Aldehydes and ketones undergo

The correct answer is Option (2) → Nucleophilic addition reaction

The carbonyl group, C=O, is polar: oxygen is more electronegative, so the carbon carries a partial positive charge and the oxygen a partial negative charge. A nucleophile therefore attacks the electrophilic carbonyl carbon from above or below the plane of the sp² carbon. The π electrons move to oxygen, giving a tetrahedral alkoxide intermediate with an sp³ carbon, which then takes a proton. The net result is addition of Nu and H across the C=O bond, a nucleophilic addition reaction. For example, ethanal adds hydrogen cyanide (base catalysed) to give a cyanohydrin:

    \( \displaystyle \ce{CH3CHO + HCN} \xrightarrow{\ce{OH-}} \ce{CH3CH(OH)CN} \)

So the characteristic reactions of aldehydes and ketones are nucleophilic addition reactions; the addition of HCN, sodium hydrogensulphite, Grignard reagents and alcohols are all of this type.

Why the other options are incorrect

• Option (1) Nucleophilic substitution reaction — Incorrect. Substitution needs a leaving group on the attacked carbon, as in alkyl halides and in acid chlorides, esters and amides. In aldehydes and ketones the carbonyl carbon carries only H or alkyl/aryl groups, which cannot leave.

• Option (3) Free radical reaction — Incorrect. Free radical substitution is typical of alkanes, as in the chlorination of methane in light; the polar C=O group reacts with ions or polar molecules instead.

• Option (4) Elimination reaction — Incorrect. Elimination removes atoms from a saturated molecule to form a multiple bond, as in the dehydration of alcohols to alkenes; aldehydes and ketones already have the C=O double bond and add to it.

NCERT (2026-27 edition):
Nucleophilic Addition Reactions – “Contrary to electrophilic addition reactions observed in alkenes, the aldehydes and ketones undergo nucleophilic addition reactions.”

Mechanism of Nucleophilic Addition Reactions – “A nucleophile attacks the electrophilic carbon atom of the polar carbonyl group from a direction approximately perpendicular to the plane of sp² hybridised orbitals of carbonyl carbon (Fig. 8.2). The hybridisation of carbon changes from sp² to sp³ in this process, and a tetrahedral alkoxide intermediate is produced. This intermediate captures a proton from the reaction medium to give the electrically neutral product. The net result is addition of Nu^– and H⁺ across the carbon oxygen double bond as shown in Fig. 8.2.”