JEE Advanced 2014 · Paper 2 · Chemistry Q21

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ChemistrySingle correct+3 / -1Question 21 of 60

The acidic hydrolysis of ether (X) shown below is fastest when

Trityl ether X, Ph₃C-OR, is hydrolysed by acid to triphenylmethanol Ph₃C-OH plus ROH.

one phenyl group is replaced by a methyl group.

one phenyl group is replaced by a para-methoxyphenyl group.

two phenyl groups are replaced by two para-methoxyphenyl groups.

no structural change is made to X.

Answer and worked solution

Official answer: C

Mechanism. Acid hydrolysis of a trityl ether is $\mathrm S_\mathrm N1$ : protonation of the ether oxygen, then loss of ROH to give the triaryl-methyl cation $\ce{Ar3C+}$ , which water captures. The rate depends on how stable that cation is.

(A) Replacing a phenyl by methyl removes a ring that delocalises the charge. Slower.
(B) One para-methoxyphenyl group stabilises the cation by +M donation. Faster than X.
(C) Two para-methoxyphenyl groups stabilise it even more. Fastest.
(D) Unchanged X is slower than (B) and (C).

(C)

Topics:Ethers Reactive intermediates

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JEE Advanced 2014 · Paper 2, Chemistry question 21