the sn1 mechanism will win this competition when the central carbon atom is surrounded by bulky groups because such groups sterically hinder the sn2 reaction. additionally, bulky substituents on the central carbon increase the rate of carbocation formation because of the relief of steric strain that occurs. the resultant carbocation is also stabilized by both inductive stabilization and hyperconjugation from attached alkyl groups. the hammond-leffler postulate says that this too will increase the rate of carbocation formation. the sn1 mechanism therefore dominates in reactions at tertiary alkyl centers and is further observed at secondary alkyl centers in the presence of weak nucleophiles.