6.15: Elimination Reactions
A nucleophile can react with an alkyl halide to give the substitution product by displacing the halogen. Or it can function as a base to give the elimination product by deprotonation of the neighboring carbon to form an alkene. In an elimination reaction, the substrate loses two groups from adjacent carbons forming at least one π bond. The carbon attached to the halogen is called the α carbon, while the adjacent carbon is called the β carbon; hence, these reactions are called β elimination or 1,2-elimination reactions.
The nucleophile acts as a Lewis base by donating a pair of electrons to a proton. Common bases used to promote elimination reactions include hydroxides (OH−), alkoxides (OR−), and amides (NH2−). In the presence of a strong base, the alkyl halide loses a proton from the β carbon and the halogen from the α carbon, enabling the formation of a π bond between the two carbon atoms.
Mechanism of Elimination Reactions
Elimination reactions commonly occur via the E2 or E1 mechanisms. The E2 mechanism takes place in a single concerted step: the abstraction of the β hydrogen by the base is accompanied by the cleavage of the α-carbon–halogen bond. Thus, the E2 reaction proceeds via one transition state.
The E1 reaction occurs in two steps. First, the alkyl halide undergoes ionization forming a carbocation intermediate and a halide ion. Next, the deprotonation of the carbocation by the base results in a π bond. Thus, in E1 reactions, the carbocation intermediate is formed via one transition state, and a second transition state exists for the deprotonation step.
Regio- and stereoselectivity
When the alkyl halide has two different β carbons, the elimination reaction can produce more than one alkene. In such cases, the more substituted (and most stable) alkene is generally observed, known as the Zaitsev product. However, in some cases, the less substituted alkene (Hofmann product) is obtained. The choice of base plays an important role in deciding which regioselective product is formed. Elimination reactions also favor the formation of trans-alkenes over the cis-isomers, making them stereoselective.