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The carbonyl group of aldehydes and ketones is highly polar. The positively charged carbon is readily attacked by electron-rich nucleophile and the negatively charged oxygen atom is attacked by an electro-deficient electrophile. Aldehydes and ketones undergo nucleophilic addition reactions in the presence of acid or a base. The mechanism involves attack of a nucleophile on the electron deficient carbonyl carbon to form a tetrahedral intermediate. A new bond is formed between the carbon and oxygen. Protonation of the anion furnished by the reaction medium yields a neutral addition product. The addition product is same whether the reaction is catalyzed by acid or a base. For example in the presence of base aldehydes and ketones react with hydrogen cyanide to form cyanohydrin.
During nucleophilic addition reaction the carbonyl carbon undergoes a change in hybridization from:

1. A \(sp^2\) to \(sp^3\)
2. B \(sp^3\) to \(sp^2\)
3. C \(sp\) to \(sp^2\)
4. D \(sp^2\) to \(sp\)