Nucleophilic Addition

Many of the reactions which carbonyl compounds undergo are nucleophilic addition reactions
The carbonyl group -C=O, in aldehydes and ketones is polarised
The oxygen atom is more electronegative than carbon drawing electron density towards itself
This leaves the carbon atom slightly positively charged and the oxygen atom slightly negatively charged
The carbonyl carbon is therefore susceptible to attack by a nucleophile, such as the cyanide ion

The carbonyl group here has a dipole with a delta positive carbon and a delta negative oxygen

7.2.3 carbonyl dipole, downloadable AS & A Level Chemistry revision notes

General Mechanism with an aldehyde: 7.2.3 Nucleophilic addition with aldehydes, downloadable AS & A Level Chemistry revision notes General Mechanism with a ketone:

In both reactions, the nucleophile (Nu) attacks the carbonyl carbon to form a negatively charged intermediate which quickly reacts with a proton

Addition of HCN to carbonyl compounds

The nucleophilic addition of hydrogen cyanide to carbonyl compounds is a two-step process, as shown below
In step 1, the cyanide ion attacks the carbonyl carbon to form a negatively charged intermediate
In step 2, the negatively charged oxygen atom in the reactive intermediate quickly reacts with aqueous H⁺ (either from HCN, water or dilute acid) to form 2-hydroxynitrile compounds,
- e.g. 2-hydroxypropanenitrile

Examiner Tip

By convention, we write the formula of an ion then its charge, e.g. CN^-.

The actual negative charge on the cyanide ion is on the carbon atom and not on the nitrogen atom.
However, when writing it together as :CN^- you will not be penalised for writing the minus charge after the N.

This reaction is important in organic synthesis, because it adds a carbon atom to the chain, increasing the chain length
The products of the reaction are hydroxynitriles
- The nitrile group is the priority functional group so it is attached to carbon 1 and results in the suffix -nitrile
- The hydroxyl group is not the priority functional group so the hydroxyl group is named using the hydroxy- prefix, rather than the -ol suffix

Forming Enantiomers

Even if a starting material does not display optical isomerism, it can still form a product which does display optical isomerism
This is the case when aldehydes and ketones undergo nucleophilic addition with hydrogen cyanide, HCN
Due to the shape of the aldehyde or ketone, the :CN^- can attack on either side of the carbonyl
- When it attacks on one side, it will produce one enantiomer and when it attacks on the other side, it will produce the other enantiomer

The reaction mixture which is produced will be a racemic mixture
- There will be a 50:50 mixture of both enantiomers, because there is a 50:50 chance of attack happening on each side
- Racemic mixtures are formed when addition reactions are done with a planar starting material, because the reaction takes place with equal probability from either side of the plane

The attack from the :CN^- has a 50:50 chance of taking place on either side of the C=O bond

A racemic mixture, or racemate, of each enantiomer is formed

The enantiomers in a racemic mixture both rotate plane polarised light, but they rotate it in opposite directions
Because there is a 50:50 mixture of both enantiomers, each rotating light in equal amounts but opposite directions, the effects on plane polarised light are cancelled out
Therefore, there will be no effect on plane polarised light with a racemic mixture
- The optical rotation of the racemic mixture is zero
This can be used as a test to determine whether a mixture is racemic
- If you know that a sample contains enantiomers of chiral compounds, and when tested there is no effect on plane polarised light, then the reaction mixture must be racemic
- If there is an effect on plane polarised light, then the sample is not racemic

Nucleophilic Addition (AQA A Level Chemistry)

Revision Note