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Before you start it would be helpful to… ALDEHYDES & KETONES Before you start it would be helpful to… know the functional groups found in organic chemistry know the arrangement of bonds around carbon atoms recall and explain the polarity of covalent bonds
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ALDEHYDES & KETONES CONTENTS Prior knowledge
Bonding in carbonyl compounds Structural differences Nomenclature
Preparation Identification Oxidation Nucleophilic addition
Reduction Before you start it would be helpful to
ALDEHYDES & KETONES Before you start it would be helpful to
know the functional groups found in organic chemistry know the
arrangement of bonds around carbon atoms recall and explain the
polarity of covalent bonds CARBONYL COMPOUNDS - STRUCTURE
Structurecarbonyl groups consists of a carbon-oxygen double bond
the bond is polar due to the difference in electronegativity
Difference ALDEHYDES -at least one H attached to the carbonyl group
C = O H C = O H CH3 CARBONYL COMPOUNDS - STRUCTURE
Structurecarbonyl groups consists of a carbon-oxygen double bond
the bond is polar due to the difference in electronegativity
Difference ALDEHYDES -at least one H attached to the carbonyl group
KETONES -two carbons attached to the carbonyl group C = O H C = O H
CH3 C = O CH3 C = O C2H5 CH3 CARBONYL COMPOUNDS - FORMULAE
MolecularC3H6O StructuralC2H5CHOCH3COCH3 Displayed Skeletal C = O H
C2H5 C = O CH3 HCCCH HOH H H HCCCO HHH H H O O CARBONYL COMPOUNDS -
NOMENCLATURE
Aldehydes C2H5CHOpropanal KetonesCH3COCH3propanone
CH3CH2COCH3butanone CH3COCH2CH2CH3pentan-2-one
CH3CH2COCH2CH3pentan-3-one C6H5COCH3phenylethanone CARBONYL
COMPOUNDS - FORMATION
ALDEHYDES Oxidation of primary (1) alcoholsRCH2OH [O] > RCHO H2O
beware of further oxidationRCHO [O] > RCOOH Reduction of
carboxylic acids RCOOH [H] > RCHO + H2O CARBONYL COMPOUNDS -
FORMATION
ALDEHYDES Oxidation of primary (1) alcoholsRCH2OH [O] > RCHO H2O
beware of further oxidationRCHO [O] > RCOOH Reduction of
carboxylic acids RCOOH [H] > RCHO + H2O KETONES secondary (2)
alcoholsRCHOHR [O] > RCOR H2O [O] means oxidizing agent like
Sodium/potassium dichromate(VI)/potassium manganate (VII) OXIDATION
OF PRIMARY ALCOHOLS
PRIMARY ALCOHOLS OXIDATION TO ALDEHYDES DISTILLATION Aldehyde has a
lower boiling point so distils off before being oxidisedfurther to
get higher yield of Aldehyde OXIDATION OF PRIMARY ALCOHOLS
If you want to produce carboxylic acid from primary alcohol, heat
underrefluxand use excess/sufficient oxidizing agent to get higer
yield ofCarboxylic acid Reactants = Primaryalcohol and
excessoxidizing agent OXIDATION OF SECONDARY ALCOHOLS
SECONDARY ALCOHOLS OXIDATION TO CARBOXYLIC ACIDS REFLUX Note :
volatile compounds Chemical compounds that transition to gas at low
temperatures Reflux condenser prevents volatilecompounds like
ethanal, ethanol andethanoic acid vapours from leavingthe flask
Aldehyde condenses back into the mixture and gets oxidised to the
acid Why 1 and 2 alcohols are easily oxidised and 3 alcohols are
not
OXIDATION OF ALCOHOLS Why 1 and 2 alcohols are easily oxidised and
3 alcohols are not For oxidation to take place easily you must have
two hydrogen atoms onadjacent C and O atoms. 1 2 3 OXIDATION OF
ALCOHOLS H H R C O + [O] R C O + H2O H H H H
Why 1 and 2 alcohols are easily oxidised and 3 alcohols are not For
oxidation to take place easily you must have two hydrogen atoms
onadjacent C and O atoms. H H RC O [O] RC O H2O HH 1 2 3 H H RC O
[O] RC O H2O RR This is possible in 1 and 2 alcohols but not in 3
alcohols. R H RC O [O] R PHYSICAL PROPERTIES OF CARBONYL
COMPOUNDS
Boiling points Carbonyl compounds are polarso induced
dipole-induced dipole as well as London forces However, hydrogen
bonding is possible between oxygen atoms and carbonyl groups and OH
group in water Simpler aldehydes such as methanal is freely
soluable in water. The simplest keytone, propanone mixes freely
with water CARBONYL COMPOUNDS - IDENTIFICATION
Method strong peak around cm-1 in the infra red spectrum Method
formation of an orange precipitate with 2,4-dinitrophenylhydrazine
Although these methods identify a carbonyl group, they cannot tell
the difference between an aldehyde or a ketone. To narrow it down
you must do a second test. 2,4-DINITROPHENYLHYDRAZINE
Structure Use reacts with carbonyl compounds (aldehydes and
ketones) used as a simple test for aldehydes and ketones makes
orange crystalline derivatives - 2,4- dinitrophenylhydrazones
C6H3(NO2)2NHNH2 How would you use the purified product to identify
the compound ??
Measure melting temperature / point Compare with
literature/database / known value CARBONYL COMPOUNDS -
IDENTIFICATION
Differentiationto distinguish aldehydes from ketones, use a mild
oxidising agent Tollens Reagent ammoniacal silver nitrate mild
oxidising agent which will oxidise aldehydes to form carboxylic
acid butnotoxidise ketones contains the diammine silver(I) ion -
[Ag(NH3)2 ]+ the silver(I) ion is reduced to silver Ag+(aq)+ e >
Ag(s) the test is known as THE SILVER MIRROR TEST Warming To liens'
reagent with an aldehyde produces a precipitate of silver which
coats clean glass with a shiny layer of silver so that it acts like
a mirror (left). There is no reaction with a ketone (right)
CARBONYL COMPOUNDS - IDENTIFICATION
Differentiationto distinguish aldehydes from ketones, use a mild
oxidising agent Fehlings Solution / Benedicts solution contains a
copper(II) complex ion giving a blue solution on warming, it will
oxidise aldehydes to form carboxylic acid butnot oxidise ketones
the copper(II) is reduced to copper(I) a red precipitate of
copper(I) oxide, Cu2O, is formed The silver mirror test is the
better alternative as it works with all aldehydes Ketones do not
react with Tollens Reagent or Fehlings Solution Fehling's reagent
is used to test for aldehydes
Fehling's reagent is used to test for aldehydes. The reagent has a
blue colour as it contains copper(II) ions. The test tube in the
middle contains Fehling's reagent that has been reduced by an
aldehyde, to form an orange-brown precipitate of copper(I) oxide.
The test tubes on the left and right contain Fehling's reagent and
ketones. Ketones are very similar to aldehydes but do not react
with Fehling's reagent, hence the colour is unchanged.
TRIIODOMETHANE (IODOFORM) REACTION WITH ALDEHYDES AND KETONES
Triiodomethane (iodoform) reaction can be used to identify the
presence of a mythylgroup next to carbonyl group Mythyl group next
to carbonyl group Note: An alternative reagent for idoform reaction
is a mixture ofpotassium iodide and sodium chlorate(I) solutions
CARBONYL COMPOUNDS - CHEMICAL PROPERTIES
OXIDATION provides a way of differentiating between aldehydes and
ketones mild oxidising agents are best aldehydes are easier to
oxidise powerful oxidising agents oxidise ketones to a mixture of
carboxylic acids ALDEHYDESeasily oxidised to acids RCHO(l) [O] >
RCOOH(l) CH3CHO(l) [O] > CH3COOH(l) KETONESoxidised under
vigorous conditions to acids with fewer carbons C2H5COCH2CH3(l) [O]
> C2H5COOH(l) CH3COOH(l) Oxidation by potassium dichromate
Acidified potassium dichromate CARBONYL COMPOUNDS - NUCLEOPHILIC
ADDITION
Mechanismoccurs with both aldehydes and ketones involves addition
to the C=O double bond unlike alkenes, they are attacked by
nucleophiles attack is at the positive carbon centre due to the
difference in electronegativities alkenes are non-polar and are
attacked by electrophiles undergoing electrophilic addition Group
Bond Polarity Attacking species Result ALKENES C=C NON-POLAR
ELECTROPHILES ADDITION CARBONYLS C=O POLAR NUCLEOPHILES ADDITION
CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
Reagenthydrogen cyanide - HCN (in the presence of KCN)
Conditionsreflux in alkaline solution Nucleophilecyanide ion CN
(Mostly KCN also added with HCN) Product(s)hydroxynitrile
(cyanohydrin) Equation CH3CHO HCN> CH3CH(OH)CN
2-hydroxypropanenitrile NotesHCN is a weak acid and has difficulty
dissociating into ions HCN H CN the reaction is catalysed by alkali
which helps produce more of the nucleophilic CN CARBONYL COMPOUNDS
- NUCLEOPHILIC ADDITION
MechanismNucleophilic addition Step 1CN acts as a nucleophile and
attacks the slightly positive C One of the C=O bonds breaks; a pair
of electrons goes onto the O STEP 1 CARBONYL COMPOUNDS -
NUCLEOPHILIC ADDITION
MechanismNucleophilic addition Step 1CN acts as a nucleophile and
attacks the slightly positive C One of the C=O bonds breaks; a pair
of electrons goes onto the O Step 2A pair of electrons is used to
form a bond with H+ Overall, there has been addition of HCN STEP 1
STEP 2 CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
MechanismNucleophilic addition Step 1CN acts as a nucleophile and
attacks the slightly positive C One of the C=O bonds breaks; a pair
of electrons goes onto the O Step 2A pair of electrons is used to
form a bond with H+ Overall, there has been addition of HCN STEP 1
STEP 2 CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
MechanismNucleophilic addition Step 1CN acts as a nucleophile and
attacks the slightly positive C One of the C=O bonds breaks; a pair
of electrons goes onto the O Step 2A pair of electrons is used to
form a bond with H+ Overall, there has been addition of HCN STEP 1
STEP 2 CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
ANIMATED MECHANISM Plane polarization of light ??
Light oscillates in many planes (directions) , when light is passed
through a Polaroid . It oscillates in only one plane Light
osccilating in many directions Optical isomerism Optical isomerism
is an example of stereo-isomerism.
It occurs when substances have the same molecular and structural
formulae, but one cannot be superimposed on the other. Put simply,
they are mirror images of each other (see the diagram below).
Molecules like this are said to be chiral (pronounced ky-ral), and
the different forms are called enantiomers. Optical isomers can
occur when there is an asymmetric carbon atom An asymmetric carbon
atom is one which is bonded to four different groups 2 enantiomers
/ Optical isomers do not superimpose
Does not superimpose CARBONYL COMPOUNDS - NUCLEOPHILIC
ADDITION
Watch out for the possibility of optical isomerism in
hydroxynitriles CN attacks from one side CN attacks from other side
CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
Watch out for the possibility of optical isomerism in
hydroxynitriles CN attacks from one side (above) CN attacks from
other side(below) ANIMATED MECHANISM TO SHOW HOW DIFFERENT ISOMERS
ARE FORMED
CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION ANIMATED MECHANISM TO
SHOW HOW DIFFERENT ISOMERS ARE FORMED 2 enantiomers form reacimic
mixture
CN attacks from both sides so forms 2 optical isomers /enantiomers
which forms a recimic mixture 2 enantiomers form reacimic mixture
How reacimic mixture forms ??
Rotate plane polarized light to anit clockwise (to the left) and is
called the () enantiomer. Rotate plane polarized light to clockwise
(to the right) and is the (+) enantiomer A mixture containing equal
concentrations of the (+) and () enantiomers is not optically
active (it will not rotate the plane of polarisation). It is called
a racemic mixture or racemate. Optical isomerism and reaction
mechanisms
The optical activity of the reactants and products of organic
reactions can help chemists to determine the mechanisms of
reactions. This is illustrated by the outcomes of the SN1 and SN2
mechanisms in nucleophilic substitution reactions. During the
one,step SN2 mechanism, the three groups that remain attached to
the central carbon atom are turned inside out. The molecule is
inverted like an umbrella in a high wind (Figure 6.12). This means
that an optically active halogenoalkane gives rise to an optically
active alcohol if substitution takes place by the SN2 mechanism. In
the two-step SN2 mechanism, a planar intermediate is formed after
the first step. However, attack by the nucleophile during the
second step can happen from either side of the planar intermediate.
The result is that starting with one optical isomer of a
halogenoalkane leads to a product which is a racemic mixture of the
two forms of the chiral alcohol. This means that the product is
optically inactive (Figure 6.13). The product of this reaction
above has a chiral centre
Theproduct of this reaction above has a chiral centre. Would you
expect the reaction to produce a solution that rotates the plane of
plane-polarized light? Explain your answer.?? No As: Reaction
site/carbonyl/aldehyde/moecule is planar Attack (equally likely)
from both sides (gives) racemic mixture CARBONYL COMPOUNDS -
REDUCTION WITH NaBH4
Reagentsodium tetrahydridoborate(III) (sodium borohydride), NaBH4
Conditionsaqueous or alcoholic solution MechanismNucleophilic
addition(also reduction as it is addition of H) NucleophileH
(hydride ion) Product(s)AlcoholsAldehydes are REDUCED to primary
(1) alcohols. Ketones are REDUCED to secondary (2) alcohols.
Equation(s)CH3CHO [H]>CH3CH2OH CH3COCH [H]>CH3CHOHCH3
NotesThe water provides a proton CARBONYL COMPOUNDS - REDUCTION
WITH LiAlH4
LiAlH4 is a powerful reducing agent whichconverts aldehydes to
primary alcohols and keytones to secondary alcohols. LiAlH4 is
easily hydrolized so, reagent is dissolved in dry ether dry ether
CARBONYL COMPOUNDS - REDUCTION WITH HYDROGEN
Reagenthydrogen Conditionscatalyst-nickel or platinum Reaction
typeHydrogenation, reduction Product(s)AlcoholsAldehydes are
REDUCED to primary (1) alcohols. Ketones are REDUCED to secondary
(2) alcohols. Equation(s)CH3CHO H >CH3CH2OH CH3COCH
H2>CH3CHOHCH3 NoteHydrogen also reduces C=C bonds CH2 = CHCHO H
>CH3CH2CH2OH
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