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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