Nitration of Hexamethylbenzene and Hexamethylbenzene-d18 in Acetic Acid. Deuterium Isotope Effect on the Product Distribution. Mechanism of Side-chain Substitution of Arenes

Abstract

Abstract The reaction in acetic acid of hexamethylbenzene and hexamethylbenzene-d18 with nitric acid in the dark has been investigated under various conditions using a high-pressure liquid chromatographic method. Pentamethylbenzyl nitrate, pentamethylphenylnitromethane, pentamethylbenzyl acetate, and pentamethylbenzyl alcohol were formed immediately after the mixing of reactants; their relative amounts remained almost unchanged up to nearly 50% conversion. The addition of sodium nitrite gave little influence on the composition of the product mixture, while urea was found to depress somewhat the formation of nitromethane. In the presence of lithium nitrate, the reaction was modestly accelerated and the nitrate formation seems to be slightly favored over the nitromethane formation. Hexamethylbenzene-d18 reacted with nitric acid at the same rate as the non-labeled hydrocarbon did, but the benzyl nitrate/phenylnitromethane ratio in the product mixture was considerably higher in the former. Based on the quantitative data obtained, the mechanism for the side-chain substitution has been discussed in terms of the SN1′ pathway: nitronium ion makes an ipso attack on the substrate to form the arenium ion, which releases a proton from the activated methyl group para to the site of attack to give the 3-methylene-6-nitro-1, 4-cyclohexadiene intermediate (7). Heterolytic fission of the C–N bond in 7 will form a benzyl cation-nitrite anion pair, which recombines at the benzylic carbon atom via a C–N bond or via a C–O bond, giving benzyl nitrite or phenylnitromethane, respectively. Benzyl nitrite will be further converted into benzyl nitrate and benzyl alcohol, while benzyl acetate will arise from the incorporation of solvent molecules into the ion-pair.