Organic Chemistry: Sn2E2 Reactions

S_N2 and E2 reactions share a number of similarities. Both require good leaving groups, and both mechanisms are concerted. S_N2 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 S_N2 reaction will dominate. On the same token, the S_N2 nucleophile needs an free path to the σ^* C-LG antibond. α and β branching block this path and reduce the proportion of S_N2 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. S_N2 reactions require good nucleophiles. Therefore a good nucleophile that is a weak base will favor S_N2 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 S_N2. Polar, aprotic solvents increase nucleophilicity, and thus increase the rate of S_N2.

S_N2

1. Requires an unhindered path to the back of the α carbon
2. α and β branching block the path and hinder S_N2
3. Requires a good nucleophile
4. Polar, aprotic solvents increase nucleophilicity
5. Bulky groups on the nucleophile decrease nucleophilicity

1. Requires an antiperiplanar β-hydrogen
2. Enhanced by α and β-branching
3. Requires a strong base