NITROSATION OF AMINES IN NONAQUEOUS SOLVENTS .1. EVIDENCE OF A STEPWISE MECHANISM

We studied the nitrosation of piperidine, morpholine, pyrrolidine, N-m ethylpiperazine, N,N'-dimethylethylenediamine and diethylamine by 2-br omoethyl nitrite, 2,2-dichloroethyl nitrite, 2,2,2-trichloroethyl nitr ite, or N-methyl-N-nitroso-p-toluenesulfonamide (MNTS) in cyclohexane, isooctane, dichloromethane, 1,4-dioxane, or tetrahydrofuran. The depe ndence of the first-order pseudoconstant k(0) on the amine concentrati on (always in excess) was sigmoid for nitrosation by alkyl nitrites an d linear or quadratic for nitrosation by MNTS. The effects on k(0) of isotopic substitution, temperature, and base catalysis by a less react ive amine were also determined. The experimental data are in keeping w ith a reaction mechanism involving a zwitterionic tetrahedral intermed iate T+/- analogous to intermediates postulated for the aminolysis of carboxylic esters in similar solvents: according to this mechanism, T/- is formed either directly from the amine and nitrosating agent (in the case of MNTS) or indirectly via a hydrogen-bonded complex between the amine and nitrosating agent (in the case of alkyl nitrites) and de composes either spontaneously or with the catalytic assistance of a se cond amine molecule. For alkyl nitrites, the rate-controlling step is the formation of T+/- at high amine concentrations and its decompositi on at low amine concentrations; for MNTS, the rate-controlling step is the formation of T+/- in more polar solvents and its decomposition in less polar solvents. An alternative mechanism, involving the formatio n of T+/- from both monomers and dimers of the amine, is ruled out.