Methanediol dehydrates to give formaldehyde, which reacts rapidly and rever
sibly with monochloramine to form N-chloroaminomethanol. Under drinking wat
er conditions, N-chloroaminomethanol undergoes a relatively slow decomposit
ion that eventually leads to the formation of cyanogen chloride (CICN) in a
pparently stoichiometric amounts. The following reaction sequence is propos
ed: CH2(OH)(2) reversible arrow CH2O + H2O; CH2O + NH2Cl reversible arrow C
H2(OH)NHCl; CH2(OH)NHCl - CH2NCl + H2O; CH2NCl --> HCl + HCN; CN- + NH2Cl H+ --> ClCN + NH3. These reactions were studied at 25.0 degrees C and an i
onic strength of 0.10 M (NaClO4). Stopped-flow photometry was used to monit
or rapid, reversible reactions, and photometry was used to study relatively
slow decomposition reactions. Equilibrium and rate constants for the addit
ion of formaldehyde to manochloramine were (6.6 +/- 1.5) x 10(5) M-1 and (2
.8 +/- 0.1) x 10(4) M-1 s(-1), respectively. The dehydration of N-chloroami
nomethanol was catalyzed by both H+ and OH-, with respective rate constants
of 277 +/- 7 and 26.9 +/- 5.6 M-1 s(-1). Under characteristic drinking wat
er conditions, the decay of N-chloroaminomethanol is the rate-limiting step
. N-Chloromethanimine, formed by the dehydration of N-chloroaminomethanol,
had a decomposition rate constant of (6.65 +/- 0.06) x 10(-4) s(-1). At the
relatively high methanediol concentrations used in this study, the interme
diary N-chlorodimethanolamine was formed by the rapid and reversible reacti
on of N-chloroaminomethanol with formaldehyde. N-Chlorodimethanolamine then
decayed relatively slowly. The following reaction sequence is proposed: CH
2(OH)NHCl + CH2O reversible arrow {CH2(OH)}(2)NCl; {CH2(OH)}(2)NCl --> CH2N
Cl + CH2O + H2O. The equilibrium and rate constants for the addition of for
maldehyde to N-chloroaminomethanol were (9.5 +/- 2.5) x 10(4) M-1 and (3.6
+/- 0.1) x 10(3) M-1 s-', respectively. The decomposition of N-chlorodimeth
anolamine was catalyzed by OH-, with a rate constant of 19.2 +/- 3.7 M-1 s(
-1). N-Chlorodimethanolamine would not be present under typical drinking wa
ter treatment conditions.