Quantitative Kinetics in the Organic Laboratory: Mechanism and Substituent Effects by NMR

The nucleophilic substitution reactions of five benzyl chloride derivatives with sodium iodide in acetone were followed by low-field 1 H NMR. Rate constants measured at room temperature reveal that all reactions are bimolecular. Both electron-withdrawing and electron-donating groups accelerate the rate, resulting in a V-shaped Hammett plot. Organic Chemistry students 1) visualize both reactant and product signals through to reaction completion, 2) measure rate constants quantitatively, 3) evaluate the mechanism from rate data, and 4) relate electronic effects to the transition state structure.