond as simply flipping vertically makes them super imposable

across a carbon-carbon double bond each carbon in turn must have different substituents

it doesn't matter whether the two groups are the same e.g. in the example on the right no amount of flipping or rotating makes them super imposable.

For geometrical isomerism to be possible both carbon atoms on the double bond must have

different atoms/groups attached to themselves, however, the carbon atoms can still both be

identical in that respect.

E and Z are used to distinguish between the two isomers (its quite EZy to do will

a little practice)

E isomers have the main grouping diagonally across the double bond

Z isomers have the main grouping on the same side of the double bond

cis and trans were previously used and in most cases E corresponds to trans and Z to cis BUT NOT ALWAYS

How science works Find out about the food industry and trans fats and the

hydrogenation of vegetable oils.

How Science

Works: A

How Science

Works: L



21

the trick is to identify what the main groupings are:

where the two atoms directly bonded to the carbons of the double bond with the largest

atomic numbers (highest priority) are diagonally opposite then it is deemed an E isomer

where the two atoms directly bonded to the carbons of the double bond with the largest

atomic numbers (highest priority) on the same side then it is deemed an Z isomer

if on one of the carbons the atoms directly bonded are identical then to establish the

highest priority grouping a tie break situation arises in which you look at the next

highest priority atom attached to each of them e.g -CH2Br beats CH2Cl and so on

take care here when using older text books as some molecules deemed trans in the old system would actually be Z in the new system i.e. across the double bond in one system does not directly yield across the double bond in the other e.g. 3-bromobut-2-ene

Example: but-2-ene

Step 1: split the alkene Step 2: assign the relative priorities. The two attached atoms are C and H, so since the atomic numbers C > H then the -CH3 group is higher priority.

Step 3: look at the relative positions of the higher priority groups : same side = Z, hence (Z)-but-2-ene.

The two attached atoms are C and H, so since the atomic numbers C > H then the -CH3 group is higher priority. Therefore the two high priority groups are on the opposite side, then this is (E)-but-2-ene.

E-Z transformations are possible given an energy source e.g. photochemistry and eyesight:

Optical Isomerism

How many functional

groups can you see

here?

How can you test to

provide evidence for

each one?

CH3

CH3

CH3

CH3

CH3

O

nerve impulse to the brain

O

C H 3 C H 3

C H 3

C H 3

C H 3

lightto eye

CH3

CH3

CH3

CH3

CH3

O

CH3

CH3

CH3

CH3

CH3

O

nerve impulse to the brain

O

C H 3 C H 3

C H 3

C H 3

C H 3

lightto eye



22

Optical isomerism exists where there is an asymmetric (i.e. chiral) carbon with four different

groups attached.

Optical isomers are non-super imposable molecules (enantiomers) which are mirror images of

one another.

dont just put mirror images as they can sometimes be superimposable where a plane of symmetry exists

if two groups are the same i.e. there is a plane of symmetry then a simple rotation yields the same spatial arrangement so they are not enantiomers

many chemicals synthesised in the lab produce equal amounts of both enantiomers (see lactic acid later) which is called a racemic mix

this is because the reagents are not stereo specific (rather like a left handed screwdriver)

however, living organisms tend to manufacture one enantiomer in preference to the other as determined by the reactive sites of the optically active enzyme used to construct it

enzymes are stereo specific reagents

the way that these enantiomeric molecules interact with biological systems can be different, for example:

carvone one enantiomer tastes of spearmint the other caraway

limonene one smells of lemons the other oranges (see if you can spot the chiral centre in

these enantiomers of limonene)

Can you give the

systematic name for this

molecule?



23

Enantiomers rotate plane polarised light in opposite directions (clockwise and

anticlockwise).

this physical property is the means by which they can be distinguished

one enantiomer rotates the polarised light clockwise (to the right) and is the (+) enantiomer;

the other rotates the polarised light anticlockwise (to the left) and is called the () enantiomer.

a racemic mix consists of a 50:50 mix of both isomers and this will therefore NOT rotate plane polarised light as the two enantiomers cancel one another out

How science works Page 62 The thalidomide tragedy


Exam Style Questions Page 64 2, 4, 5


Chemguide isomerism

How Science

Works: L



24

Aldehydes and Ketones

Physical properties

lower Mr aldehydes and ketones are miscible with water due to polar C=O bonds ability to hydrogen bond with water molecules

higher Mr molecules have greater VdW with one another due to increasing size of hydrocarbon tail hence miscibility is reduced for energetic reasons

Preparation

primary alcohol aldehyde heat with K2Cr2O7(aq)/H2SO4(aq), distil off aldehyde (boiling point lower than alcohol)

secondary alcohol ketone reflux under heat with K2Cr2O7(aq)/H2SO4(aq)

orange dichromate(VI) ions (Cr2O72-aq)) are reduced to green chromium(III) ions (Cr3+(aq))

Distinguishing Between Aldehydes and Ketones

ACIDIFIED POTASSIUM DICHROMATE SOLUTION

tests are based on the fact that aldehydes can be easily oxidised to a carboxylic acid while ketones cannot be.

Aldehyde + [O] Carboxylic Acid

reflux with K2Cr2O7(aq)/H2SO4(aq) orange dichromate(VI) ions (Cr2O72-(aq)) are reduced to

green chromium(III) ions (Cr3+(aq))

Care: The above test is only applicable if it is clear that the unknown sample is not a primary

or secondary alcohol.



25

NOTE: PRIMARY AND SECONDARY ALCOHOLS CANNOT BE OXIDISED BY

FEHLINGS OR TOLLENS HENCE DO NOT GIVE POSITIVE RESULTS WITH THE TESTS BELOW.

writing balanced redox equations under alkaline conditions is a little more involved than acidic conditions but you will find an excellent strategy on CHEMGUIDE see the link below

FEHLINGS TEST

warm with Fehlings solution blue complexed Cu2+(aq) is reduced to brick (an alkaline solution of a complexed red Cu2O(s)

copper ion) (this is the basis of the test for reducing sugars)

RCHO(aq) + 2Cu2+

(aq) + 4OH-(aq) RCOOH(aq) + Cu2O(s) + 2H2O(l)

TOLLENS REAGENT

warm with Tollens reagent complex ion [Ag(NH3)2]+(aq) is reduced to Ag(s), (aqueous silver nitrate in xs ammonia) hence the silver mirror effect

RCHO(aq) + 2[Ag(NH3)2]+(aq) + 2OH-(l) RCOOH(aq) + 2Ag(s) + 4NH3(aq) + H2O

strictly speaking you will get the carboxylate anion RCOO-(aq) under alkaline conditions rather than the carboxylic acid itself

additionally its worth knowing that methanoic acid (which has a hydrogen present HCOOH) can be oxidised to carbon dioxide via carbonic acid H2CO3 which then easily breaks down

into CO2 and H2O (see if you can work out the equations)

you should be aware of changes in the IR spectra during the oxidation reactions of compounds containing one or more oxygen atoms (and also the reduction reactions of said

molecules)


A2 Chemistry (Nelson Thornes) AQA 66 67, 69

A2 Chemistry (Heinemann) AQA 65 67 Q 1 on page 66

Chemguide Ionic alkaline, Tollens, Fehlings



26

Nucleophilic Addition

Reduction of Aldehydes and Ketones

polar nature of the carbonyl group - you would be wise to revise electro negativity and the nature of nucleophiles

aldehyde + 2[H] primary alcohol sodium tetrahydridoborate(III) (NaBH4) in water (+ ethanol as a universal solvent

ketone + 2[H] secondary alcohol for higher (longer chain) members)

BH4- provides the hydride ion, H- which acts as a nucleophile

mechanism is required (nucleophilic attack by hydride ion)

intermediate ion then gains H+ from the water present in the aqueous solvent just show H+ in the mechanism itself

Reaction with Hydrogen Cyanide

ethanal + HCN 2-hydroxypropanenitrile

HCN is made in situ using acidified sodium or potassium cyanide

HCN is a toxic gas so there are health and safety issues in its use

cyanide ions are toxic so this will not be done in the lab

note: an extra carbon is introduced into the chain so this is an important synthesis step

mechanism for reaction = nucleophilic addition

trigonal planar carbonyl group can be attacked from either side

hence products from aldehydes other than methanal exhibit optical isomerism i.e. there is an asymmetric (i.e. chiral) carbon with four different groups attached hence two non-

super imposable molecules (enantiomers) exist which are mirror images of one another.

a racemic mix is produced i.e. one that is 50:50 of both isomers and this will therefore NOT rotate plane polarised light in this case as the two enantiomers cancel one another out

symmetrical ketones do not yield enantiomers as no chiral centre is present

hydroxynitriles can be converted to a carboxylic acid by undergoing acid hydrolysis

this involves refluxing with a dilute acid (below I have shown the organic as named for clarity - although you MUST give its formula - as the other particles will always be the same

irrespective of the number of carbons in the chain)

the optical properties are preserved in the carboxylic acid

2-hydroxypropanenitrile + 2H2O + H+ 2-hydroxypropanoic acid + NH4+(aq) (lactic acid)

NA

NA



27

lactic acid synthesised this way will exist as racemates BUT that produced biologically will not be since enzymes are stereo specific yielding only one optical isomer hence this DOES

rotate plane polarised light.

Summary Questions Page 63 1 3 Page 70 1 - 4


A2 Chemistry (Nelson Thornes) AQA 60 63, 68, 70

A2 Chemistry (Heinemann) AQA 66 70, 57 62 Q 2 3 on pages 68 Q 4 -6 on page 70

Q 1, 3, 5 on page 82

Q 4 on page 149

Chemguide Aldehydes, Isomerism



28

Carboxylic Acids and Esters

Nomenclature

be aware of different ways to write the acid functional groups RCOOH, RCO2H etc

benzenecarboxylic acid C6H5CO2H

general structural formula of esters: RCO2R` e.g. CH3CO2CH2 CH2CH3

you should be able to work out the acid/alcohol used to make an ester and visa versa

esters and carboxylic acids are functional group isomers (easily distinguished by IR or nmr see later - or a simple chemical test using sodium carbonate and testing for evolved CO2)

Summary Questions Page 73 1 4

Physical properties

lower members of the carboxylic acids and esters are miscible with water due to hydrogen bonding with water

higher members are less miscible with water as the extent of VdW with themselves becomes prevalent (they are more soluble in sodium hydroxide solution do you know why?)

most carboxylic acids are crystalline solids (hydrogen bonding) whilst esters are typically oils and fats (no hydrogen bonding) compared to similar sized hydrocarbons

melting points can be used to identify and determine the purity (to some extent) of organic solids


A2 Chemistry (Heinemann) AQA 71 75 Q 7 10 on pages 71 - 74

Chemguide Carboxylic acids, Esters

Synthesis of carboxylic acids

primary alcohol or aldehyde carboxylic acid reflux under heat with xs K2Cr2O7(aq) and H2SO4(aq)

orange dichromate(VI) ions (Cr2O72-aq)) are reduced to green chromium(III) ions (Cr3+(aq))

A word about REAGENTS: When the examiner asks for a reagent then it is the name on the bottle

NOT the active particle introduced e.g. H+(aq) is not a reagent but H2SO4(aq) is.



29

Reactions of Carboxylic Acids and Esters

carboxylic acids are weak acids understanding required in terms of equilibria

chlorine-substituted ethanoic acids are more acidic due to negative inductive (I) effect of chlorine atoms due to their relatively high electronegativity

ethanoic acid + sodium hydroxide sodium ethanoate + water

ACID + BASE SALT + WATER

ethanoic acid + sodium carbonate sodium ethanoate + water + carbon dioxide

the latter is a useful test for the presence of COOH for which you will OBSERVE effervescence and that the gas evolved turns limewater (Ca(OH)2(aq)) cloudy and thus

INFER that CO2(g) was produced suggesting a carboxylic acid.

Ca(OH)2(aq) + CO2(g) CaCO3(s) + H2O(l)

acid catalysed esterification with an alcohol is relatively slow and gives a poor yield

ethanol + ethanoic acid ethyl ethanoate + water reflux under heat with cH2SO4

catalyst

acid + alcohol ester + water


Hydrolysis of Esters

ethyl ethanoate + water ethanoic acid + ethanol sulphuric acid catalyst

initiated by nucleophilic attack by the water molecule on the C+ of the carbonyl group

acid catalysed hydrolysis not complete due to an equilibrium being established

alkali (hot NaOH) catalysed (saponification) hydrolysis is quicker and goes to completion

sodium salt of the carboxylic acid is produced since the acid produced reacts with the sodium

hydroxide which will drive the equilibrium RHS as acid is removed from the system

adding xs sulphuric acid protonates the alkanoate anion carboxylic acid


A2 Chemistry (Heinemann) AQA 71 74 Q 11 on page 74

Chemguide Carboxylic acids, esters hydrolysis



30

Uses of Esters

solvents e.g. ethyl ethanoate for nail varnish

they evaporate relatively easily since there is no hydrogen bonding

esters have pleasant smells (unlike carboxylic acids which typically have unpleasant smells rancid fats ask to smell some butanoic acid youll get the idea) so are used in perfumes and in the food industry as flavourings:

butyl butanoate ethyl pentanoate 3-methylbutyl ethanoate

The cost of synthesising esters is often far less expensive than extracting them from natural sources

2-methoxyphenol is a waste product from the paper industry and can be used to make methyl vanillin (4-hydroxy-3-methoxybenzaldehyde) an artificial vanilla by the end of module 4 you might be able to draw its structure and suggest a possible synthesis strategy (although the

industrial process is more complex)

plasticizers added to plastics (e.g. PVC) to make them softer and more flexible as they weaken the IMF between polymer strands allowing them to slide over each other more

readily (loss over time makes the plastic brittle)

some phthalate based plasticizers have a possible association with birth defects although this is still a subject of some disagreement

Fats and Oils

animal fats + vegetable oils are triesters of propane-1,2,3-triol (glycerol) and fatty acids (long chained carboxylic acids)

they are triglycerides - three alcohol groups esterified by up to three different carboxylic acids (take care with the hydrolysis equation regarding the actual products e.g. under alkaline

conditions the anion of each acid group will be formed and each of these could be different)

oils have a higher degree of unsaturation cf fats (research E-Z isomerism and trans fats)

a greater number of double bonds reduces the relative flexibility (due to restricted rotation) of the molecule which in turn reduces the overlap efficiency of intermolecular forces

if a triglyceride undergoes alkaline hydrolysis (NaOH(aq))(saponification) then the salt of a fatty acid is produced e.g. sodium octadecanoate which is also known as sodium stearate and

better known as a soap

being ionic, soaps are soluble

the carboxylate anion RCOO- released is miscible with water (due to the hydrophilic carboxylate

group) and miscible with grease (due to its

hydrophobic tail) hence can solvate grease into

water allowing its removal

soap is precipitated out of solution by adding xs common salt (salting out) which can be understood from an equilibria point of view

How Science

Works: I



31

Glycerol (glycerine)

the co-product of soap making, propane-1,2,3-triol (glycerol) has numerous uses:

it is a useful solvent e.g. in medicines, food colouring

its extensive hydrogen bonding makes it good at retaining water thus preventing drying out

(e.g. facial creams)

and can be used to make nitro-glycerine (which mixed with finely divided silicon(iv) oxide =

dynamite)

Biodiesel

a renewable fuel made from oils obtained from vegetable matter (e.g. rape seed) which generally consist of a combination of any three of five common carbon chains linked by a

glycerol structure

it can also be made from animal fat and waste oil

small industrial plants tend to be batch processes but larger plants can use continuous flow methods which are more economical

methyl esters are produced by reacting these oils with methanol and a strong alkali at around 60oC

this is called base-catalysed transesterification

the methyl groups replace the glycerol structure on each of the fatty acids

Note: there may be three different methyl esters produced but the general formula of each is:

CH3OOCCxHy where x and y depend on chain length and degree of unsaturation

(this can be written the other way around CxHyCOOCH3)

the ester produced does not readily mix with the propane-1,2,3-triol co product so can be separated using a separating tank or a centrifuge

any remaining glycerol can be extracted using water (hydrogen bonding)

there may be some soap bi-product so further processing will be necessary to achieve a level of purity acceptable for a biofuel

rape seed (the yellow stuff you see in fields) produces rape methyl ester (RME) which is very similar to the diesel obtained from crude oil

this can be used directly or as a small % of filling station diesel



A2 Chemistry (Heinemann) AQA 75

Chemguide Fats and oils, soap, glycerol

How Science

Works: K



32

Acylation using Acyl Chlorides or Acid Anhydrides

nomenclature named after the parent carboxylic acid hence deemed acid derivatives (as are esters and amides)

Acyl chlorides: -oic acid replaced with: -oyl chloride

X = Cl e.g. ethanoyl chloride = an acyl chloride

Acid anhydrides e.g. ethanoic anhydride

X = OR

both are readily attacked by nucleophiles (book error on 79) due to very polar carbonyl group the polarity of which is increased by the electron withdrawing effect of X

thus this group of compounds are more useful than carboxylic acids in synthesis due to their high reactivity due to the enhanced + of the carbonyl carbon and since -X is a good leaving group cf OH

used to join an acyl group to the oxygen of water, alcohol or phenol or the nitrogen of ammonia or an amine

(ethanoylation is specifically when R = CH3)

H+ is eliminated in each case in the final step

Hydrolysis

ethanoyl chloride + water ethanoic acid + hydrogen chloride

very exothermic reaction, steamy fumes of hydrogen chloride are produced even when exposed to air (due to the reaction between HCl and water vapour), hence anhydrous

conditions essential with acyl chlorides and must be stated in exams

reaction is faster than with haloalkanes due to additional polarising effect of C=O

hydrogen chloride fumes can be tested for using:

conc. ammonia a drop at the end of a glass rod will create a white

smoke of ammonium chloride

silver nitrate solution a drop at the end of a glass rod will go cloudy as white

silver chloride is precipitated

mechanism is nucleophilic addition-elimination (condensation)

NOTE: H is not abstracted by Cl- (think why HCl is a strong acid!)

Ethanoic anhydride + water ethanoic acid + ethanoic acid

NAE

O

R C

:Nu

O

R C

X



33

Reaction with Alcohols and Phenols

ethanoyl chloride + ethanol ethyl ethanoate + HCl

yield of ester better than with carboxylic acid since reaction goes to completion

NOTE: acyl chlorides form esters with the phenol group unlike carboxylic acids. The

lone pair of the O in phenol is less readily available since these electrons are

delocalised into the ring system hence reducing the electron density and the thus

the effectiveness of phenol as a nucleophile (higher activation energy)

mechanism for reaction with an alcohol - ester formation is very similar to that with water

treat the alcohol as RO-H cf water as HO-H so RO- is added to the carbon rather than HO-

NOTE: H is not abstracted by Cl-

ethanoic anhydride + ethanol ethyl ethanoate + ethanoic acid

Reactions with Ammonia and Amines

ethanoyl chloride + ammonia ethanamide (a primary amide) + hydrogen chloride

violent reaction with aqueous ammonia at room temperature

ethanamide is the only product as further substitution does not occur due to strong electron withdrawing effect of C=O which makes the lone pair of the nitrogen less readily available

than with amines (cf haloalkanes + ammonia)

ethanoyl chloride + ethylamine N-ethylethanamide + hydrogen chloride (primary amine) (secondary amide)

ethanoyl chloride + phenylamine N-phenylethanamide + hydrogen chloride

(i) the product (an acyl derivative) is a white crystalline solid with a sharp melting point - can be recrystallised and used in the identification of the original amine

(ii) suggest reagents and mechanism for the synthesis of paracetamol N-(4-hydroxyphenyl)ethanamide

ethanoic anhydride + ammonia ethanamide + ethanoic acid

Summary Questions Page 82 1 4 Exam Style Questions Page 83 1, 3


A2 Chemistry (Heinemann) AQA 76 83 Q 12 - 15 on pages 77 81 Q 1 8 pages 82 - 83

Chemguide Acyl

NAE

NAE

NAE



34

The Synthesis of Aspirin

(aspirin is 2-ethanoyloxybenzoic acid dont panic you wont be asked for this on the exam)

the benefits of willow bark, which contains salicylic acid (2-hydroxybenzoic acid) a similar compound to aspirin, have been known for millennia e.g. Hippocrates (~460 B.C - 377 B.C.),

African Hottentots and North American Indians

it acts as an analgesic (pain killer) and has an anti-pyretic effect (body temperature)

salicylic acid was first isolated around 1829

the next step was to find a way to synthesise it rather than rely on extraction from a natural source as this can be problematic:

the source might be rare, or seasonal, or have a low concentration, or have harmful

contaminants

in 1860 it was synthesised from phenol (a by product of the production of town gas from coal) using the Kolbe process (NaOH and high pressure CO2)

but the problem was that it was tough on stomachs so alternatives with a similar structure (hence retaining the benefits) were searched for

aspirin itself had been synthetically produced in 1853 by a French chemist named Charles Frederic Gerhardt but he didnt realise its potential and took it no further

in 1898, a German chemist named Felix Hoffmann rediscovered Gerhardt's formula

he gave it to his father who was suffering from the pain of arthritis and with good results (he had tried other formulations before that!!) so convinced the German pharmaceutical company

Bayer to patent it in 1900 (the patent was ignored by the allies during WW1 and thereafter

along with the patent they held for heroin!)

its sales increased dramatically during the Spanish Flu epidemic of 1918

aspirin can be synthesised from salicylic acid using ethanoyl chloride or ethanoic anhydride

ethanoyl chloride + 2-hydroxybenzoic acid aspirin + HCl (salicylic acid)

Ethanoic anhydride + 2-hydroxybenzoic acid aspirin + ethanoic acid (salicylic acid)

both are more readily attacked by nucleophiles than the corresponding acid

acid anhydrides offer certain advantages over acyl chlorides, despite being less reactive, in that they are:

cheaper, less corrosive (no HCl liberated), less readily hydrolysed

How science works Page 82 Aspirin


A2 Chemistry (Nelson Thornes) AQA 82

A2 Chemistry (Heinemann) AQA 81

Q 14 on page 81

Chemguide Aspirin

How Science

Works: I, J

NAE



35

Aromatic Chemistry involves compounds containing a benzene ring (aka arenes look out for C6H5-)

empirical formula CH, Mr =78, molecular formula C6H6 Structure and Stability of Benzene

hydrogenation with 3 moles of H2 suggests the equivalence of 3 carbon-carbon double bonds

How science works Page 85 Kekules dream

Kekule did propose a cyclic structure but it could not account for some major aspects of the chemistry of Benzene:

1. no electrophilic addition reactions (e.g. with Br2(aq) in the dark) unlike alkenes 2. you dont get two isomeric (1,2) disubstituted compounds 3. X-ray diffraction studies found intermediate between double and single and equal C-C

bond length i.e. a symmetrical structure

4. enthalpy of hydrogenation (208 kjmol-1) is less than 3 x cyclohexene (360 kjmol-1)

initially a resonance hybrid structure was suggested

the true structure and shape of benzene can be explained in terms of

the delocalisation of the electrons of 6 x 2p orbitals (whilst its not on the syllabus, knowledge of orbital

hybridisation would be helpful have a look on Chemguide or in an older A-

level book)

delocalised electrons increase

relative stability (less electron - electron repulsion)

Summary Questions Page 86 1 3 Exam Style Questions Page 93 6


A2 Chemistry (Heinemann) AQA 84 - 87

Q 1 2 on page 87

Chemguide Bonding benzene

How Science

Works: A

Why is it drawn like this?

How Science

Works: F



36

Physical properties

non-polar colourless liquid and does not mix with water (no hydrogen bonding)

boiling point similar to 6 carbon aliphatic hydrocarbons but melting point is higher a planar structure allows better packing therefore more effective VdW.

Nomenclature

C6H5- phenyl group and simple monosubstituted aromatic compounds (arenes)

normally named as derivatives of benzene so benzene often forms the root of the name mono-substituted arenes are generally of formula C6H5X e.g. benzaldehyde C6H5CHO and

yield a peak of 77 on mass spectra due to the fragment C6H5+ (see later)

methylbenzene (toluene) chlorobenzene nitrobenzene benzaldehyde

benzenecarboxylic acid (benzoic acid) ethylbenzene (chloromethyl)benzene

some names also use phenyl or variations of it when the benzene is regarded as a side chain

phenol (instead of hydroxybenzene) phenylamine (aniline) (instead of

aminobenzene)

4-hydroxyphenyl ethanoate phenylethene (instead of ethenylbenzene)

aromatic compounds with more than one substituent

1 lowest numbering possible 2 alphabetical order for substituents (ignore di, tri etc)

2-hydroxybenzoic acid 2,4,6-trinitrotoluene 2,4,6-trinitrophenol (picric acid)

benzene-1,4-dicarboxylic acid (Terephthalic acid)

take heart naming aromatic compounds is complex but you will only have to deal with simple examples as it is far more important that you understand the chemistry!



Chemguide Naming aromatic



37

Dont use H2 for cHCL/Sn

Reactions of aromatic compounds

Electrophilic Substitution

high electron density in the ring attracts electrophiles

Br2(aq) is not decolourised (in the dark) electrophile must be powerful (+ve not just +)

why benzene resists attack by poor electrophiles stability of benzene compared to alkenes

electrophilic substitution rather than addition (c.f. alkenes) since this retains the relatively stable benzene ring structure hence is energetically more favourable

Nitration of Benzene and Methylbenzene

benzene nitrobenzene (a yellow oil) cH2SO4/cHNO3 refluxed at 50oC (nitrating mixture)

mechanism of formation of NO2+ the nitronium (old name = nitryl) cation (sulphuric acid acts as a homogeneous catalyst)

methylbenzene 2(and 4)-nitromethylbenzene cH2SO4/cHNO3 refluxed at 50oC

the methyl group is 2, 4, and 6 directing and activates the ring towards electrophilic substitution (hence faster rate) since it donates electron density into the ring thus making it

relatively less stable and more susceptible to attack by electrophiles.

further substitution requires more vigorous conditions (higher acid conc. and temperature) as the nitro group deactivates the ring towards electrophilic substitution by withdrawing electron

density from the ring (i.e. increasing the extent of delocalisation thus further stabilising the

ring) TNT is an explosive

nitrobenzene (a yellow oil) phenylamine cHCl/Sn or H2/Ni

C6H5NO2 + 6[H] C6H5NH2 + 2H2O

the above reaction is much simplified (whilst this equation is acceptable do you see why the initial product would not be phenylamine? a possible A* question perhaps?)

phenylamine is important for the production of AZO dyes since the end of 19th century (these replaced the old technique of mordant dying why not read about this it is interesting)

How Science Works Page 90 TNT

Exam Style Questions Page 92 1, 3


A2 Chemistry (Heinemann) AQA 88 - 89

Q 3, 4 on page 89

Chemguide Electrophilic substitution, nitration

ES

ES

ES



38

Friedel-Crafts Acylation

role of the AlCl3 - halogen carrier why is it a lewis acid

anhydrous conditions - AlCl3 readily hydrolysed by water before it does its job

electrophilic substitution mechanism proceeds via an acylium ion intermediate

you should be able to write a full balanced equation for the above

benzene + ethanoyl chloride phenylethanone warm with AlCl3 catalyst anhydrous conditions


Exam Style Questions Pages 92 93 2 - 5

A2 Chemistry (Nelson Thornes) AQA 91

A2 Chemistry (Heinemann) AQA 91, 184

Q 7, 8 on page 91

Q 1 8 on pages 94 - 95

Chemguide Acylation



39

Amines

nomenclature of primary, secondary and tertiary amines (by the way amine has ONE m in it!!!)

note how this differs from alcohols and haloalkanes

amino can sometimes be used (see amino acids later on)

Physical properties

boiling points are elevated by the ability to hydrogen bond but are lower than similar sized alcohols due to the relative electronegativity of O and N compared (ASK if you dont understand the significance of this)

lower members are gases

liquid amines smell like rotting (fishy) flesh adding acid removes this smell WHY?

smaller primary amines are water soluble (hydrogen bonding) producing alkaline solutions

solubility decreases with chain length (as with alcohols) due to increased mutual VdW

phenylamine is not very soluble in water as the VdW between the rings is significant compared to hydrogen bonding between the amine group and water

produce alkaline solutions in water when they dissolve


Basic properties

lone pair on N can be a nucleophile, base or ligand depending on the context

how good it is depends on the availability of that lone pair i.e. what the N is bonded to

Brnsted-Lowry bases proton acceptors, Lewis base = lone pair donor

ethylamine is more basic than ammonia due to +I inductive effect of the alkyl group (its also a better nucleophile than ammonia - see alkyl halides)

this explains the relatively more basic nature of small secondary amines but this does not hold true for tertiary amines where reduced solubility is a factor

phenylamine is less basic than ammonia since the lone pair on the nitrogen is less available due to delocalisation in the ring structure diagram

(phenylmethyl)amine is as basic as primary amines as the N is NOT bonded directly to the ring (note brackets in name why ?)

acid amides are relatively poor bases/nucleophiles/ligands as the strongly electronegative oxygen causes the lone pair on the nitrogen to be more withdrawn

pKa value of conjugate acid increases (i.e. is poorer) with increased basicity of the conjugate amine



40

amines react with acids to form salts in a similar manner to ammonia

ethylamine + hydrochloric acid ethylammonium chloride + water

solvation of insoluble phenylamine achieved by the addition of HCl to form a soluble salt phenylammonium chloride (reversed by adding NaOH)

phenylamine + hydrochloric acid phenylammonium chloride + water


Exam Style Questions Pages 108 9 1


A2 Chemistry (Heinemann) AQA 96 97, 99 - 100 Q 1, 2 on pages 96 97 Q 5, 6 on pages 99 - 100

Q 4 on page 104

Chemguide Amine name, amine base



41

Preparation

From haloalkanes

bromoethane ethylamine alcoholic solution of NH3 under pressure (lots of by-products so not a good method)

mechanism NH3 acts as nucleophile

excess ammonia is used to improve the yield of the primary amine

if xs bromoethane is used, since the ethylamine produced is also a nucleophile (stronger than ammonia due to the +I inductive effect of the alkyl group) it can react with the xs

bromoethane to give diethylamine

further substitution can then occur to produce: triethylamine and tetraethylammonium bromide (a quaternary ammonium salt cf ammonium ions)

note that acyl chlorides only yield primary amide (mechanism reminder) lone pair withdrawn by strong d+ on C due to the polarity of C=O caused by the electronegativity

of the O

From nitriles

Ethanenitrile ethylamine reduction by H2/Ni

RCN + 4[H] RCH2NH2

(note: at AS you were also told that acid hydrolysis of nitrile yields a carboxylic acid)

LiAlH4 but not NaBH4 (not a powerful enough reducing agent) can also be used (dont put H2 in

balanced equation in this case!!)

Aromatic Amines

benzene nitrobenzene (a yellow oil) cH2SO4/cHNO3 refluxed at 50oC (nitrating mixture)

nitrobenzene (a yellow oil) phenylamine reduction cHCl/Sn (or H2/Ni)

phenylammonium chloride* is initially produced, and addition of NaOH yields phenylamine (via deprotonation) which is then obtained by steam distillation

* strictly speaking its actually phenylammonium hexachlorostannate(IV)

NS



42

Uses of Amines

quaternary ammonium salts are used as cationic surfactants (long carbon chains help) in fabric

conditioners and hair conditioning products

the positive charges present will repel and add body to the hair/fabric whilst the tail section associates with the fabric

if you are particularly interested in laundry see: http://www.scienceinthebox.com/en_UK/glossary/

surfactants_en.html

http://www.chemistryquestion.com/English/Questions/ChemistryInDailyLife/27c_nonionic_s

urfactant.html

aryl amines are used in synthetic dyes

aryl amines are used to make certain drugs e.g. paracetamol

amines are used to make polymers e.g. polyurethanes (cavity wall insulation) and polyamides (nylons)

How Science Works Pages 100 - 1 Sulfa drugs

Summary Questions Page 101 1, 2

Exam Style Questions Pages 108 2 - 4


A2 Chemistry (Heinemann) AQA 97 98, 101 - 105 Q 7 on page 103

Q 13, 5-8 on pages 104 - 105

Chemguide Amine preparation, amine uses



43

C

N H

C OOH

H

CH

2

CH 2

CH 2

Amino acids

there are 20 important naturally occurring amino acids (amine is on the C next to the acid

group CO2H)

you should recognise that there is a CHIRAL centre hence amino acids exhibit optical isomerism (name the exception)

some amino acids have been identified in space (those of you who are interested in science might read the next link)

http://www.newscientist.com/article/dn7895-space-radiation-may-select-amino-acids-for-life.html

the amino acid proline is a 2o amine all the others are primary amines

Extra Info for potential medical/biochemistry/pharmacy students:

amino acids in proteins are all L-isomers

this does not necessarily mean plane polarised light is rotated the same way

if L(+) then D(-) for a given amino acid and L(-)/D(+) dont exist in our natural system

CORN law I have provided some info about this on my site it is not on the exam but it may be helpful at your interview for medical school given the biochemical significance


Page 107 1

R can vary

R = H glycine

R = CH3 alanine

R = CO(OH)CH2

aspartic acid

etc



44

Zwitterions

the amino group and the carboxyl group of each amino acid are both ionisable

the acidic carboxyl group (with a pKa of about 3 its a weak acid) is deprotonated

the basic amino group (with a pKa of around 9 its a weak base) is protonated

amino acids thus exist as zwitterions with both a positive and negative charge present

Zwitterions are amphoteric i.e. they exhibit both acidic and basic properties in solution because of the two functional groups

they thus form salts with both acids and bases (note all similar groups ionised as appropriate)

amino acids can thus act as buffers (hence regulate pH)

charge on the zwitterion ion depends on pH, -ve at high pH and +ve at low pH

at a given pH, the isoelectric point of the amino acid there will be no overall charge

as the + and cancel

this pH varies from amino acid to amino acid and provides a means of separating

them using a technique called

electrophoresis.

Extra Info for potential medical/biochemistry/pharmacy students:

you might be interested to find out how electrophoresis works by looking at this link

http://www.saburchill.com/IBbiology/chapters01/003.html

Amino acids exist as zwitterions in the solid state and thus have strongly ionic character

this explains their high solubility in polar solvents e.g. water

it also explains the high MPt (white crystalline solid when pure)

MPt too high to be accounted for by hydrogen bonding alone - supporting existence of Zwitterions and ionic nature of amino acids in the solid state

Exam Style Questions Pages 108 5, 8, 11

Adding OH-

pH increasing

Adding H+

pH decreasing



45



46

Condensation Polymerisation and hydrolysis

peptide (amide) links are formed between 2 amino acids by condensation reactions (where a small molecule such as water is eliminated) to form a dipeptide

Polypeptide (~50 amino acids) are formed by condensation polymerisation and catalysed by

enzymes

sequence of amino acids in a protein is called its primary protein structure

hydrogen bonding between C=O and N-H

controls shape - secondary protein structure - -

helix (coiled) and -pleated sheet (folded)

tertiary structure involves further bending and twisting (a good analogy is a knotted,

multicoloured telephone coil)

the stretching of wool is dependent on hydrogen bonding the length of which can be

reversibly increased up to a limit (very hot water

can break these ruining the fluffyness)

hydrolysis of proteins is achieved by refluxing with acid, base or enzyme catalyst (cf hydrolysis of an amide) in effect reversing the process shown in the diagram above

the liberated amino acids can then be separated by paper chromatography (developed by treating with ninhydrin which colours amino acids violet

some enzymes are selective and only partially hydrolyse certain proteins enabling amino acid sequences to be identified

enzymes are themselves proteins and are very specific in what they catalyse (substrate) due to their shape

their shape is dependent on hydrogen bonding hence their activity is sensitive to elevated temperatures where they are denatured

How Science Works Pages 107 Robots in the lab


Exam Style Questions Pages 109 6, 8 - 10


A2 Chemistry (Heinemann) AQA 111 114, 57 62 Q 8 - 12 on pages 111 113 Q 2 4, 5 - 6 on pages 115 - 116

Chemguide Zwitterions, peptide



47

Polymerisation

Addition Polymerisation

addition across the carbon-carbon double bond of a single unsaturated monomer

occurs by a free radical mechanism (details not required) started by an initiator (e.g. a peroxide) which is incorporated at the start/end of the polymer chain

you should be able to work out monomer from polymer and visa-versa (tip use the >C=C< form of the monomer i.e. rewrite it)

if asked for a single repeating unit dont show brackets otherwise use [C-C]-n format

ethene poly(ethene)

propene poly(propene)

chloroethane poly(chloroethene) (PVC)

phenylethene poly(phenylethene) (polystyrene)

tetrafluoroethene poly(tetrafluoroethene) (PTFE or Teflon)

methyl 2-methylpropenoate perspex

Summary Questions Page 114 1-4

Exam Style Questions Pages 122 4, 5


A2 Chemistry (Heinemann) AQA 106 -108

Q 1-3 on page 106

Q 7, 8 on page 116

Chemguide Addition polymerisation

heat, pressure, catalyst

in all cases



48

Condensation Polymerisation

Polyesters

two different types of monomers, one a diol, the other a dicarboxylic acid

this polymer is connected by ester linkages

you should be able to draw repeating units and determine the monomers used to make a given

polymer colour coding diagrams helps here

condition usually involve heat + catalyst

look for CO2- in structural formula

balanced equation (dont forget 2n H2O !!!!)

ethane-1,2-diol + benzene-1,4-dicarboxylic acid PET (ethylene glycol) (terephthalic acid)

the chain repeat unit of PET is:

commonly just called polyester PET, poly(ethylene terephthalate) was initially used as a

fibre (e.g. Terylene and Dacron)

it is now used extensively in plastic containers e.g. for fizzy drink bottles it does not smash on impact

you can read about the invention of polyester at:

http://inventors.about.com/library/inventors/blpolyester.htm

those of you doing textiles might also like to visit the polyester story at

http://schwartz.eng.auburn.edu/polyester/polyester.home.html

poly(2-hydroxypropanoic acid) used in surgery as its broken down by enzymes/body fluids over a number of days (why is this good?)

how would you synthesise it from ethene?

you can find out more about plastics from renewable raw materials and biologically degradable plastics at this site (its also a good example of (or idea for) an EPQ project!):

http://www.rsc.org/education/teachers/learnnet/green/docs/plastics.doc

You will find it useful to look at the reaction

pathways template that I have put on line

How Science

Works: I



49

Polyamides

two different types of monomers, one a diamine, the other a dicarboxylic acid

this polymer is connected by amide linkages

an diacyl chloride can be used instead of the acid but what would be the pros and cons of this

nylon-6,6 is so called as both monomers have 6 carbons

nylon is a polyamide (just like proteins)

1,6-diaminohexane + hexanedioic acid nylon-6,6

you might be interested in the history of nylon its an ideal stocking filler for Xmas see: http://www.cha4mot.com/p_jc_dph.html and http://inventors.about.com/od/nstartinventions/a/nylon.htm

polyamides have extensive hydrogen bonding between

parallel strands (cf protein

structure)

when nylon is spun into fibres, amide groups on

adjacent chains form

hydrogen bonds making

nylon yarn strong.

kevlar is an example of an aromatic polyamide and is made from the monomers benzene-1,4-dicarboxylic acid and benzene-1,4-diamine

the molecule is flat because of the aromatic groups

the uses of Kevlar are related to its strength (its several times stronger than steel)

this is related to the packing together of sheets of molecules held together by hydrogen bonds formed between N H groups and C = O groups on adjacent molecules

here is a scuba diving site that has an excellent overview of natural and synthetic polymers have a look it is very good: http://njscuba.net/artifacts/matl_polymers.html

How Science

Works: I

Medieval chainmail similarity?



50

Environmental Issues

polyalkenes are saturated, have no polar bonds and have strong C-C and C-H bonds hence they are relatively unreactive e.g. with acids, alkali or oxidants

this in turn makes them difficult to dispose of

non biodegradability means they last a long time but you can use landfill not ideal though

combustion yields toxic and greenhouse gases e.g CO and carbon particulates, NO2 and HCN from polyurethane in older upholstery or HCl and dioxins released through combustion

of halogenated plastics such as PVC and of course there will always be CO2 produced

recycling is expensive as the plastics must be identified and separated from other waste

energy production is an option but as this involves combustion there will be CO2 produced

using as chemical feedstock after cracking

use biodegradable/photodegradable polymers instead is an option

polyesters and polyamides both have polar bonds hence they are potentially biodegradable

they are broken down by (catalytic) hydrolysis in acid/alkaline solution or with enzymes although this can take a long time

recycling these materials e.g. terylene will save on natural resources and energy in their initial production (as well as reducing the need for landfill), however, as they are biodegradable

their structural integrity will diminish after repetitive usage

as with polyalkanes the cost of collection, separation and transportation (require energy and are labour intensive) must be taken into account

How Science Works Pages 120 - 1 Hermann Staudinger + Q 1,2


Page 121 1-4

Exam Style Questions Pages 121 - 123 1, 2, 3, 6, 7


A2 Chemistry (Heinemann) AQA 108 -110

Q 4 - 8 on page 109 - 110

Q 1, 5 on pages 115 - 116

Chemguide Condensation polymerisation

How Science

Works: J



51

Aliphatic synthesis You should be able to write full chemical equations, and identify the type for all the reactions listed:

Oxidation, Reduction, Addition, Elimination, Addition-Elimination (Condensation),

Substitution, Hydration, Dehydration, Hydrogenation, Dehydrogenation, Hydrolysis

Mechanisms: Nucleophilic Addition NA, Nucleophilic Substitution NS, Electrophilic Addition EA,

Nucleophilic Addition-Elimination NAE, Free Radical Substitution FRS or Elimination E.

Alkanes haloalkane, alkene

methane + Cl2 chloromethane FRS

alkanes can be used to produce alkenes by thermal cracking

Alkenes haloalkane, alkane, alcohol, alkoxyalkane

ethene + HBr bromoethane EA

ethene + Br2 1,2-dibromoethane EA

ethene + H2 ethane Ni catalyst, ~200oC (catalytic hydrogenation)

ethene + H2O ethanol EA

Method 1 cH3PO4 on a silica support with high temperature and pressure,

Method 2 first react with cold cH2SO4, then warm with water

Alkyl Halides amine, alcohol, nitrile, alkene

bromoethane + NH3 ethylamine + HBr NS heat with xs ammonia under pressure to minimise further substitution

bromoethane + OH-(aq) ethanol + Br- NS reflux with dilute NaOH dissolved in water

bromoethane + CN- propanenitrile + Br- NS boil under reflux with alcoholic NaCN or KCN (NOT HCN)

propanenitrile + 4[H] propylamine reduction by H2/Ni

bromoethane + OH- ethene + H2O + Br- E reflux with NaOH dissolved in ethanol



52

Alcohols carbonyl (aldehyde and ketone), carboxylic acid, ester, haloalkane, alkene

ethanol + [O] ethanal + H2O mild conditions - K2Cr2O7(aq)/H2SO4(aq) dist

Documents

Unit 4 Chemistry notes