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

  1. Requires an unhindered path to the back of the α carbon
  2. α and β branching block the path and hinder SN2
  3. Requires a good nucleophile
  4. Polar, aprotic solvents increase nucleophilicity
  5. Bulky groups on the nucleophile decrease nucleophilicity
E2
  1. Requires an antiperiplanar β-hydrogen
  2. Enhanced by α and β-branching
  3. Requires a strong base

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