The Hofmann Rearrangement. I. Kinetic Substituent Effects of ortho-, meta-, and para-Substituted N-Bromobenzamides

The kinetic substituent effects on the Hofmann rearrangements of ortho-, meta-, and para-substituted N-bromobenzamides were studied in an aqueous sodium hydroxide solution. The rate constants of the meta- and para-derivatives were correlated by means of our equation: logk⁄k0=−2.39(σ0+0.41ΔσR+)+0.002 The reaction rates of ortho-derivatives were found to be 2–20 times faster than those of the corresponding para-derivatives. The reaction mechanism of this rearrangement was discussed in terms of LFER, the ortho/para rate ratios, and the activation parameters; as a result, the concerted mechanism (the participation of the phenyl group in electron-deficient nitrogen at the transition state) was favored rather than the two-step mechanism. The similarity of the reaction mechanism of the Hofmann to the Lossen rearrangement was also shown on the basis of LFER.