KINETICS AND MECHANISMS OF THE DECOMPOSITION OF N-CHLORO-ALPHA-AMINO ACIDS AND DICHLORAMINE IN AQUEOUS SOLUTIONS

VINCENT CYRIL HAND, Purdue University

Abstract

The decomposition of N-chloro-(alpha)-amino acids in neutral solutions gives Cl('-); imines, which hydrolyze rapidly to amine and carbonyl products; and CO(,2). The reactions are first order in {N-Cl-amino acid} and independent of pH. At 25.0(DEGREES)C the rate constants (s('-1)) are: N-Cl-glycine, 4.2 x 10('-6); N-Cl-sarcosine, 5.1 x 10('-5); N-Cl-threonine, 2 x 10('-4); N-Cl-alanine, 2.7 x 10('-4); N-Cl-proline, 8.8 x 10('-3); N-Cl-(alpha)-amino-isobutyric acid, 1.3 x 10('-2); N-Cl-amino-1-carboxycyclohexane, 9.0 x 10('-2). A concerted fragmentation mechanism is proposed to account for the products, the large change in reactivity, and the large positive (DELTA)S('(DBLDAG)) values (30-61 J/K(.)mol). Dichloramine (NHCl(,2)) solutions which are free of chloramines and NH(,3) are prepared by passing monochloramine (NH(,2)Cl) through a cation exchange resin. In the pH range 4 to 12, NHCl(,2) decomposes autocatalytically to trichloramine (NCl(,3)) and other products. The rate of reaction increases with increasing pH and increasing hypochlorite (OCl('-)) concentration. When NHCl(,2) reacts with excess OCl('-) in the pH range 5.5 to 12, NHCl(,2) is chlorinated by HOCl while a general base simultaneously removes the proton from NHCl(,2). Third order rate constants (M('-2) s('-1)) for the general base HPO(,4)('=), OCl('-), CO(,3)('=), and OH('-) are 1.6 x 10('4), 1 x 10('5), 6 x 10('6), and 3.3 x 10('9), respectively. Direct addition of acid to NH(,2)Cl solution produces NHCl(,2) and NH(,4)('+), with an equilibrium constant of 1 x 10('9) M('-1). Hypochlorite reacts much more rapidly with NH(,3) than with NHCl(,2), so that NHCl(,2) decomposition is five orders of magnitude slower when NH(,3) is present. Dichloramine decomposes to NH(,2)Cl and other products in neutral and basic solutions containing NH(,3). Monochloramine is produced by two separate pathways. One of these has a rate which is first order in {NHCl(,2)} and first order in {NH(,4)('+)} with a rate constant of 3 x 10('-5) M('-1)s('-1). The second pathway is first order in {NHCl(,2)} and first order in {OH('-)} with a rate constant of 150 M('-1)s('-1). Dichloramine is decomposed without producing NH(,2)Cl by a third mechanism which is first order in {OH('-)}. The molar absorptivity of dichloramine at its (lamda)(,max) of 294 nm is 176 M('-1) cm('-1). For trichloramine the value is 195 M('-1) cm('-1) at 336 nm. The formation of trichloramine proceeds with the loss of ('(TURN))25% of the initial oxidizing power.

Degree

Ph.D.

Subject Area

Chemistry

COinS