Electrophilic aromatic substitution. Part 31. The kinetics and products of nitration of naphthalene, biphenyl, and some reactive monocyclic aromatic substrates in aqueous phosphoric acid containing nitric acid or propyl nitrate

Abstract

Rate coefficients for nitration of naphthalene, 1- and 2-nitronaphthalene, biphenyl, 2-, 3-, and 4-nitrobiphenyl, anisole, and 2- and 4-nitroanisole in 90–97% H₃PO₄, and of biphenyl and 2- and 4-nitrobiphenyl in 69–72% H₂SO₄, have been measured. With results previously reported, these confirm that aromatics more reactive than benzene are nitrated in H₃PO₄ at the rate of encounter of aromatic and nitronium ion. This rate is slightly higher for the bicyclic than for the monocyclic compounds. Rate coefficients for solvolysis of propyl nitrate in H₃PO₄ are closely similar to those for nitronium ion formation from nitric acid, suggesting that these reactions have similar mechanisms. Kinetics and products of nitration of naphthalene with propyl nitrate in place of nitric acid, show that propyl nitrate is first solvolysed, followed by normal nitronium ion nitration of naphthalene. Product yields following nitration in 94–97% H₃PO₄ of benzene, toluene, anisole, chlorobenzene, naphthalene, and biphenyl, and for nitration in 69–72% H₂SO₄ of biphenyl are reported. Isomer ratios from nitration in phosphoric acid of toluene, anisole, naphthalene, and biphenyl reveal a marked preference for nitration at the site remote from the substituent, when comparison is made with nitration in sulphuric acid of comparable acidity. This is not the case for chlorobenzene, the least reactive of the substrates studied. It is suggested that viscosity can affect rates of translation within encounter pairs, so that unhindered positions of highly reactive substrates are relatively favoured in the more viscous solvent.