Carboxylic Acids: Types of Reactions
Carboxylic acid exhibits a number of reactions depending on the involvement of a specific part of the -COOH group. It mainly includes nucleophilic acyl substitution reaction, in which a hydroxy group of -COOH is replaced by another nucleophile, and the acidic behavior of carboxylic acids, which is due to the cleavage of O-H bond in -COOH group. The former reaction leads to the conversion of carboxylic acid into a variety of different derivatives/categories of organic compounds, whereas the latter converts it into salts which find applications in many industries, like soap and detergent synthesis and pharmaceutical industries. Other important types of chemical reactions involve reduction of -COOH group, decarboxylation, electrolysis and substitution reactions in the hydrocarbon part at α-carbon atom.
Oxidation of 5-methylhexan-3-one using strong oxidising agent KMnO4 results in formation of which of the following mixture of acids ?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → CH3CH2COOH, CH3CH(CH3)COOH, CH3COOH and CH3CH(CH3)CH2COOH only.
Ketones resist mild oxidation, but strong oxidising agents such as \( \ce{KMnO4} \), at higher temperature, break a C–C bond next to the carbonyl group. In an unsymmetrical ketone the bond on either side of the carbonyl group can break, so a mixture of carboxylic acids forms, each with fewer carbons than the ketone.
5-Methylhexan-3-one has seven carbons, with the carbonyl group at C-3:
\( \displaystyle \ce{CH3-CH2-CO-CH2-CH(CH3)-CH3} \)
Breaking the bond between C-2 and C-3 gives an acid with 2 carbons and an acid with 5 carbons:
\( \displaystyle \ce{CH3CH2COCH2CH(CH3)CH3} \xrightarrow[\Delta]{\ce{KMnO4}} \ce{CH3COOH + CH3CH(CH3)CH2COOH} \)
Breaking the bond between C-3 and C-4 gives an acid with 3 carbons and an acid with 4 carbons:
\( \displaystyle \ce{CH3CH2COCH2CH(CH3)CH3} \xrightarrow[\Delta]{\ce{KMnO4}} \ce{CH3CH2COOH + CH3CH(CH3)COOH} \)
Both cleavages take place, so the mixture contains all four acids: ethanoic, 3-methylbutanoic, propanoic and 2-methylpropanoic acid.
Shortcut: Each cleavage gives two acids whose carbon counts add up to 7 (2 + 5 and 3 + 4). Cleavage on both sides of the carbonyl group gives four acids, and only Option (1) lists four.
Why the other options are incorrect
Option (2) CH3CH(CH3)COOH, CH3CH2COOH and CH3COOH only. — Incorrect. Acetic acid (2 carbons) forms only together with its partner \( \ce{CH3CH(CH3)CH2COOH} \) (5 carbons), which is missing here.
Option (3) CH3CH2COOH and CH3CH(CH3)COOH only. — Incorrect. These are only the products of cleavage between C-3 and C-4. The bond on the other side of the carbonyl group also breaks.
Option (4) CH3CH(CH3)COOH, CH3COOH and CH3CH(CH3)CH2COOH only. — Incorrect. 2-Methylpropanoic acid (4 carbons) forms only together with propanoic acid (3 carbons), which is missing here.
NCERT (2026-27 edition):
Oxidation – …
