Pyridine-catalyzed halogenation of aromatic compounds. III. Chlorination and the relative stabilities of trichloride and tribromide in chloroform

Abstract

The mechanism of catalysis of aromatic chlorination by pyridine was investigated by measuring the rates of chlorination of toluene in acetic acid with and without added pyridine, pyridinium nitrate, or lithium chloride. All three increase the rate of chlorination slightly and to about the same extent. The effect on the second-order rate constant for chlorination of p-chloranisole in chloroform was determined for pyridine, pyridinium chloride, N-ethylpiperidinium chloride, tetra-n-butylammonium chloride, and tetra-n-butylammonium perchlorate. Again, all additives increase the rate constant, with the increase being least for pyridine. It is concluded that, for chlorination in dilute solution, pyridine has no specific catalytic effect, but probably increases the rate by increasing the polarity of the medium. Tetra-n-butylammonium chloride was observed to have a decreasing accelerating effect on chlorination in chloroform as its concentration is increased. This was thought to be the result of complexation of chlorine as trichloride. The formation constant for tetra-n-butylammonium trichloride in chloroform was found by the Benesi–Hildebrand spectrophotometric method to be 17 ± 3 M−1. This, together with the previously determined value of 9 × 104 M−1 for the corresponding tribromide, shows that the order of stability of trihalides in nonaqueous solvents is [Formula: see text] contrary to an earlier report.