0.3 E2 elimination  (Page 6/3)

The e2 elimination reaction

Objective

This experiment illustrates the base-induced dehydrohalogenation of alkyl halides with strong base for the preparation of alkenes. The stereo and regiochemical effect of the size of the base is investigated, and the product mixture is analyzed by gas chromatography.

Background information

Base-induced elimination (dehydrohalogenation) of alkyl halides is a general reaction for preparing alkenes. This process is often referred to as an E2 elimination, since a hydrogen atom is always removed in addition to a halide (leaving group): E2 indicates a bimolecular elimination reaction, where rate = k [B][R-LG].

This pathway is a concerted process with the following characteristics:

Let's look at how the various components of the reaction influence the reaction pathway:

Effects of r-

In an E2 reaction, the reaction transforms two ${\text{sp}}^3$ C atoms into two ${\text{sp}}^2$ C atoms. This moves the substituents further apart, decreasing any steric interactions. So, more highly substituted systems undergo E2 eliminations more rapidly. This is the same reactivity trend as seen in E1 reactions.

-LG The C-LG bond is broken during the rate-determining step, so the rate does depend on the nature of the leaving group. However, if a leaving group is too good, then an E1 reaction may result.

B Since the base is involved in the rate-determining step, the nature of the base is very important in an E2 reaction. More reactive bases will favor an E2 reaction.

The e2 pathway is most common with:

a) high concentration of a strong base b) poor leaving groups c) R-LG that do not lead to stable carbocations (where the E1 mechanism will occur)

A high concentration of a strong base in a relatively nonpolar solvent is used to carry out the dehydrohalogenation reaction. Combinations include sodium methoxide in methanol, sodium ethoxide in ethanol, potassium isopropoxide in isopropanol, and potassium tert-butoxide in tert-butanol or dimethyl sulfoxide (DMSO).

Elimination reactions nearly always yield an isomeric mixture of alkenes. Under the reaction conditions, the elimination is regioselective and follows the Zaitsev rule when more than one route is available for the elimination of HX from an unsymmetrical alkyl halide. Consequently, the reaction proceeds in the direction that yields the most highly substituted alkene. In cases where cis or trans alkenes can be formed, the reaction exhibits stereoselectivity, where the more stable trans isomer is the major product. For example,

Mechanism of the e2 reaction:

Experimental evidence indicates that the five atoms involved in the E2 elimination reaction must lie in the same plane. The anti-periplanar conformation is preferred. This conformation is necessary so that orbital overlap can occur in order for the $\pi$ bond to be generated in the alkene. The ${\text{sp}}^3$ -hybridized atomic orbitals on carbon that comprise the C-H and C-X $\sigma$ bonds broken in the reaction develop into the $\rho$ orbitals comprising the $\pi$ bond of the alkene formed: