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
General Mechanism with an aldehyde:
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
Exam 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
