If there is steric crowding on the substrate near the leaving group , such as at a tertiary carbon center , the substitution will involve an SN1 rather than an SN2 mechanism , ( an SN1 would also be more likely in this case because a sufficiently stable carbocation intermediary could be formed ) .
If there is steric crowding on the substrate near the leaving group , such as at a tertiary carbon center , the substitution will use an SN1 rather than an SN2 mechanism , ( an SN1 would also be more likely with blocked molecules because a sufficiently stable carbocation intermediary could be formed . )