Electrophilic aromatic substitution. Part 16. The nitration of anisole, o-methylanisole, and p-methylanisole in aqueous sulphuric acid

Abstract

In the quantitative mononitration of anisole in 54–82% sulphuric acid at 25° the o : p ratio varies from 1.8 to 0.7. It is suggested that the rate-limiting step is the formation of an encounter pair between the nitronium ion and an anisole molecule which is hydrogen-bonded to a hydronium ion. The change in the o : p ratio may be due to competition between direct formation of Wheland intermediates from the hydrogen-bonded encounter pair, and loss of the hydronium ion to give a nitroniurn ion–anisole encounter pair, with subsequent formation of Wheland intermediates. With o- and p-methylanisole the products, and changes in product ratios with acidity are interpreted by considering the fates of the ipso-Wheland intermediates formed at C–Me. 4-Methyl-2-nitrophenol is an important product of the nitration of p-methylanisole, and results from ipso-attack by nitronium at C–Me, followed by attack of water and loss of methoxy.