Sulfonation of three Symmetrical 2,6-Dialkylphenols, 2,6-Dichlorophenol, Phenol, and 2,6-Dimethylanisole. Sulfation and Sulfonation Product Distribution and Mechanisms

Article abstract of DOI:10.1021/jo00199a034

The sulfonation of four symmetrically substituted 2,6-disubstituted phenols, phenol, 2,6-dimethylanisole with SO3 in aprotic solvents was studied.With the phenols the initial product is the phenyl hydrogen sulfate, which is slowly converted into the phenolsulfonic acids via O-desulfonation and subsequent C-sulfonation if the phenol is in excess and via C-sulfonation and subsequent O-desulfonation if the SO3 is in excess.The ratio of partial rate factors for 3- and 4-sulfonation (f3/f4) for 2,6-dimethylphenol (1) in nitromethan strongly decreases on replacing the methyl groups by i-Pr, t-Bu (steric effects), and Cl (electronic effects).The strong increase of f3/f4 on increasing the SO3:1 ratio from 0.9 to 6.0 is ascribed to increasing sulfonation of the phenyl hydrogensulfate for which because of steric inhibition of resonance f3/f4 is higher (3.9) than that of the phenol 1 (<0.01) and of 2,6-dimethylanisole (5) (<0.01).The protic sulfonation of 1 and 5 in concentrated sulfuric acid was also studied.The large variations in f3/f4 with increasing sulfuric acid concentration are discussed in terms of steric inhibition of resonance for the entities undergoing sulfonation, viz., 1, its hydrogen sulfate, and 5 by the various sulfonating entities in the sulfuric acid range 75 - 107 percent H2SO4.