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.

When an alkyl halide reacts with a nucleophile, the nucleophile can displace the halogen to give the substitution product or it can abstract a neighboring hydrogen to form an alkene through an elimination reaction.

In elimination reactions, nucleophiles function as Lewis bases by donating a pair of electrons to a proton. Some of the common bases used to promote elimination reactions include hydroxides such as sodium hydroxide, alkoxides like potassium tert-butoxide, or alcohols like ethanol.

Elimination reactions typically involve the loss of small molecular fragments from a substrate to form at least one π bond. In alkyl halides, the elimination reaction proceeds with the loss of one hydrogen atom and one halogen atom, hence the name dehydrohalogenation.

Since the carbon bonded to the leaving group is an α carbon and the hydrogen on the adjacent carbon is a β hydrogen, these reactions are often called β-elimination or 1,2-elimination reactions.

Most elimination reactions occur via an E2 or E1 mechanism.

For E2 reactions, strong bases like sodium ethoxide are used. The concerted mechanism is initiated by the deprotonation of the β carbon followed by the departure of the halide leaving group leading to the formation of a π bond between the α and β positions.

In contrast, the E1 reaction proceeds in two steps. The first involves the departure of the leaving group to form a carbocation intermediate, followed by deprotonation of the carbocation by the base to form a π bond.

With alkyl halides containing two different β carbons, elimination reactions can produce more than one alkene. Here, the more substituted alkene is the most stable and is called the Zaitsev product, while the less substituted alkene is called the Hofmann product. Thus, elimination reactions are said to be regioselective.

Additionally, elimination reactions favor the formation of trans-alkenes over the cis-isomers, making them stereoselective.