Consider the following statements about the chemical reactions of aldehydes and ketones.
(A) In the electrophilic substitution of aromatic aldehydes and ketones, the carbonyl group acts as an activating and meta-directing group.
(B) Cross aldol condensation between two different aldehydes that both contain α-hydrogen atoms gives a mixture of four products.
(C) The oxidation of ketones involves carbon-carbon bond cleavage to afford a mixture of carboxylic acids having more carbon atoms than the parent ketone.
(D) The aldol and ketol readily lose water to give α,β-unsaturated carbonyl compounds.
Which of the statements given above are incorrect? Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → (A) and (C) only
The carbonyl group withdraws electrons. This decides how it affects the benzene ring, why its α-hydrogens take part in aldol reactions, and how ketones break up on strong oxidation.
(A) In the electrophilic substitution of aromatic aldehydes and ketones, the carbonyl group acts as an activating and meta-directing group. — Incorrect statement. The carbonyl group is deactivating and meta-directing; it pulls electrons out of the ring, so it cannot activate it.
(B) Cross aldol condensation between two different aldehydes that both contain α-hydrogen atoms gives a mixture of four products. — Correct statement. With ethanal and propanal, each can react with itself or with the other, giving two self and two cross products.
(C) The oxidation of ketones involves carbon-carbon bond cleavage to afford a mixture of carboxylic acids having more carbon atoms than the parent ketone. — Incorrect statement. Breaking a C–C bond splits the molecule, so the acids have a lesser number of carbon atoms than the ketone.
(D) The aldol and ketol readily lose water to give α,β-unsaturated carbonyl compounds. — Correct statement. These are the aldol condensation products, such as but-2-enal from 3-hydroxybutanal.
NCERT (2026-27 edition):
8.4 Chemical Reactions – “(ii) Electrophilic substitution reaction: Aromatic aldehydes and ketones undergo electrophilic substitution at the ring in which the carbonyl group acts as a deactivating and meta-directing group.”
“When aldol condensation is carried out between two different aldehydes and / or ketones, it is called cross aldol condensation. If both of them contain α-hydrogen atoms, it gives a mixture of four products.”
“Their oxidation involves carbon-carbon bond cleavage to afford a mixture of carboxylic acids having lesser number of carbon atoms than the parent ketone.”
“The aldol and ketol readily lose water to give α,β-unsaturated carbonyl compounds which are aldol condensation products and the reaction is called Aldol condensation.”