As described previously, primary alkyl halides generally undergo substitution reactions with simple nucleophiles by an SN2 mechanism. Secondary alkyl halides, often react with simple basic nucleophiles to give a mixture of products arising from both substitution and elimination.
As with substitution reactions, the rate at which elimination reactions proceed can be proportional to both the concentration of the base and the concentration of the reactant alkyl halide (an “E2” reaction (elimination bimolecular), or the rate can be proportional only to the alkyl halide (an “E1” reaction (elimination unimolecular).
The mechanism for the E2 reaction is best described as concerted with the reaction coordinate passing through a single energy maximum with no distinct intermediate. The transition state for this reaction is described by the structure shown below in which partial bonds exist between the attacking base, the hydrogen which is abstracted, and departing halogen.
The geometry of this transition state requires that the halogen be anti and coplanar with the hydrogen which is being removed (also termed “antiperiplanar”).
This fact is important to remember since the stereochemistry of the resulting alkene (Z or E; cis or trans) is often controlled by the hydrogen which is removed in the elimination reaction.
The Essay on Synthesis of an Alkyl Halide
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In the reaction shown below, the hydrogen on the carbon bearing the methyl group cannot become anti-to the halogen, hence the elimination occurs on the secondary carbon, to give the unfavored less substituted alkene.
The rate-limiting transition state in the E1 reaction is again, carbocation formation, and the transition state is generally described as shown below. Factors which control whether E1 or E2 mechanisms will be observed again relate simply to the stability of the intermediate carbocation.
Control of the reaction pathway between substitution and elimination is generally accomplished by careful choice of the reactants; strong, sterically hindered bases tend to favor elimination, while weak, unhindered nucleophiles tend to favor substitution. The choice for a “strong, hindered base” is generally tert-butoxide anion in tert-butanol as solvent, and it is generally safe to assume that a potential substitution/elimination reaction showing these conditions will proceed with elimination.
