Read the passage carefully and answer the following questions.
Aldehydes, carboxylic acids and ketones are widespread in the animal and plant kingdoms. They play a pivotal role in various biochemical processes of life. Their presence in nature adds fragrance and flavor. These compounds are widely used in food products, pharmaceuticals, paints, resins and other important product industries. These compounds are prepared by various laboratory methods, which mainly include oxidation, formylation, acylation and reduction. Due to the polar nature of the carbonyl group in aldehydes and ketones, they can exhibit different reactions like nucleophilic addition. They do exhibit redox and various condensation reactions which lead to the formation of various important compounds. On the other hand, the carboxylic acids are mainly prepared by oxidation and hydrolysis of different compounds. The carboxylic acid consists of a carbonyl group and the hydroxy group (attached to the carbonyl carbon atom). This makes it possible for the carboxylic acid to participate in various chemical reactions which involve cleavage of the C-OH bond and the O-H bond along with the reactions involving the complete -COOH group.
Which of the following compounds will give butanone on oxidation with chromic anhydride (\( CrO_3 \))?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Butan-2-ol
Oxidation of an alcohol removes the H of the –OH group and the H on the carbon carrying it. The class of the alcohol therefore decides the product: a primary alcohol gives an aldehyde (which can be oxidised further to a carboxylic acid), a secondary alcohol gives a ketone, and a tertiary alcohol, with no H on that carbon, resists oxidation. Chromic anhydride (\( \ce{CrO3} \)) oxidises secondary alcohols to ketones.
Butanone, \( \ce{CH3COCH2CH3} \), has a straight four-carbon chain with the C=O at carbon 2. The alcohol with the same skeleton and the –OH on carbon 2 is butan-2-ol, a secondary alcohol:
\( \displaystyle \ce{CH3CH(OH)CH2CH3} \xrightarrow{\ce{CrO3}} \ce{CH3COCH2CH3} \)
So butanone is obtained from butan-2-ol.
Why the other options are incorrect
Option (1) Butan-1-ol — Incorrect. It is a primary alcohol, so it gives butanal (and then butanoic acid), not a ketone:
\( \displaystyle \ce{CH3CH2CH2CH2OH} \xrightarrow{[\mathrm{O}]} \ce{CH3CH2CH2CHO} \)
Option (3) 2-Methyl propane — Incorrect. It is an alkane with no –OH group, and its branched chain could never give the straight chain of butanone.
Option (4) 2-Methyl butane — Incorrect. It is also an alkane with no –OH group, and it has five carbon atoms, one more than butanone.
Note: The NCERT book (2026-27) prints “chromic anhyride”; it should read “chromic anhydride”.
NCERT (2026-27 edition):
Oxidation – “Oxidation of alcohols involves the formation of a carbon- oxygen double bond with cleavage of an O-H and C-H bonds. … Depending on the oxidising agent used, a primary alcohol is oxidised to an aldehyde which in turn is oxidised to a carboxylic acid. … Secondary alcohols are oxidised to ketones by chromic anhyride (CrO3).”