What is the final product when benzyl alcohol is oxidised with an excess of acidified potassium dichromate?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Benzoic acid
A primary alcohol is oxidised first to an aldehyde and then to a carboxylic acid. Which product is obtained depends on the oxidising agent: a strong oxidant used in excess, such as acidified potassium dichromate, carries the oxidation through to the acid.
Benzyl alcohol, \( \ce{C6H5CH2OH} \), is a primary alcohol. With excess acidified dichromate both stages take place:
\( \displaystyle \ce{C6H5CH2OH + [O]} \xrightarrow{\ce{K2Cr2O7}/\ce{H^+}} \ce{C6H5CHO + H2O} \)
\( \displaystyle \ce{C6H5CHO + [O]} \xrightarrow{\ce{K2Cr2O7}/\ce{H^+}} \ce{C6H5COOH} \)
Because the oxidant is in excess, the benzaldehyde formed is not left over; the final product is benzoic acid.
Why the other options are incorrect
Option (1) Benzaldehyde — Incorrect. Benzaldehyde is the intermediate. It is isolated only with a milder reagent such as PCC or \( \ce{CrO3} \) in an anhydrous medium, not with excess acidified dichromate.
Option (3) Methyl benzoate — Incorrect. \( \ce{C6H5COOCH3} \) is an ester. It forms when benzoic acid is heated with methanol and a little conc. \( \ce{H2SO4} \) (esterification); no methanol is present here.
Option (4) Benzophenone — Incorrect. \( \ce{C6H5COC6H5} \) is a ketone with two phenyl groups (13 carbon atoms). Oxidation of a primary alcohol never gives a ketone, and benzyl alcohol has only 7 carbon atoms.
NCERT (2026-27 edition):
8.7 Methods of Preparation of Carboxylic Acids
7.4.4 Chemical Reactions – “… Depending on the oxidising agent used, a primary alcohol is oxidised to an aldehyde which in turn is oxidised to a carboxylic acid. Strong oxidising agents such as acidified potassium permanganate are used for getting carboxylic acids from alcohols directly.”