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

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

matt101 · Answer

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.