Which of the following is incorrect?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → (CH3)3C–OH with Cu at 573 K giving acetone and CH4
When alcohol vapours are passed over heated copper at 573 K, primary and secondary alcohols are dehydrogenated (to an aldehyde or a ketone), while tertiary alcohols undergo dehydration, because they have no hydrogen on the carbon carrying the OH group.
Option (1) — A correct reaction. Ethanol is dehydrated to ethene by concentrated \( \ce{H2SO4} \) at 443 K:
\( \displaystyle \ce{C2H5OH} \xrightarrow[\text{443 K}]{\text{conc. }\ce{H2SO4}} \ce{CH2=CH2 + H2O} \)
Option (2) — An incorrect reaction. 2-Methylpropan-2-ol is a tertiary alcohol, so over Cu at 573 K it is dehydrated to 2-methylpropene, not split into acetone and methane:
\( \displaystyle \ce{(CH3)3C-OH} \xrightarrow[\text{573 K}]{\ce{Cu}} \ce{(CH3)2C=CH2 + H2O} \)
Option (3) — A correct reaction. Secondary alcohols dehydrate under milder conditions:
\( \displaystyle \ce{CH3CH(OH)CH3} \xrightarrow[\text{440 K}]{85\%\ \ce{H3PO4}} \ce{CH3CH=CH2 + H2O} \)
Option (4) — A correct reaction. Ethanol, a primary alcohol, is dehydrogenated by Cu at 573 K:
\( \displaystyle \ce{CH3CH2OH} \xrightarrow[\text{573 K}]{\ce{Cu}} \ce{CH3CHO + H2} \)
NCERT (2026-27 edition):
Dehydration – “… Ethanol undergoes dehydration by heating it with concentrated H2SO4 at 443 K. … Secondary and tertiary alcohols are dehydrated under milder conditions.”
Oxidation – “… 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.”