CHLORINATION OF AMINO ACIDS : REACTION PATHWAYS AND REACTION 4 RATES 5 6

18 Chlorination of amino acids can result in the formation of organic monochloramines or organic 19 dichloramines, depending on the chlorine to amino acid ratio (Cl:AA). After formation, organic 20 chloramines degrade into aldehydes, nitriles and N-chloraldimines. In this paper, the formation 21 of organic chloramines from chlorination of lysine, tyrosine and valine were investigated. 22 Chlorination of tyrosine and lysine demonstrated that the presence of a reactive secondary 23 group can increase the Cl:AA ratio required for the formation of N,N-dichloramines, and 24 potentially alter the reaction pathways between chlorine and amino acids, resulting in the 25 formation of unexpected by-products. In a detailed investigation, we report rate constants for 26 all reactions in the chlorination of valine, for the first time, using experimental results and 27 modelling. At Cl:AA = 2.8, the chlorine was found to first react quickly with valine (5.4x10 28 M s) to form N-monochlorovaline, with a slower subsequent reaction with N29 monochlorovaline to form N,N-dichlorovaline (4.9x10 M s), although some N30 monochlorovaline degraded into isobutyraldehyde (1.0x10 s). The N,N-dichlorovaline then 31 competitively degraded into isobutyronitrile (1.3x10 s) and N-chloroisobutyraldimine 32 (1.2x10 s). In conventional drinking water disinfection, N-chloroisobutyraldimine can 33 potentially be formed in concentrations higher than its odour threshold concentration, resulting 34 in aesthetic challenges and an unknown health risk. 35 36