Which of the following compounds is most reactive towards nucleophilic addition reaction?

Reactivity in case of aldehydes and ketones is decided by two factors:

(1) Steric hindrance

(2) Electrophilicity of carbon with oxygen

(1) Steric hindrance: It is the congestion created by the presence of surrounding atoms or ligands which may slow down or even prevent a reaction from occurring. In Ketones, there is large hindrance caused by the bulky alkyl groups attached to \(C=O\); due to which incoming nucleophiles do not have the required space to form bonds with carbon. This lowers the reactivity of ketones in larger amounts than aldehydes.

Due to the large and bulky size of the phenyl group, steric hindrance caused by benzaldehyde is more than acetaldehyde. Similarly, acetophenone has a phenyl group present in its structure; the hindrance created by it is more than that created by methyl groups of acetone. That is why, acetaldehyde is more reactive than benzaldehyde and acetone is more reactive than acetophenone.

(2) Electrophilicity of carbonyl carbon: Due to the electronegativity of oxygen the double-bonded carbon attached to it acquires a partial positive charge, which makes the attack of nucleophiles easier. But the presence of electron-donating groups nullifies the positive charge created. Thus, the presence of electron-donating groups decreases the nucleophilic addition reaction.

Hence, the order of reactivity of all compounds is: