Consider the following statements about the oxidation and dehydrogenation of alcohols.
(A) When the vapours of a primary alcohol are passed over heated copper at 573 K, an aldehyde is formed.
(B) When the vapours of a secondary alcohol are passed over heated copper at 573 K, an aldehyde is formed.
(C) When the vapours of a tertiary alcohol are passed over heated copper at 573 K, dehydration takes place.
(D) Tertiary alcohols are oxidised to ketones by chromic anhydride (CrO3).
Which of the statements given above are correct? Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (A) and (C) only
Oxidation of an alcohol forms a C=O bond by removing hydrogen from the O–H and from the C–H of the carbon carrying –OH. A primary or secondary alcohol has such a C–H bond; a tertiary alcohol has none, so it is not oxidised in this way.
(A) When the vapours of a primary alcohol are passed over heated copper at 573 K, an aldehyde is formed. — True. Dehydrogenation takes place: \( \ce{RCH2OH} \xrightarrow[\text{573 K}]{\ce{Cu}} \ce{RCHO} \).
(B) When the vapours of a secondary alcohol are passed over heated copper at 573 K, an aldehyde is formed. — False. The carbon carrying –OH is joined to two alkyl groups, so dehydrogenation gives a ketone: \( \ce{R-CH(OH)-R'} \xrightarrow[\text{573 K}]{\ce{Cu}} \ce{R-CO-R'} \).
(C) When the vapours of a tertiary alcohol are passed over heated copper at 573 K, dehydration takes place. — True. With no hydrogen to lose from that carbon, the alcohol loses water and gives an alkene, e.g. 2-methylpropan-2-ol gives 2-methylpropene: \( \ce{(CH3)3C-OH} \xrightarrow[\text{573 K}]{\ce{Cu}} \ce{CH2=C(CH3)2 + H2O} \).
(D) Tertiary alcohols are oxidised to ketones by chromic anhydride (CrO3). — False. It is secondary alcohols that CrO3 oxidises to ketones. Tertiary alcohols do not undergo oxidation; only strong oxidising agents at high temperature break their C–C bonds.
NCERT (2026-27 edition):
7.4.4 Chemical Reactions – “Secondary alcohols are oxidised to ketones by chromic anhyride (CrO3).”
“Tertiary alcohols do not undergo oxidation reaction. Under strong reaction conditions such as strong oxidising agents (KMnO4) and elevated temperatures, cleavage of various C-C bonds takes place and a mixture of carboxylic acids containing lesser number of carbon atoms is formed.”
“When the vapours of a primary or a secondary alcohol are passed over heated copper at 573 K, dehydrogenation takes place and an aldehyde or a ketone is formed while tertiary alcohols undergo dehydration.”
\( \displaystyle \ce{RCH2OH} \xrightarrow[\text{573 K}]{\ce{Cu}} \ce{RCHO} \)
\( \displaystyle \ce{R-CH(OH)-R'} \xrightarrow[\text{573 K}]{\ce{Cu}} \ce{R-CO-R'} \)
\( \displaystyle \ce{CH3-C(CH3)2-OH} \xrightarrow[\text{573 K}]{\ce{Cu}} \ce{CH3-C(CH3)=CH2} \)