Which of the following pairs of methods converts an alkyl halide into a carboxylic acid having one carbon atom more than the alkyl halide?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Conversion into the nitrile followed by hydrolysis; conversion into the Grignard reagent followed by reaction with carbon dioxide and acidification
To go up the series by one carbon, the method must bring in a new carbon atom that ends up as the –COOH carbon. The cyanide ion and carbon dioxide both do this.
In the first method, the alkyl halide gives a nitrile with the cyanide ion, and the –CN carbon becomes the –COOH carbon on hydrolysis:
\( \displaystyle \ce{R-X} \xrightarrow{\ce{KCN}} \ce{R-CN} \xrightarrow{\ce{H3O+}} \ce{R-COOH} \)
In the second method, the Grignard reagent adds to carbon dioxide, and the salt formed gives the acid on acidification:
\( \displaystyle \ce{R-X} \xrightarrow[\text{dry ether}]{\ce{Mg}} \ce{R-MgX} \xrightarrow[\text{(ii) }\ce{H3O+}]{\text{(i) }\ce{CO2}} \ce{R-COOH} \)
Why the other options are incorrect
Option (2) Hydrolysis to the alcohol followed by oxidation; conversion into the nitrile followed by hydrolysis — Incorrect. Hydrolysis to the alcohol and then oxidation keeps the same number of carbon atoms: \( \ce{RCH2X -> RCH2OH -> RCOOH} \). Only the nitrile route in this pair adds a carbon.
Option (3) Conversion into the Grignard reagent followed by reaction with carbon dioxide and acidification; hydrolysis to the alcohol followed by oxidation — Incorrect. The Grignard route adds a carbon, but the alcohol route paired with it does not.
Option (4) Hydrolysis to the alcohol followed by oxidation; elimination to the alkene followed by oxidation — Incorrect. Neither method brings in a new carbon atom: the alcohol and the alkene have the same carbon skeleton as the alkyl halide, and oxidation cannot lengthen a chain.
NCERT (2026-27 edition):
8.7 Methods of Preparation of Carboxylic Acids – “As we know, the Grignard reagents and nitriles can be prepared from alkyl halides (refer Unit 6, Class XII). 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).”