Aromatic sulphonation. Part 91. The sulphonation of anisole, phenol, phenyl methanesulphonate, potassium phenyl sulphate, and a series of methyl-, bromo-, and chloro-substituted anisoles and phenols in concentrated aqueous sulphuric acid

Abstract

The (homogeneous) sulphonation of a number of aromatic ethers and alcohols, viz. anisole (1), 3- methyl-(2), 4-methyl-(3). 2-bromo-(4), 4-bromo-(5), 2-chloro-(6), and 4-chloro-anisole (7), phenol (8), 2-methyl-(9), 3-methyl-(10), 4-methyl-(11), 4-bromo-(12), 2-chloro-(13), and 4- chloro-phenol (14), phenyl methanesulphonate (15), and potassium phenyl sulphate (16) in concentrated aqueous sulphuric acid at 25.0 °C has been studied, and rates and isomer distributions have been determined. The sulphonation is first-order in the aromatic substrate, and from the rate measurements it is concluded that the species undergoing sulphonation in the phenyl ring is an unprotonated substrate species. In the lower acid concentrations the sulphonating entity is H₃SO₄⁺. With increasing sulphuric acid concentration there is a gradual change-over in the sulphonating entity from H₃SO₄⁺ to H₂S₂O₇. The acid concentrations of equal rate contribution by the two entities for anisole and phenol are 87 and 90 ± 1%, respectively. Sulphonation on the oxygen atom (i.e., sulphation) does not occur. The o/p-ratios for (1) and (8) do not vary over the studied sulphuric acid range of 75–90% H₂SO₄. Partial rate factors for the 2- and 4-position of (1) and (8) are reported. The very low partial rate factors for the 4-substitution of (1) and (8) and the observed extreme suppression and compression of the reactivities of the substrates (1)–(14) are ascribed to hydrogen bonding of the substrates with the acidic solvent species present. It is tentatively suggested that the relatively high contents of sulphonation ortho to –OR with anisole (36%) and phenol (48%) are due to specific complexation of the substrates with the sulphonating electrophile.