On the intermediacy of phenyl hydrogen sulfates in the sulfonation of phenols. Sulfonation of phenol, anisole, methyl phenyl sulfate, the 2‐halogenophenols, a series of phenyl methanesulfonates together with 2,6‐dimethylaniline and its N‐methylsulfonyl derivative

The sulfonation of methyl phenyl sulfate (4) with concentrated aqueous sulfuric acid at 25°C yields the 4‐sulfonic acid. This initial product then decomposes to give phenol‐4‐sulfonic acid, which is subsequently sulfonated to phenol‐2,4‐disulfonic acid. From the first‐order‐rate coefficients obtained for sulfonation of phenyl methanesulfonate (3) and (4) in 93.2% H2SO4, for which acid concentration the sulfonating entity is H2S2O7, the σp+ values of the OSO2Me and OSO2OMe substituents have been determined to be 0.40 and 0.46, respectively. The sulfonation of both 2‐chlorophenyl (10) and 2‐methylphenyl methanesulfonate (14) with 2.0 equiv of SO3 in nitromethane at 0.0°C yields the 4‐sulfonic acid as the exclusive product, whereas 2‐methoxyphenyl methanesulfonate (13), under the same conditions, forms exclusively the 5‐sulfonic acid. 2‐Chloro‐ (6), 2‐bromo‐ (7) and 2‐iodophenol (8) with 1.0 and 4.0 equiv of SO3 in nitromethane yield exclusively the 4‐sulfonic acid, while 2‐fluorophenol (5) with 2.0 and 5.0 equiv of SO3 yields in addition the 5‐sulfonic acid to the extent of 4 and 30%, respectively. In the reactions of 2,6‐dimethylphenol (15) and 2,6‐dimethylaniline (17) with at least 1 equiv of SO3 in nitromethane, the 3‐SO3H/4‐SO3H ratio was found to increase on increasing the relative amount of SO3. For both 15 and 17, the limiting 3‐SO3H/4‐SO3H ratio at high SO3/ArOH ratios is 86/14 and this value is considered to be the ratio for the sulfonation of both 2,6‐dimethylphenyl hydrogen sulfate and 2,6‐dimethylaniline‐N‐sulfonic acid. This assumption is supported by the observation that the sulfonation of 2,6‐dimethylphenyl methanesulfonate (16) and N‐(methylsulfonyl)‐2,6‐dimethyl‐aniline (18) with SO3 leads to a similar 3‐SO3H/4‐SO3H ratio.