Slow proton transfer in the nitration of anthracene; determination of the kinetic isotope effect of hydrogen in the nitration of naphthalene and anthracene with nitronium tetrafluoroborate

The kinetic isotope effect of hydrogen in the nitration of anthracene with nitronium tetrafluoroborate has been determined, using anthracene-9-d as substrate. In sulfolane as solvent at 30° and in acetonitrile at 0°, kH/kD was found to be equal to 2.6 ± 0.3 and 6.1 ± 0.6, respectively. The kinetic isotope effect of hydrogen in the nitration of naphthalene in these two solvents under similar reaction conditions, using naphthalene-1,4-d2 as substrate, were found to be 1.15 ± 0.05 and 1.08 ± 0.05, respectively. UV spectral evidence was obtained that the nitration of anthracene with nitronium tetrafluoroborate in sulfolane as solvent proceeds via an intermediate which is suggested to be the 9-nitro-9-anthracenonium ion. This intermediate is formed directly on mixing the reagents. The proton transfer from this intermediate with formation of 9-nitroanthracene is a relatively slow process.