Scope and Mechanism

Many secondary and tertiary halides undergo E1 elimination in competition with the SN1 reaction in neutral or acidic solutions. For example, when tert-butyl chloride solvolyzes in 80% aqueous ethanol at 25^o, it gives 83% tert-butyl alcohol by substitution and 17% 2-methylpropene by elimination:

The ratio of substitution and elimination remains constant throughout the reaction, which means that each process has the kinetic order with respect to the concentration of tert-butyl halide. The SN1 and E1 reactions have a common rate-determining step, namely, slow ionization of the halide. The solvent then has the choice of attacking the intermediate carbocation at the positive carbon to effect substitution, or at a β hydrogen to effect elimination:

Factors influencing the E1 reactions are expected to be similar to those for the SN1 reactions. An ionizing solvent is necessary, and for easy reaction the RX compound must have a good leaving group and form a relatively stable R^⊕ cation. Therefore the E1 orders of reaction rates are X=I > Br >Cl > F and tertiary R > secondary R > primary R.

With halides such as 2-chloro-2-methylbutane, which can give different alkenes depending on the direction of elimination, the E1 reaction is like the E2 reaction in tending to favor the most stable or highly substituted alkene: