An organic compound (A) (molecular formula C8H16O2) was hydrolysed with dilute sulphuric acid to give a carboxylic acid (B) and an alcohol (C). Oxidation of (C) with chromic acid produced (B). (C) on dehydration gives a mixture of but-1-ene and but-2-ene, both of which on reduction give (D). (C) on oxidation with pyridinium chlorochromate (PCC) gives (E), which is an aldehyde.
Identify the compound (B) in the reaction sequence given in the passage.
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Butanoic acid
Reading the reaction sequence in the passage:
Compounds (B) and (C) have the same chain: the ester C8H16O2 is hydrolysed to an acid (B) and an alcohol (C), and oxidation of (C) gives (B). Oxidation does not change the number of carbons, so each has half of the 8 carbon atoms: 4 carbon atoms.
Compound (C) is butan-1-ol: it is oxidised to an acid with the same chain, so it is a primary alcohol, and its dehydration gives unbranched alkenes, a mixture of but-1-ene \( \ce{CH3CH2CH=CH2} \) and but-2-ene \( \ce{CH3CH=CHCH3} \).
Compound (B) is its oxidation product: chromic acid (chromium trioxide in acidic medium) turns the primary alcohol into the carboxylic acid with the same chain.
\( \displaystyle \ce{CH3CH2CH2CH2OH} \xrightarrow{\ce{H2CrO4}} \ce{CH3CH2CH2COOH} \)
So (B) is butanoic acid, and the ester (A) is butyl butanoate:
\( \displaystyle \ce{CH3CH2CH2COOCH2CH2CH2CH3 + H2O} \xrightarrow{\ce{H^+}} \ce{CH3CH2CH2COOH + CH3CH2CH2CH2OH} \)
Why the other options are incorrect
Option (2) Propanoic acid — Incorrect. It has 3 carbon atoms, so it cannot come from oxidation of a four-carbon alcohol.
Option (3) Pentanoic acid — Incorrect. It has 5 carbon atoms; with butan-1-ol it would give an ester C9H18O2, not C8H16O2.
Option (4) Acetic acid — Incorrect. It has only 2 carbon atoms; it is the oxidation product of ethanol.
NCERT (2026-27 edition):
8.7 Methods of Preparation of Carboxylic Acids: 1. From primary alcohols and aldehydes – “Primary alcohols are readily oxidised to carboxylic acids with common oxidising agents such as potassium permanganate (KMnO4) in neutral, acidic or alkaline media or by potassium dichromate (K2Cr2O7) and chromium trioxide (CrO3) in acidic media (Jones reagent).”