MISS CHOHAN – UNIT 2 EDEXCEL NOTES FSG

ALCOHOLS Alcohols contain the -OH functional group. General formula – CnH2n+1OH

Naming Alcohols The first part of the name of an alcohol is according to the longest carbon atom sequence. The second part

of the name is –ol. A number will be included to indicate the position of the alcohol group.

If the compound has an –OH group in addition to other functional group that need a suffix ending then the –OH can be named with the prefix hydroxy):

If there are two or more –OH groups then di, tri are used. Add the `e` on to the stem name though.

Bond Angles In Alcohols

All the H-C-H bonds and C-C-O are 109.50 (tetrahedral shape), because there are 4 bond pairs of electrons repelling to a position of minimum repulsion.

The H-O-C bond is 104.50 (bent line shape), because there are 2 bond pairs of electrons and 2 lone pairs repelling to a position of minimum repulsion. Lone pairs repel more than bond pairs so the bond angle is reduced.

Types Of Alcohols

There are three types of alcohol; primary, secondary and tertiary. They are classified according to the number of carbon groups attached to the carbon with the OH group.

MISS CHOHAN – UNIT 2 EDEXCEL NOTES FSG

Physical properties of alcohols

The OH group can take part in hydrogen-bonding, both as a donor (H is sufficiently +) and as an acceptor (through the two lone pairs on the oxygen atom). As a result alcohols have higher melting and boiling points than hydrocarbons of comparable molar mass.

They are also more soluble in water because of H-bonding: ethanol is miscible with water in all proportions. As the non-polar hydrocarbon chain becomes longer, it becomes harder for water to dissolve the alcohol: from C4 alcohols are less soluble, and don’t mix with water.

Reactions of Alcohols

1. Combustion All alcohols undergo combustion to form carbon dioxide and water. For example the equation for the combustion of butanol is as follows;

C4H9OH + 6O2 4CO2 + 5H2OAlcohols combust with a clean flame

2. Reaction With SodiumAll alcohols react with sodium.

2ROH + 2Na 2RONa + H2This equation is similar to the reaction of sodium with acid. The salt formed here is an alkoxide.e.g. Ethanol and sodium 2C2H5OH + 2Na 2 C2H5ONa + H2

Sodium ethoxideNote: This reaction is used to test for alcohols.

3. Nucleophilic Substitution Reactions of Alcohols To Form Halogenoalkanes

Various halogenating compounds can be used to substitute the –OH group for a halogen

a) Reaction with phosphorous (V) halide

ROH + PCl5 RCl + POCl3 + HCl

e.g. Propanol and phosphorus pentachlorideCH3CH2CH2OH + PCl5 CH3CH2CH2Cl + POCl3 + HCl

Note: This reaction with PCl5 (phosphorous(v)chloride) can be used as a test for alcohols. You would observe white misty fumes of HCl produced.

So,

3CH3CH2CH2OH + PCl3 3CH3CH2CH2Cl + H3PO4

CH3CH2CH2OH + SOCl2 CH3CH2CH2Cl + SO2 + HCl C2H5OH (l) + HCl (g) C2H5Cl (l) + H2O (l)

Observations

Effervescence The mixture gets

hot Sodium dissolves A white solid is

formed

NOTE:PCl5 / PCl3 / conc HCl / SOCl2 / mixture of NaCl + H2SO4 can all be uses for substituting a Cl

MISS CHOHAN – UNIT 2 EDEXCEL NOTES FSG

b) Forming Bromo and Iodoalalkanes

i.e.C2H5OH + HBr C2H5Br + H2OPI3 + 3 CH3CH2OH 3CH3CH2 I + H3PO3(The phosphorous (III) halide can be produced in situ by reacting red phosphorus and the halogen).

The relative reactivity’s of alcohols in halogenation are tertiary > secondary > primary alcohol.

4. Oxidation Of Alcohols

Alcohols can be oxidised with a mixture of dilute sulphuric acid with sodium or potassium dichromate(VII) solution, which together act as oxidising agents.

The exact reaction, however, depends on the type of alcohol, i.e. whether it is primary, secondary, or tertiary, and on the conditions.

In writing equations for these oxidation reactions [O] is used to represent the oxidising agent. When alcohols do react with acidified potassium or sodium dichromate VI a colour change from

orange to green is seen.

NOTE:For Br and I it is best to use PI5 , PI3 and Br equivalents. It is not suitable to use NaBr, or NaI + H2SO4 to produce HBr or HI because the sulphuric acid with oxidise the hydrogen halides to other products

10 - Primary alcohols form an aldehyde, and then on further oxidation, form carboxylic acids.

20 - Secondary alcohols form ketones, but no further oxidation takes place.

30 - Tertiary alcohols do not react with oxidizing agents.

Cr2O72-/H+

Heat/distil

Cr2O72-/H+

Heat/reflux

Cr2O72-/H+

Heat/reflux

MISS CHOHAN – UNIT 2 EDEXCEL NOTES FSG

Equations Showing Partial oxidation of 10 AlcoholsObservation:

the orange dichromate ion (Cr2O72-)

reduces to the green Cr 3+ ion

Note:

An aldehyde’s name ends in –al It always has the C=O bond on the first carbon of the chain so it does not need an extra number

Equations Showing The full Oxidation Of 10 AlcoholsObservation:

the orange dichromate ion (Cr2O7

2-) reduces to the green Cr3+ ion

Equations Showing The Oxidation of 20 Alcohols

Observation:

The orange dichromate ion (Cr2O7

2-) reduces to the green Cr3+ ion

When ketones have 5C’s or more in a chain then it needs a number to show the position of the double bond. E.g. pentan-2-one

MISS CHOHAN – UNIT 2 EDEXCEL NOTES FSG

Partial Oxidation Of Alcohols To Aldehydes (Distillation)

e.g. ethanol to ethanal.CH3CH2OH + [O] CH3CHO + H2O

To make the aldehyde, the one reagent is added dropwise to the other and the product is distilled off as it forms.

By distilling the aldehyde off as it forms, it means it will not undergo further oxidation to the acid.

Complete Oxidation Of Alcohols To Carboxylic Acids

e.g. ethanol to ethanoic acid. The mixture of reagents is heated under reflux. CH3CH2OH + 2[O] CH3CO2H + H2O

Heating under reflux

The apparatus shown here is used a large number of organic preparations.

The reaction mixture is placed in the pear shaped flask. It has a reflux condenser* fitted. This means that as the reactants are heated and the volatile liquids boil off, they are converted back to liquid in the condenser and return to the flask.

*A reflux condenser “is not a special type of condenser, it is an ordinary condenser fitted so that reflux takes place.

Note

the bulb of the thermometer should be at the T junction connecting to the condenser to measure the correct boiling point

the water goes in the bottom of the condenser to go against gravity. This allows more efficient cooling and prevents back flow of water.

NOTE:

Reflux is used when heating organic reaction mixtures for long periods. The condenser prevents organic vapours from escaping by condensing them back to liquids.

Never seal the end of the condenser as the build up of gas pressure could cause the apparatus to explode. This is true of any apparatus where volatile liquids are heated including the distillation set up

Anti-bumping granules are added to the flask in both distillation and reflux to prevent vigorous, uneven boiling.

MISS CHOHAN – UNIT 2 EDEXCEL NOTES FSG

Once the carboxylic has been formed, it needs to be separated from the reaction mixture and other products. This is done by distillation.i.e.

Distillation is used to separate a volatile product from a mixture of involatile substances, or substances that have a boiling point of at least 50oC higher than the component being collected

Distinguishing between Aldehydes and Ketones The fact that aldehydes can be further oxidised to carboxylic acids whereas ketones cannot be further oxidised is the chemical basis for tests that are commonly used to distinguish between aldehydes and ketones

Fehling’s (Benedict’s) solution

Reagent: Fehling’s Solution containing blue Cu2+ ions.

Conditions: heat gently

Reaction: aldehydes only are oxidised by Fehling’s solution into a carboxylic acid and the copper ions are reduced to copper(I) oxide

Observation: Aldehydes : Blue Cu2+ ions in solution change to a red precipitate of Cu2O. Ketones do not react Fehling’s (Benedict’s) solution

CH3CHO + 2Cu2+ + 2H2O CH3COOH + Cu2O + 4H+

NOTE:The presence of a carboxylic acid can be tested by addition of sodium carbonate. It will fizz and produce carbon dioxide

MISS CHOHAN – UNIT 2 EDEXCEL NOTES FSG

Purifying an organic liquid Put the distillate of impure product into a separating funnel. Wash product by

adding either:-1. sodium hydrogencarbonate solution , shaking and releasing the pressure

from CO2 produced. Sodium hydrogencarbonate will neutralise any remaining reactant acid.

2. Saturated sodium chloride solution. Sodium chloride will help separate the organic layer from the aqueous layer

Allow the layers to separate in the funnel, and then run and discard the aqueous layer.

Run the organic layer into a clean, dry conical flask and add three spatula loads of drying agent (anhydrous sodium sulphate) to dry the organic liquid.

The drying agent should:

a) be insoluble in the organic liquid b) not react with the organic liquid

• Carefully decant the liquid into the distillation flask

•Distill to collect pure product

MISS CHOHAN – UNIT 2 EDEXCEL NOTES FSG

MISS CHOHAN – UNIT 2 EDEXCEL NOTES FSG

MISS CHOHAN – UNIT 2 EDEXCEL NOTES FSG

MISS CHOHAN – UNIT 2 EDEXCEL NOTES FSG

MISS CHOHAN – UNIT 2 EDEXCEL NOTES FSG

MISS CHOHAN – UNIT 2 EDEXCEL NOTES FSG

MISS CHOHAN – UNIT 2 EDEXCEL NOTES FSG

MISS CHOHAN – UNIT 2 EDEXCEL NOTES FSG

MISS CHOHAN – UNIT 2 EDEXCEL NOTES FSG

MISS CHOHAN – UNIT 2 EDEXCEL NOTES FSG

MISS CHOHAN – UNIT 2 EDEXCEL NOTES FSG

MISS CHOHAN – UNIT 2 EDEXCEL NOTES FSG

MISS CHOHAN – UNIT 2 EDEXCEL NOTES FSG

MISS CHOHAN – UNIT 2 EDEXCEL NOTES FSG

MISS CHOHAN – UNIT 2 EDEXCEL NOTES FSG

MISS CHOHAN – UNIT 2 EDEXCEL NOTES FSG

MISS CHOHAN – UNIT 2 EDEXCEL NOTES FSG

MISS CHOHAN – UNIT 2 EDEXCEL NOTES FSG