Which of the following conditions favor the SN1 mechanism as opposed to the SN2 mechanism? I. tertiary alkyl halide
II. primary alkyl halide
III. polar solvent

a. only I
b. only Il
c. I and Ill
d. Il and Ill

c. I and Ill

Explanation:

SN1 mechanism is a unimolecular nucleophillic substitution reaction mechanism. "Unimolecular" implies that only one molecule is involved in the rate determining step and that is the alkyl halide. The rate determining step of SN1 reaction is the formation of a carbocation which subsequently reacts with a nucleophile in the fast step.

SN1 reactions occur mostly with tertiary alkyl halides due to sterric reasons. SN2 mechanism involves a crowded transition state because it is a concerted reaction mechanism. This is not possible in a tertiary alkyl halide due to the presence of bulky groups around the tertiary carbon atom hence tertiary alkyl halides do not undergo nucleophilic reaction by SN2 mechanism.

Secondly, polar solvents aid in the ionization of the alkyl halide thus assisting in the formation of the carbocation which participates in the second step of the reaction.