An elimination reactions are in which two substituents are removed from a molecule in either a one or two-step mechanism.
Generally, an alkyl derivative, under appropriate conditions, will eliminate HX, where X is commonly a halide, ester, or -onium function, provided that there is a hydrogen located on the carbon adjacent to that bearing the X function: ,
An important feature of many elimination reactions is that they occur with the same combinations of reagents that cause nucleophilic substitution. In fact, elimination and substitution often are competitive reactions. Therefore it should be no surprise that substitution and elimination have closely related mechanisms.
THE E2 Elimination Reactions
An E2 elimination reactions is a bimolecular elimination. It is a one-step process of elimination with a single transition state. E2 elimination is is typical for primary and secondary alkyl halides.
Kinetics and Mechanism of Elimination Reactions
E2 elimination reactions starts when a base takes away H⊕ from the carbon that is adjacent to the carbon attached to the leaving group. The pair of electrons from the C-H bond move to occupy the p orbital between the H-C-C-LG. A pi bond is formed. Leaving group leaves. Here no intermediate is formed.
The conditions used for substitution reactions by the SN2 mechanism very often lead to elimination. The reaction of 2-bromopropane with sodium ethoxide in ethanol provides a good example:
Elimination to give propene competes with substitution to give ethyl isopropyl ether. Furthermore, the rate of elimination, like the rate of substitution, is proportional to the concentrations of 2-bromopropane and ethoxide ion. Thus elimination here is a second-order reaction.
As to the mechanism of this kind of elimination, the attacking base, eOC2H5, removes a proton from the /3-..carbon more or less simultaneously with the formation of the double bond and the loss of bromide ion from the neighboring carbon:
E2 elimination reactions occur most rapidly when the H–C bond and C–LG bonds involved are co-planar, most often at 180o or antiperiplanar. E2 elimination reactions are controlled by this stereochemical requirements also.
Elimination reactions have been shown to occur with many organics, the ones of most importance to environmental engineers and scientists are those with halogenated organics. The most common of these reactions are characterized by the release of HX (dehydrohalogenation) and the formation of a double bond.