Primary hydrogen isotope effect in the nitration of polymethylbenzenes with nitric acid

The side-chain nitrooxylation (formation of benzyl nitrate) of specifically deuterated polymethylbenzenes with nitric acid in nitromethane has been investigated. The kinetic isotope effect (KIE) of hydrogen in the reaction was determined by means of MS and NMR spectroscopy. For 6-X-Pentamethylbenzenes (1, X=CH3; 2, X=H; 3, X=COCH3; 4, X=COC(CH3)3), KIE was 2.88(00C), 1.69 and 1.74 (0 and 250C), 2.01 (250C), and 1.46 (250C), respectively, by the intramolecular competition between the methyl group and the trideuteriomethyl group. For 1,2,3,5-tetramethylbenzene (5), the intermolecular KIE obtained by means of high performance liquid chromatograph (HPLC) was 1.2(00C). No hydrogen exchange was detected in the course of the reaction. These relatively large primary KIEs indicate that the proton removal is partly involved in the rate-determination. This is quite different from ordinary ring nitration of simple arenes. A range of the KIE values is discussed in terms of proton acidity, stability of the arenium ion, as well as steric circumstance. (author)