Isobutyl chloride undergoes hydrolysis in aqueous solution to yield tert-butanol and chloride ion:
(CH3)3CCl+H2O⟶(CH3)3COH+H++Cl−.
Two different kinetic regimes have been reported, depending on temperature.
Regime 1 (298 K): rate=k1[(CH3)3CCl]
Regime 2 (345 K): rate=k2[(CH3)3CCl][H2O]
Chemists have suggested two elementary possibilities for the rate-limiting step.
Step X: (CH3)3CCl→(CH3)3C++Cl−
Step Y: (CH3)3CCl+H2O→(CH3)3C++Cl−+H2O
(The carbocation reacts rapidly with water in a subsequent step to form tert-butanol.)
(a) For each regime, indicate which of the two proposed elementary steps (X or Y) could serve as the rate-limiting step. Support your answer by comparing the molecularity of the step with the experimentally observed rate law.
(b) Propose a mechanistic explanation for why a change in temperature might shift the rate-limiting step from one elementary process to the other.
(c) Design an additional kinetic experiment (other than temperature variation) that could help confirm which step is rate-limiting under a given set of conditions. Describe the experimental variable you would change, the expected outcome for each mechanism, and how the results would distinguish between the two possibilities.
(d) Explain why any mechanism that contains both water and isobutyl chloride in its slow step would be incompatible with the first-order rate law observed at 298 K.