Electrophilic aromatic substitution. Part 23. The nitration of phenol and the cresols in aqueous sulphuric acid

Abstract

The quantitative mononitration of phenol and o-, m-, and p-cresol by nitric acid in 58–80% sulphuric acid involves reaction with the nitronium ion at or very close to the encounter-controlled rate. For phenol the o : p ratio changes smoothly from 2.4 to 0.9 over the range 56–83% sulphuric acid. For o-cresol the ratio of 2-methyl-6-nitrophenol to the 4-nitro isomer changes from 1.5 to 0.8 over 50–83% sulphuric acid, and for m-cresol the ratios of both 3-methyl-2- and 3-methyl-6-nitrophenol to the 4-nitro isomer fall over 58–81% sulphuric acid (0.6 to 0.2 and 1.5 to 0.7 respectively). These changes, and those for some related methyl ethers reported previously, are discussed. The nitration of p-cresol in 68–72% sulphuric acid involves ca. 40%ipso substitution at C–Me and the 4-methyl-4-nitrocyclohexadienone formed undergoes an acid-catalysed rearrangement to 4-methyl-2-nitrophenol. 2-Methyl-6-nitrophenol is established as an important product of nitration of o-methylanisole and a mechanism involving demethoxylation of a Wheland intermediate, formed by ipso-substitution at C–Me, and a subsequent specific 1,3-rearrangement is proposed.