Flash photolysis of 2,2,6,6-tetramethyl-N-chloropiperidine in aqueous solution at pH 2-8 generated the 2,2,6,6-tetramethylpiperidinyl radical which was protonated very rapidly to give the 2,2,6,6-tetramethylpiperidinium radical; the latter decayed under second-order kinetics with rate constants ranging from 6.9 × 105 cm s-1 at pH 7 to 7.7 × 104 cm s-1 at pH 4. The aminium radical abstracted hydrogen from methanol and ethanol with the rate constants of 66 and 244 M-1 s-1, respectively, and added to cyclohexene with a rate constant of 1.1 × 104 M-1 s-1. These rate constants, in particular that for addition to cyclohexene, were much smaller than those of the corresponding piperidiniumyl radical reaction and attributed to steric crowding around the radical center. The pKa of the aminium radical in water was determined to be 7.5-8.0 and its absorption curves in acetonitrile and in 10% acetonitrile-water were determined. In acetonitrile containing dilute sulfuric acid, the photolytically generated amino radical appeared to be protonated slowly having a lifetime in the order of 100 μs.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry