KINETICS OF CHLORINE TRANSFER FROM MONOCHLORAMINE TO AMINO NITROGEN AND THE DECOMPOSITION OF N-CHLORAMINO ACIDS AND N-CHLORODIPEPTIDES

MARTIN PATRICK SNYDER, Purdue University

Abstract

Monochloramine, NH(,2)Cl, transfers chlorine to the amino nitrogen of methyl amine, amino acids and dipeptides by direct atom transfer. The equilibrium constants for the reactions are favorable and the reactions have first-order dependences in total monochloramine and in total amine concentration. The observed rate constants are independent of hydrogen ion concentration between -log {H('+)} of 4 to 8 but decrease, in general, in more acidic or basic solutions. The results indicate that NH(,3)Cl('+) is the reactive chlorinating species. The resolved second-order rate constants increase as the amine nucleophilicity increases but reach a limiting value at 2.4 x 10('8) M('-1)s('-1) (25.0(DEGREES)C, 0.50 M(NaClO(,4))). This limit is interpreted as electrophilic bond breaking of the NH(,3)-Cl('+) bond. The influence of temperature on the reaction parameters is evaluated. Chlorine transfer is a significant pathway for loss of NH(,2)Cl in the environment. N-Chloramino dipeptides are more stable than their chloramino acid analogs in neutral pH solution but are less stable in basic solutions. The decomposition reactions in OH('-) solutions proceed by a sequential two-step mechanism. The first step is hydrogen abstraction from the (alpha)-carbon of the chloramine by OH('-) followed by rearrangement with loss of Cl('-) to give an intermediate imine. The second step is the relatively slower base hydrolysis of the imine to give NH(,3) and an (alpha)-dicarbonyl as products. In neutral pH solutions, the chloramino dipeptides undergo a slow general-base catalyzed decomposition which is first order in the chloramine (k(,d) = 10('-5) to 10('-6) s('-1)) to give the same products as the decomposition reaction in OH('-) solutions. Chloramino acids, on the other hand, undergo a more favorable oxidative decarboxylation reaction (k(,d) = 10('-2) to 10('-5) s('-1)) to give NH(,3), Cl('-), CO(,2), and an aldehyde or ketone as products. Steric enhancement of the oxidative decarboxylation rate is reported for the first time. An analytical method for the determination of NH(,2)Cl in the presence of N-chloramino acids and N-chlorodipeptides is evaluated. Pretreatment of a test sample with hydroxide solution for a fixed time interval (0.2 M OH('-) for 20 min.) removes the oxidizing residual of the organic chloramines but does not affect the NH(,2)Cl residual significantly. After readjustment of the sample pH, sample analysis by the DPD Colorimetric Method shows that 92% recovery of NH(,2)Cl is achieved at the 0.2-2.0 ppm level. Some organic chloramines, such as CH(,3)NHCl, are not removed by the pretreatment. Interference by NHCl(,2) is minimized by an ether extraction but the recovery of NH(,2)Cl becomes less than 90%.

Degree

Ph.D.

Subject Area

Analytical chemistry

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