Aromatic sulfonation 85. Halogen directing and steric effects in the sulfonation of the twelve halogenotoluenes and some related compounds

Abstract

Abstract. The isomer distributions for the sulfonation of the twelve halogenotoluenes and some trisubstituted halogenomethylbenzenes, with both 98.4% H,SO, at 25°C and sulfur trioxide in nitromethane at O"C, have been determined and found to be very similar. The predominantly para-directing effect of the halogen substituent dominates over that of the methyl substituent: with the 2-halogenotoluenes, the degree of 5-substitution decreases from 2 9 0 % for the fluoro to 50% for the iodo compound. The 2-to 3-sulfonation ratio of the 4-halogenotoluenes strongly increases on going from fluorine (0.5) to iodine (7). The ratio of the partial rate factors for the sulfonation of a halogenobenzene ortho and meta to halogen varies from 17 1 for the fluoro to 1.5 $ -0.4 for the iodo substituent. In competition to the sulfonation, 2-and 4-iodotoluene undergo deiodination. The latter process is more important with the 4-isomer and with the protic sulfonating reagent. With the aprotic reagent, the reaction proceeds by direct sulfodeiodination, whereas with the sulfuric acid reagent, it proceeds by initial protiodeiodination and subsequent sulfodeprotonation.