β-Lactam degradation catalysed by Cd2+ ion in methanol

Abstract Kinetic schemes are established for degradation catalysed by Cd 2+ ions in methanolic medium for penicillin G, penicillin V and cephalothin, a cephalosporin. Methanolysis of penicillin V and cephalothin occurs with the formation of a single substrate-metal ion intermediate complex, SM, while degradation of penicillin G occurs with the initial formation of two complexes with different stoichiometry, SM and S 2 M. In each case, degradation is of first order with respect to SM with rate constant values equal to 0.079 min −1 , 0.120 min −1 and 0.166 min −1 at 20, 25 and 30°C, respectively, for penicillin G; 0.061 min −1 at 20°C for penicillin V; and 2.0×10 −3 min −1 at 20°C for cephalothin. Activation energy for the decomposition process of the SM intermediate for penicillin G was calculated to be about 5.5×10 4 J/mol. Equilibrium constant values between SM compound and S 2 M at 20°C (77.1 l/mol), 25°C (45.3 l/mol) and at 30°C (25.7 l/mol) were also calculated as well as the normal enthalpy of this equilibrium. With respect to the reaction products there is evidence that Cd 2+ becomes part of their structure, forming complexes between Cd 2+ and the product resulting from antibiotic methanolysis (L). Some characteristics of these complexes are discussed.