Macroscopic and microscopic ionization constants of the thiol and amine groups in 1-methyl-4-mercaptopiperidine

The acid dissociation constants of 1-methyl-4-mercaptopiperidine (p K 1 = 9.51, p K 2 = 11.33), the 1,1-dimethyl-4-mercaptopiperidinium ion (p K A = 9.59) and 1-methyl-4-(methylthio)piperidine (p K B = 10.18) have been determined potentiometrically in 3 M sodium perchlorate (10% methanol) medium. The ultraviolet absorption of the mercaptide ion has been used to determine the relative proton affinity of the sulphur and nitrogen functions in 1-methyl-4-mercaptopiperidine under the same conditions, and its four microscopic constants (p K a = 9.49, p K b = 10.23, p K c = 11.34, p K d = 10.60) have been calculated; p K A has also been determined spectrophotometrically. From the results obtained, it can be concluded that the thiol group is more acidic than the amine group and that the Adams relation, K a + K b = K 1, holds very well when it is assumed that the spectrophotometric values for K a, and K b, can be replaced by K A and K B respectively.