Chemical Kinetics

The ability to generate reaction coordinate diagrams for classic reactions from organic chemistry is an important skill. For example, the reaction coordinate diagram for S_N2 substitution has one maximum because there is only one activated complex. The leaving group departure coincides with nucleophile approach.

With S_N1 substitution, however, the reaction coordinate diagram has two maxima. Leaving group departure occurs first followed by nucleophile approach. Because the activation energy corresponding to leaving group departure is typically higher than the activation energy of nucleophile approach in S_N1 substitution, leaving group departure is the rate determining step. Therefore, only the concentration of the alkyl halide substrate appears in the rate expression.