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Kinetics of oxidation of acidic amino acids and their monoamides by N-chloroarylsulphonamides in aqueous acidic medium
Gowda, B. Thimme,Shetty, Mahesha
, p. 848 - 864 (2007/10/03)
Twelve sodium salts of N-chloroarylsulphonamides were employed as oxidants for studying the kinetics of oxidation of two acidic amino acids (aspartic and glutamic acid) and their monoamides (aspargine and glutamine) in aqueous acidic medium under various conditions, to see how the oxidative strength of these reagents vary with substitution. The sodium salts of N-chloroarylsulphonamides employed are of the general formulae i-X-C6H4SO 2. NaNCl·xH2O (where i-X = 4-C2H 5, 4-F, 4-Cl or 4-Br) and i-X-j-Y-C6H3SO 2NaNCl·xH2O [where i-X-j-Y = 2,3-(CH 3)2, 2,4-(CH3)2, 2,5-(CH 3)2, 2-CH3-4-Cl, 2,4-Cl2 and 3,4-Cl2]. The reactions show second-order kinetics in [oxidant], fractional order in [amino acid] and an inverse dependence on [H+], Addition of the reduced product of the oxidants or variation in ionic strength of the medium has no significant effect on the rates of oxidations. Mechanisms in conformity with the observed kinetics are discussed. The effective oxidizing species of the oxidants is Cl+ in different forms. The oxidizing strengths of N-chloroarylsulphonamides depend on the ease with which Cl + is released from them. The study reveals that the introduction of electron-withdrawing groups such as halides to the benzene ring eases the release of Cl+ from the reagent and hence increases the oxidizing strengths of the N-chloroarylsulphonamides. The effect of substituents on E a of the reactions was analysed by optimising Ea with reference to log A, and log A with reference to Ea of the parent oxidant. Copyright
