Organic synthesis and applications
In aldehydes, carbonyl group is bonded to carbon and hydrogen while in ketones, it is bonded to two carbon atoms. The carbonyl carbon atom is sp2-hybridised and a trigonal coplanar structure. The bond angles are approximately 120°. Methanal is gas at room temperature. Ethanal is a volatile liquid. Other aldehydes and ketones are liquid or solid at room temperature. The boiling points of aldehydes and ketones are higher than hydrocarbons and ether of comparable molecular masses. It is due to weak molecular association in aldehydes and ketones arising out of the dipole-dipole interactions. Their boiling points are lower than those of alcohols of similar molecular masses due to absence of intermolecular hydrogen bonding. The lower members of aldehydes and ketones are miscible with water because they form hydrogen bonding with water. However, the solubility of aldehydes and ketones decreases rapidly on increasing the length of alkyl chain.
Aldehydes are generally more reactive than ketones in the nucleophilic addition reactions due to steric and electronic reasons. An organic compound (A) with molecular formula C8H8O forms an orange-red precipitate with 2,4-DNP reagent and gives yellow precipitate on heating with iodine in the presence of sodium hydroxide. It neither reduces Tollens' or Fehling's reagent, nor does it decolourise bromine water or Baeyer's reagent. On drastic oxidation with chromic acid, it gives a carboxylic acid (B) having molecular formula C7H6O2.
The characteristic reactions of carbonyl compounds are:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → nucleophilic addition reactions.
The carbonyl group (C=O) is polar: oxygen is more electronegative, so the carbon carries a partial positive charge and is attacked by nucleophiles. The carbonyl carbon is sp2 and planar, so the nucleophile can approach from either face. Aldehydes and ketones have no good leaving group on this carbon, so instead of a substitution:
the nucleophile adds to the carbon, the C=O π bond breaks and a tetrahedral alkoxide ion forms;
the alkoxide ion takes up a proton to give the neutral addition product.
\( \displaystyle \ce{R2C=O + HCN} \xrightarrow{\ce{OH^-}} \ce{R2C(OH)CN} \)
Addition of HCN (cyanohydrins), \( \ce{NaHSO3} \) (bisulphite adducts), alcohols (hemiacetals and acetals) and Grignard reagents are all of this type. So the characteristic reactions of carbonyl compounds are nucleophilic addition reactions.
Why the other options are incorrect
Option (1) nucleophilic substitution reactions — Incorrect. These need a leaving group on the attacked carbon, as in haloalkanes (SN1/SN2) and acid chlorides; aldehydes and ketones have only H or alkyl groups there.
Option (2) nucleophilic elimination reactions — Incorrect. There is no reaction class called nucleophilic elimination; carbonyl compounds add the nucleophile across C=O, and any loss of water comes after the addition (as with ammonia derivatives).
Option (4) Pericyclic reaction — Incorrect. These are concerted ring-forming or ring-opening reactions of π systems, such as the Diels–Alder reaction, not the typical reactions of C=O.
NCERT (2026-27 edition):
Nucleophilic Addition Reactions
