Match List-I with List-II.
| List-I (Starting compound and reagents) | List-II (Carboxylic acid formed) |
|---|---|
| (A) Ethylmagnesium bromide; dry ice; then H3O+ | (I) Benzoic acid |
| (B) Ethanol; acidified KMnO4 | (II) Ethanoic acid |
| (C) Propylbenzene; KMnO4–KOH, heat; then H3O+ | (III) Phenylethanoic acid |
| (D) Benzylmagnesium chloride; dry ice; then H3O+ | (IV) Propanoic acid |
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → A-IV, B-II, C-I, D-III
Count the carbon atoms. A Grignard reagent with carbon dioxide adds one carbon atom, which becomes the –COOH carbon. Oxidation of a primary alcohol keeps the carbon chain. In the vigorous oxidation of an alkylbenzene, the whole side chain is oxidised, so only one carbon is left on the ring.
(A) Ethylmagnesium bromide; dry ice; then H3O+ — This matches (IV) Propanoic acid. Two carbons of the ethyl group plus the carbon of CO2 give a three-carbon acid:
\( \displaystyle \ce{CH3CH2MgBr + CO2} \xrightarrow{\text{dry ether}} \ce{CH3CH2COOMgBr} \xrightarrow{\ce{H3O+}} \ce{CH3CH2COOH} \)
(B) Ethanol; acidified KMnO4 — This matches (II) Ethanoic acid. A primary alcohol is oxidised to the acid with the same number of carbon atoms:
\( \displaystyle \ce{CH3CH2OH} \xrightarrow{\ce{KMnO4}/\ce{H+}} \ce{CH3COOH} \)
(C) Propylbenzene; KMnO4–KOH, heat; then H3O+ — This matches (I) Benzoic acid. The entire propyl chain is oxidised, whatever its length:
\( \displaystyle \ce{C6H5CH2CH2CH3} \xrightarrow[\text{heat}]{\ce{KMnO4}\text{-}\ce{KOH}} \ce{C6H5COOK} \xrightarrow{\ce{H3O+}} \ce{C6H5COOH} \)
(D) Benzylmagnesium chloride; dry ice; then H3O+ — This matches (III) Phenylethanoic acid. The benzyl group, \( \ce{C6H5CH2-} \), gains one carbon as –COOH:
\( \displaystyle \ce{C6H5CH2MgCl + CO2} \xrightarrow{\text{dry ether}} \ce{C6H5CH2COOMgCl} \xrightarrow{\ce{H3O+}} \ce{C6H5CH2COOH} \)
NCERT (2026-27 edition):
8.7 Methods of Preparation of Carboxylic Acids – “Primary alcohols are readily oxidised to carboxylic acids with common oxidising agents such as potassium permanganate (KMnO4) in neutral, acidic or alkaline media …”
“Aromatic carboxylic acids can be prepared by vigorous oxidation of alkyl benzenes with chromic acid or acidic or alkaline potassium permanganate. The entire side chain is oxidised to the carboxyl group irrespective of length of the side chain.”
“Grignard reagents react with carbon dioxide (dry ice) to form salts of carboxylic acids which in turn give corresponding carboxylic acids after acidification with mineral acid.” … “The above methods (3 and 4) are useful for converting alkyl halides into corresponding carboxylic acids having one carbon atom more than that present in alkyl halides (ascending the series).”