Question

When an isomer Y of molecular formula C4H9Br undergoes hydrolysis in aqueous alkali to form an alcohol C4H9OH, the rate of reaction is found to be unaffected by changes in the concentration of OH– ions present. Which is the most likely molecular structure of Y? A CH3CH2CH2CH2Br B CH3CH2CHBrCH3 C (CH3)2CHCH2Br D (CH3)3CBr

Answer 1

Answer: D.  (CH3)3CBr

Explanation:  As it is clearly written in the question itself that when an isomer Y of molecular formula $C_{4}H_{9}Br$ undergoes hydrolysis in aqueous alkali to form an alcohol $C_{4}H_{9}OH$, the rate of reactio is found to be unaffected by changes in the concentration of $OH^{-}$ present, This explains that the rate of reaction is unaffected by the presence of the nucleophile o the catalyst.

Thus it is the example of Substitution reaction Unimolecular in which the reaction depends only on the structure of the reactant not on the catalyst or the nucleophile.

Thus out of the given options, (CH3)3CBr  is the correct answer.

Answer 2

I would say D because the Br is bonded to a tertiary carbon (a carbon bonded to three other carbon atoms). This makes the bond more stable compared to if the Br was bonded to a primary or secondary carbon, and so it is unlikely for spontaneous substitution of Br for OH to occur. Hydrolysis occurs through reaction intermediates that make the making and breaking of C-OH and C-Br bonds respectively more favourable.