Selenium and Sulfur in Exchange Reactions: A Comparative Study

Cysteamine reduces selenocystamine to form hemiselenocystamine and then cystamine. The rate constants are k(1) = 1.3 x 10(5) M-1 s(-1); k(-1) = 2.6 x 10(7) M-1 s(-1); k(2) = 11 M-1 s(-1); and k(-2) = 1.4 x 10(3) M-1 s(-1), respectively. Rate constants for reactions of cysteine/selenocystine are similar. Reaction rates of selenium as a nucleophile and as an electrophile are 2-3 and 4 orders of magnitude higher, respectively, than those of sulfur. Sulfides and selenides are comparable as leaving groups.