The aldehydes and ketones undergo many nucleophilic reactions. Aldehydes are generally more reactive than Ketones in nucleophilic addition reactions due to steric and electronic reasons. The aldehydes and ketones undergo a number of reactions due to acidic nature of α-Hydrogen also. Aldol and cross aldol reactions occur in carbonyl compounds which contain α-Hydrogen. Aldehydes and ketones having at least one α-Hydrogen undergo reaction in dil. alkali to form β-hydroxy aldehyde or β-hydroxy ketone. This is aldol reaction. Aldol or ketol lose water to form α,β unsaturated carbonyl compounds which is called aldol condensation. Carbonyl compounds react with ammonia derivatives such as NH2OH, NH2NHCONH2, 2,4 DNP to give products useful in their identification.
Aldehydes react with one equivalent of monohydric alcohols in the presence of dry hydrogen chloride to give:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → A hemiacetal
One equivalent of an alcohol adds across the C=O group of an aldehyde in the presence of dry HCl and gives an alkoxyalcohol, the hemiacetal; a second molecule of alcohol would turn it into an acetal.
\( \displaystyle \ce{RCHO + R'OH} \xrightarrow{\text{dry HCl}} \ce{RCH(OH)OR'} \quad (\text{hemiacetal}) \)
Why the other options are incorrect
Option (1) An acetal — Incorrect. It forms when the hemiacetal reacts with a second molecule of alcohol, giving a gem-dialkoxy compound, RCH(OR′)2.
Option (3) An oxime — Incorrect. It is formed with hydroxylamine (\( \ce{NH2OH} \)), not with an alcohol.
Option (4) A ketal — Incorrect. Ketals are formed from ketones (e.g. with ethylene glycol), not from aldehydes.
NCERT (2026-27 edition):
Addition of Alcohols – “Aldehydes react with one equivalent of monohydric alcohol in the presence of dry hydrogen chloride to yield alkoxyalcohol intermediate, known as hemiacetals, which further react with one more molecule of alcohol to give a gem-dialkoxy compound known as acetal as shown in the reaction.”