How to Favor an E2 Mechanism
1. Use a
secondary or tertiary alkyl halide if possible.
Why: Because
steric hindrance in the substrate will inhibit substitution.
2. When a
synthesis must begin with a primary alkyl halide, use a bulky base.
Why: Because the
steric bulk of the base will inhibit substitution.
3. Use a high
concentration of a strong and nonpolarizable base such as an alkoxide.
Why: Because a
weak and polarizable base would not drive the reaction toward a bimolecular
reaction, thereby allowing unimolecular processes (such as SN1 or E1 reactions)
to compete.
4. Sodium
ethoxide in ethanol(EtONa/EtOH)and potassium tert-butoxide intertbutyl
alcohol(t-BuOK/t-BuOH)are bases typically used to promote E2 reactions.
Why: Because they
meet criterion 3 above. Note that in each case the alkoxide base is dissolved
in its corresponding alcohol. (Potassium hydroxide dissolved in ethanol or
tert-butyl alcohol is also sometimes used, in which case the active base
includes both the alkoxide and hydroxide species present at equilibrium.)
5. Use elevated
temperature because heat generally favors elimination over substitution.
Why: Because
elimination reactions are entropically favored over substitution reactions
(because the products are greater in number than the reactants). Hence ∆S° in the
Gibbs free-energy equation, ∆ G°=∆ H°
- T ∆S° is significant, and ∆S° will be increased
by higher temperature since T is a coefficient, leading to a more negative (favorable)
∆G°.
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