Electrophilic aromatic substitution. Part 30. The kinetics and products of the solvolyses in aqueous sulphuric acids of 5-chloro-2-methyl-2-nitrocyclohexa-3,5-dienyl acetate: the occurrence of AAC2 and AAL 1 solvolyses, and of an acid-catalysed elimination of nitrous acid, and the relationship of the AAL 1 solvolysis to the nitration of 4-chlorotoluene

Abstract

Good first-order kinetics of solvolysis of the above-named diene in water and in 6.5–43.6% H₂SO₄ at 25°C, and in water and in 15.2–58.8% H₂SO₄ at 5°C have been observed. The yields of 4-chlorotoluene, 5-chloro-2-methylphenyl acetate, 5-chloro-2-methylphenol, 4-chloro-2-nitrotoluene, 4-chloro-3-nitrotoluene, and 4-methyl-2-nitrophenol produced in water and in 21.5–92.4% H₂SO₄ at 25°C in the presence of sulphanilic acid or hydrazinium sulphate, and additionally of 2- and 4-nitroanisole when anisole was also added, have been measured. The solvolysis proceeds by an acid-catalysed elimination of nitrous acid (confirming a tentative conclusion in another case ¹), which competes with A_AC2 and A_AL1 ester solvolyses. With increasing acidity the solvolyses become dominant, the A_AL1 reaction increasingly so. The small yield of 4-chloro-3-nitrotoluene comes from a thermal reaction of the diene unrelated to the elimination and solvolyses. The A_AL1 reaction generates the ipso-Wheland intermediate (W_i^Me) that is also formed in the nitration of 4-chlorotoluene. The intermediate reacts by return to 4-chlorotoluene and nitronium ion (which can be captured by anisole), by 1,2- and 1,4-nucleophilic capture by water (giving 5-chloro-2-methylphenol and 4-methyl-2-nitrophenol, respectively), and by 1,2-rearrangement to 4-chloro-2-nitrotoluene. The first of these reactions never accounts for more than about 12% of the W_i^Me and competition between capture and rearrangement moves strongly in favour of the latter with increasing acidity. Re-examination of the nitration of 4-chlorotoluene has revealed products arising from 1,2- and 1,4-capture of W_i^Me, previously overlooked. An improved assessment of positional reactivities shows 59% of primary attack by nitronium ion to occur at C–Me in 63% H₂SO₄.