3.3.7 and 3.3.8
Nucleophilic addition to carbonyl compounds
Optical isomerism
Preparation of an organic solid:
o Purification of a crude organic solid - recrystallisation
o Purity test – melting point
o Identification - infra-red spectroscopy
Specification sections
3.3.7 Optical isomerism
Optical isomerism is a form of stereoisomerism and occurs as a result of chirality in molecules, limited to
molecules with a single chiral centre.
An asymmetric carbon atom is chiral and gives rise to optical isomers (enantiomers), which exist as non
superimposable mirror images and differ in their effect on plane polarised light.
A mixture of equal amounts of enantiomers is called a racemic mixture (racemate).
Students should be able to:
draw the structural formulas and displayed formulas of enantiomers
understand how racemic mixtures (racemates) are formed and why they are optically inactive.
explain why a stereospecific active site can only bond to one enantiomeric form of a substrate or drug.
(from 3.3.13.3 enzymes)
3.3.8 Aldehydes and ketones
Aldehydes are readily oxidised to carboxylic acids.
Chemical tests to distinguish between aldehydes and ketones including Fehling’s solution and Tollens’
reagent.
Aldehydes can be reduced to primary alcohols, and ketones to secondary alcohols, using NaBH 4 in
aqueous solution.
These reduction reactions are examples of nucleophilic addition.
The nucleophilic addition reactions of carbonyl compounds with KCN, followed by dilute acid, to produce
hydroxynitriles.
Aldehydes and unsymmetrical ketones form mixtures of enantiomers when they react with KCN followed
by dilute acid.
The hazards of using KCN.
Students should be able to:
write overall equations for reduction reactions using [H] as the reductant
outline the nucleophilic addition mechanism for reduction reactions with NaBH 4 (the nucleophile should
be shown as H–)
write overall equations for the formation of hydroxynitriles using HCN
outline the nucleophilic addition mechanism for the reaction with KCN followed by dilute acid
explain why nucleophilic addition reactions of KCN, followed by dilute acid, can produce a mixture of
enantiomers.
Page 1 of 12
, Reduction of aldehydes and ketones
Example equation:
CH3 CH3
\ |
C = O + 2[H] H - C - OH
/ |
H H
ethanal ethanol
Condensed equation: CH3CHO + 2[H] → CH3CH2OH
Adehydes reduced to primary alcohols
Ketones reduced to secondary alcohols.
Reagent and conditions:
NaBH4 dissolved in water is added at room temperature to the carbonyl compound and the
reaction mixture allowed to stand.
OR
• H2 in the presence of a catalyst e.g. nickel at 150OC
Note about reagents:
• NaBH4 is a selective reducing agent for carbonyl compounds i.e. it will not reduce other
groups such as C=C, -CN and –NO2
• H2 is not specific and will reduce all these groups
Questions:
1. Write balanced equations (using [H]) for the reduction of the following with NaBH4. In each
case name the product and classify it as a primary or secondary alcohol.
(a) propanal (b) propanone (c) pentan-3-one.
2. Give the structural formula and name of the carbonyl compound that produces the following
alcohols upon reduction:
(a) butan-1-ol
(b) butan-2-ol.
3. Draw the structural formula of the product when CH2 = CHCOCH3 is reduced with
(a) NaBH4
(b) H2 in the presence of a catalyst.
4. A carbonyl containing compound W is converted to an alcohol X. X is converted to 2-
methylpropene. Write structural formulae equations for the two reactions and in each case
name the type of reaction.
5. The BH4 ion is oxidised to the H2BO3 ion when it reduces carbonyl compounds. Attempt to
construct a half-equation for this oxidation reaction.
Page 2 of 12
Nucleophilic addition to carbonyl compounds
Optical isomerism
Preparation of an organic solid:
o Purification of a crude organic solid - recrystallisation
o Purity test – melting point
o Identification - infra-red spectroscopy
Specification sections
3.3.7 Optical isomerism
Optical isomerism is a form of stereoisomerism and occurs as a result of chirality in molecules, limited to
molecules with a single chiral centre.
An asymmetric carbon atom is chiral and gives rise to optical isomers (enantiomers), which exist as non
superimposable mirror images and differ in their effect on plane polarised light.
A mixture of equal amounts of enantiomers is called a racemic mixture (racemate).
Students should be able to:
draw the structural formulas and displayed formulas of enantiomers
understand how racemic mixtures (racemates) are formed and why they are optically inactive.
explain why a stereospecific active site can only bond to one enantiomeric form of a substrate or drug.
(from 3.3.13.3 enzymes)
3.3.8 Aldehydes and ketones
Aldehydes are readily oxidised to carboxylic acids.
Chemical tests to distinguish between aldehydes and ketones including Fehling’s solution and Tollens’
reagent.
Aldehydes can be reduced to primary alcohols, and ketones to secondary alcohols, using NaBH 4 in
aqueous solution.
These reduction reactions are examples of nucleophilic addition.
The nucleophilic addition reactions of carbonyl compounds with KCN, followed by dilute acid, to produce
hydroxynitriles.
Aldehydes and unsymmetrical ketones form mixtures of enantiomers when they react with KCN followed
by dilute acid.
The hazards of using KCN.
Students should be able to:
write overall equations for reduction reactions using [H] as the reductant
outline the nucleophilic addition mechanism for reduction reactions with NaBH 4 (the nucleophile should
be shown as H–)
write overall equations for the formation of hydroxynitriles using HCN
outline the nucleophilic addition mechanism for the reaction with KCN followed by dilute acid
explain why nucleophilic addition reactions of KCN, followed by dilute acid, can produce a mixture of
enantiomers.
Page 1 of 12
, Reduction of aldehydes and ketones
Example equation:
CH3 CH3
\ |
C = O + 2[H] H - C - OH
/ |
H H
ethanal ethanol
Condensed equation: CH3CHO + 2[H] → CH3CH2OH
Adehydes reduced to primary alcohols
Ketones reduced to secondary alcohols.
Reagent and conditions:
NaBH4 dissolved in water is added at room temperature to the carbonyl compound and the
reaction mixture allowed to stand.
OR
• H2 in the presence of a catalyst e.g. nickel at 150OC
Note about reagents:
• NaBH4 is a selective reducing agent for carbonyl compounds i.e. it will not reduce other
groups such as C=C, -CN and –NO2
• H2 is not specific and will reduce all these groups
Questions:
1. Write balanced equations (using [H]) for the reduction of the following with NaBH4. In each
case name the product and classify it as a primary or secondary alcohol.
(a) propanal (b) propanone (c) pentan-3-one.
2. Give the structural formula and name of the carbonyl compound that produces the following
alcohols upon reduction:
(a) butan-1-ol
(b) butan-2-ol.
3. Draw the structural formula of the product when CH2 = CHCOCH3 is reduced with
(a) NaBH4
(b) H2 in the presence of a catalyst.
4. A carbonyl containing compound W is converted to an alcohol X. X is converted to 2-
methylpropene. Write structural formulae equations for the two reactions and in each case
name the type of reaction.
5. The BH4 ion is oxidised to the H2BO3 ion when it reduces carbonyl compounds. Attempt to
construct a half-equation for this oxidation reaction.
Page 2 of 12
