Kinetics and Mechanism

The conditions used for substitution reactions by the S_N2 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 (it may be helpful to review Section 8-4 at this point):

rate of substitution =kS[RBr][^⊖OC2H5]

rate of elimination =kE[RBr][^⊖OC2H5]

As to the mechanism of this kind of elimination, the attacking base, ^⊖OC2H5, removes a proton from the β carbon more or less simultaneously with the formation of the double bond and the loss of bromide ion from the neighboring carbon:

The abbreviation for this mechanism is E2, E for elimination and 2 for bimolecular, there being two reactants involved in the transition state.