Question

Acid-catalysed dehydration of which of the following compounds is fastest?
A. Tert-butanol
B. Sec-butanol
C. N-butanol
D. All dehydrate at the similar rate.

Answer 1

The acid catalysed dehydration of alcohols takes place though ${S_N}2$ mechanism. It involves the formation of a carbocation.

Complete Step by step Solution:
- Alcohols dehydrate in the presence of strong acids like sulphuric or phosphoric acids to form alkenes. The reaction takes place through the formation of a carbocation intermediate. The acid molecule releases a proton which attacks the oxygen atom of the hydroxyl group of the alcohol and hence it leaves as a water molecule, leaving a positive charge on the carbon atom. To satiate the positive charge, the carbon-hydrogen bond from the adjacent carbon atom is broken and hence a carbon-carbon double bond is formed and hence the hydrogen atom leaves as a proton.
- Due to the formation of the carbocation in the intermediate step, the order of preference of the alcohols will be: Tert-butanol > Sec-butanol> N-butanol.
This is because the methyl groups in the tert-butyl group stabilizes the positive charge on the carbocation due to the positive inductive effect.
Hence, the fastest reaction will be for tert-butanol.

So, the correct answer is A.

Notes: As the dehydration of alcohols takes place through the formation of the carbocation, so this reaction follows ${{\text{S}}_{\text{N}}}1$ mechanism. In this mechanism, the polar protic solvents are best suited for the reaction to satisfy the positive charge on the carbocation. Such an example is water itself as the negative electron density on the oxygen atom of the hydroxyl group satisfies the positive charge on the carbocation.