Elimination Reaction - E2 Mechanism

E2 Mechanism

During the 1920s, Sir Christopher Ingold proposed a model to explain a peculiar type of chemical reaction; the E2 mechanism. E2 stands for bimolecular elimination.

The fundamental elements of the reaction are as follows:

• One step mechanism in which carbon-hydrogen and carbon-halogen bonds break to form a double bond. C=C Pi bond.

Specificities

• E2 is a one-step process of elimination with a single transition state.
• Typically undergone by primary or secondary substituted alkyl halides
• The reaction rate, influenced by both the alkyl halide and the base (bimolecular), is second order.
• Because E2 mechanism results in formation of a pi bond, the two leaving groups (often a hydrogen and a halogen) need to be antiperiplanar. An antiperiplanar transition state has staggered conformation with lower energy than a synperiplanar transition state which is in eclipsed conformation with higher energy. The reaction mechanism involving staggered conformation is more favorable for E2 reactions (unlike E1 reactions).
• E2 typically uses a strong base, It needs a chemical strong enough to pull off a weakly acidic hydrogen.
• In order for the pi bond to be created, the hybridization of carbons need to be lowered from sp3 to sp2.
• The C-H bond is weakened in the rate determining step and therefore a primary deuterium isotope effect much larger than 1 (commonly 2-6) is observed.
• E2 is very similar to the S_N2 reaction mechanism.

An example of this type of reaction in scheme 1 is the reaction of isobutylbromide with potassium ethoxide in ethanol. The reaction products are isobutylene, ethanol and potassium bromide.