Those addition reactions which involve the initial attack by a nucleophile are known as nuclephilic addition reactions. Additions on aldehydes and ketones (i.e. on carbonyl group) are common examples of nucleophilic addition reactions. A typical nucleophilic addition reactions is the addition of hydrogen cyanide on aldehydes and ketones.
Mechanism : Mechanism of the above reaction involves the following steps.
Step 1. HCN gives proton (H+) and cyanide ion. \(H - CN \longrightarrow H ^+ CN ^-\)
Step 2. The carbonyl group undergoes electromeric effect at the requirement of the attacking reagent. The cyanide ion (nucleophile) attacks the positively charged carbonyl carbon to form the corresponding anion.
It is important to note that the positively charged carbon, being a less stable arrangement, is attacked first while the negatively charged oxygen, being a more stable arrangement, is attacked later on. This confirms the nucleophilic character of the reaction.
Step 3. The proton (electrophile) attacks on the anion to form the adddition product.
It is important to note that the reaction is catalysed by bases and retarded by acids. This leads to the conclusion that the addendum is the cyanide ion, CN- and not HCN directly.
