SN2 and 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