SN2 vs. E2
SN2 and E2 reactions share a number of similarities. Both require good
leaving groups, and both mechanisms are concerted.
SN2
reactions require a good nucleophile and E2 reactions require a
strong base. However, a good nucleophile is often a strong base. Since the two reactions share many
of the same conditions, they often compete with each other. The the outcome of the competition is
determined by three factors: the presence of antiperiplanar
β-hydrogens, the degree of α and β branching, and the nucleophilicity vs.
basicity of the reactant species.
In order for an E2 elimination to occur, there must be antiperiplanar β-hydrogens to
eliminate. If there are none, the SN2 reaction will dominate. On the same token, the
SN2 nucleophile needs an free path to the σ* C-LG antibond. α and
β branching block this path and reduce the proportion of SN2 relative to E2.
E2 occurs even with extensive branching because it relies on the β-hydrogens, which are
much more accessible than the σ* C-LG antibond.
The identity of the nucleophile or base also determines which mechanism is favored. E2
reactions require strong bases. SN2 reactions require good nucleophiles. Therefore a good
nucleophile that is a weak base will favor SN2 while a weak nucleophile that is a strong base
will favor E2. Bulky nucleophiles have a hard time getting to the α-carbon, and
thus increase the proportion of E2 to SN2. Polar,
aprotic solvents increase
nucleophilicity, and thus increase the rate of SN2.
SN2
- Requires an unhindered path to the back of the α carbon
-
α and β branching block the path and hinder SN2
- Requires a good nucleophile
- Polar, aprotic solvents increase nucleophilicity
- Bulky groups on the nucleophile decrease nucleophilicity
E2
- Requires an antiperiplanar β-hydrogen
- Enhanced by α and β-branching
- Requires a strong base
