SN2 and
E2 reactions require a good
nucleophile or a strong base.
SN1 and
E1 reactions occur with strong bases with molecules whose
α-carbon is
secondary or tertiary and in the
absence of good nucleophiles.
This reaction yields
SN1 and
E1 products:
If we disregard ethanol's poor nucleophilicity and weak basicity, this reaction looks very much like an
SN2 or
E2. The fact that it is not a bimolecular reaction becomes apparent in
the
SN1/
E1 rate law.
Remember that the rate law shows which molecules are present in the transition
state of the rate-
limiting step. Since
SN1 and
E1 share the same rate law (including
k), it is
reasonable to assume that both reactions go through the same rate-limiting transition state. That transition
state involves a forming carbocation.
Once the carbocation intermediate forms, the two reactions follow divergent pathways. In the
SN1 pathway, ethanol acts as a nucleophile. In the
E1 pathway, ethanol is a base.
A base/nucleophile as weak as ethanol can substitute or eliminate because the carbocation is an incredibly
reactive species. Without the carbocation or a very good leaving group, SN1 and
E1 would be impossible.
In the broader context of all substitution and elimination reactions, remember that
SN1 and
E1 will not occur in the presence of a strong base or a good nucleophile. In these cases,
E2 and SN2 dominate their unimolecular cousins.
