Read the following passage given below and answer the question.
Aldehydes and ketones undergo nucleophilic addition reactions. A nucleophile attacks the electrophilic carbon atom of polar carbonyl group from perpendicular to plane of \(sp^2\) hybridized orbitals of carbonyl carbon. The hybridization of carbon changes from \(sp^2\) to \(sp^3\) and a tetrahedral alkoxide intermediate is formed. This captures \(H^+\) to give neutral product. In carboxylic acids, carbonyl carbon is less electrophilic than carbonyl carbon of aldehydes and ketones.
The optimum pH for carrying out the reaction of aldehyde with hydroxylamine to form oxime is
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → pH = 4 - 5
Hydroxylamine adds to the carbonyl group, and the adduct loses water to give the oxime:
\( \displaystyle \ce{RCHO + NH2OH} \xrightarrow{\ce{H+}} \ce{RCH=NOH + H2O} \)
The reaction is catalysed by acid: protonation of the carbonyl oxygen makes the carbonyl carbon more electrophilic. But the nucleophile \( \ce{NH2OH} \) must keep its lone pair free. A weakly acidic medium (pH about 4–5) meets both needs, so the reaction is fastest there.
Option (1) pH = 1 — Incorrect. In strongly acidic solution hydroxylamine is almost completely protonated to \( \ce{NH3^{+}OH} \), which has no free lone pair and cannot attack the carbonyl carbon.
Option (3) pH = 12 — Incorrect. In strongly basic solution there is no acid to protonate the carbonyl oxygen or to help the loss of water, so the reaction is slow.
Option (4) Any pH — Incorrect. The rate depends strongly on pH, so the pH must be controlled in the weakly acidic range.
NCERT (2026-27 edition):
Addition of Ammonia and Its Derivatives – “Nucleophiles, such as ammonia and its derivatives H2N-Z add to the carbonyl group of aldehydes and ketones. The reaction is reversible and catalysed by acid. The equilibrium favours the product formation due to rapid dehydration of the intermediate to form >C=N-Z.”