Environ. Sci. Technol. 1985, 19, 810-814
Transfer of Active Chlorine from Chloramine to Nitrogenous Organic Compounds. 2. Mechanism Russell A. Isaac" and J. Carrel1 Morris
Division of Applied Sciences, Harvard University, Cambridge, Massachusetts 02138 In previous papers (I, 2) the authors described the kinetics of Cl' (i.e., active chlorine) transfer from NH2Cl (chloramine) to nitrogenous organic compounds. The reactions were determined to be second order-first order in each reactant. At 25 "C, observed rate constants ranged from 0.140 (dimethylamine, pH 6 and 8) to 13.8 (mol/L)-l s-l (morpholine, pH 7). The present paper presents two qechanisms that are consistent with the data: (1)NH3C1+ plus common basic form of nitrogenous compound; (2) NH2Cl plus common basic form of nitrogenous compound plus H+ (i.e., general acid catalysis). For amino acids, the common basic form is the anion, and for amines it is the neutral molecule. In addition, a positive correlation between the fundamental rate constants for the transfer reactions and the basicity of the nitrogenous substrates is demonstrated.
Mechanism In previous papers (1, 2), the authors described the kinetics of C1+ (i.e., active chlorine) transfer from NH2Cl to nitrogenous organic compounds. There are two plausible pathways for C1+ transfer from NHzCl to organic N. One is by hydrolysis: "$1 + HzO NH, + HOCl (slow) (la) N compound + HOCl N-chloro compound + H 2 0 (fast) (lb) Because hydrolysis, the rate-determining step, is a firstorder reaction, the rate of disappearance of NH2Cl is first order as is the appearance of the N-chloro compound. Thus, the overall reaction would be first order according to this scenario. The second pathway is direct transfer: NH2Cl + N compound N-chloro compound + NH,
--
-
(2) If the hydrolysis mechanism is valid, then the transfer rate in the presence of excess nitrogenous organic compound should be equal to the hydrolysis rate of NH2Cl as determined by Granstrom (3) and Gray ( 4 ) and should be independent of the nature and concentration of nitrogenous organic reactants. This was found to be not the case when the initial concentration of nitrogenous organic compound was varied. Instead the observed first-order rate constants were proportional to the initial concentration of N compound as illustrated in Table I (runs GG7GG15). In addition, nearly a 100-foldvariation in rate was observed for different N compounds (e.g., morpholine at pH 7.18 and dimethylamine at pH 6.2 and 7.87). Also the specific rates of reaction on a pseudo-first-order basis, i.e., k(obsd) X [N compound], which ranged from 1 X s-l for dimethylamine to >2 X s-l for glycine, are greater than the chloramine hydrolysis rate, 2.1 X s-l at 25 OC. Further support for the second-order reaction was obtained by conducting transfer reactions with equimolar
* To whom correspondence should be addressed at the Massachusetts Division of Water Pollution Control, Lyman School, Westborough, MA 01581. 810
Environ. Sci. Technol., Voi. 19,No. 9, 1985
concentrations of glycine-NH2C1, serine-NH2C1, and glycylglycine-NHzC1. The observed second-order rate constants calculated from sets of glycine, serine, and glycylglycine experiments employing pseudo-first-order conditions and those at the same pH but equimolar in the reactants reveal that in two cases the specific rates based on the first-order analysis were smaller than those based on the second-order analysis, ranging in the three instances from 63% to 100% as discussed later. In spite of these variations, the fit of the data to plots consistent with second-order techniques was good for experiments under second-order conditions as presented previously (I) and as typified by Figure 1. Therefore, the working hypothesis remained that the transfer reactions under the experimental conditions were second order overall, first order in each reactant. On a more general level, it also was felt that a clearer assessment of differences would ensue from considering entire data sets rather than portions of them. This approach involved testing of a general explanation or hypothesis to account for the observations as discussed later in this section. The ionic strength of the solution is important for calculating reacton rate constants for both plausible mechanisms which involve charged species (i.e., NH3C1+and/or anions of the amino acids). Ionic strength was calculated through Davies' modification of the Debye-Huckel expression: -log f* = 0.509Z2 [d/(1 + (0 - 0.211
(3)
where f k is the activity coefficient for an ion, Z is the ionic charge, and I is the ionic strength (mol/L). Activity coefficients appear in the equations for both mechanisms presented in Table 11. The Davies' expression is considered reasonable for the ionic strengths (0.0259-0.143 mol/L) employed in this work. Although a more accurate expression for calculating activity coefficients has become available recently (5), expression 3 remains reasonable, given the precision of the data and since the practical value of the derived rate constants for assessing chlorination reactions in the context of environmental questions remains unimpaired. Several possible mechanisms can account for the observed features of the direct transfer of C1+ from NHzCl to organic nitrogen. The nitrogenous compounds investigated fall into two categories: those that form zwitterions and anions and those that form primarily neutral and positive species under the experimental conditions in these studies. The first group includes the amino acids glycine, alanine, serine, and sarcosine, as well as glycylglycine. The second group consists of amines, On the basis of the inconclusive results of amide bond chlorination by NH&l in the N-acetylglycine experiments, it is likely that the reaction between glycylglycine and NH2Cl involves chlorination of the amino group rather than the peptide N. The data reveal fairly constant observed reaction rate coefficients in the neutral pH range with a decrease at high pH values as illustrated in Table I and Figure 2 for NH&l plus glycylglycine.
0013-936X/85/0919-0810$01.50/0
0 1985 American Chemical Society
______
Table I. Glycylglycine plus NH2Cl: Summary of Results
run
pH
T,"C
GG1 GG2 GG3 GG4 GG5 GG6 GG7 GG8 GG9 GGlO GGll GG12 GG13 GG14 GG15 GG16 GG17 GG18 GG19 GG20 GG21 GG22 GG23 GG24 GG25 GG26 GG27
6.24d 6.15 6.16 6.49 6.47 6.48 7.08 7.08 7.07 7.00 7.00 7.00 6.97 7.05 7.06 7.03 7.03 7.04 7.51 7.52 7.51 8.11 8.11 8.09 8.52 8.52 N.M.e
23 24 24 24 24 25 25 25 27 24 25 26 24 26 26 24 24 25 24 24 23 24 24 24 26 26 26
[NH&l]" xi03,mol/L
[GlyGly]" xi03, mol/L
k'(obsd)
15.2
0.945
15.3
0.813 0.461
4.62
0.461
2.31
0.461
1.15
0.461
0.462 15.1
1.05 0.912
15.1
0.967
15.2
nb
-re
7 7 7 7 8 8 5 7 5 6 7 5 6 7 7 9 12 9 6 7 6 17 17 17 13 13
0,993 0.998 0.994 0.991 0.994 0.996 0.999 >0.999 >0.999 >0.999 0.995 0.982 0.995 0.995 0.979 0.994 0.995 0.994 0.996 0.998 0.995 0.995 0.998 0.999 0.998 0.999 0.998
11
r = linear correlation coefficient.
a Before mixing. Number of points. measurement. e Not measured. 4.0
I
x103,
s-l
39.2 47.4 54.0 59.5 53.3 66.3 15.7 13.0 16.1 5.051 4.35 4.51 1.46 1.50 1.68
29.1 30.4 33.0 22.9 22.0 24.4 16.1 15.3 18.5
k(obsd), (rnol/L)-' s-l 5.20 6.27 7.15 7.65 6.'97 8.67 7.11 5.93 7.31 4.82 4.16 4.29 2.95 3.01 3.39 5.38 5.22 5.54 3.85 4.02 4.37 3.03 2.91 3.23 2.12 2.01 2.44
k(obsd) av
point on Figure 2
6.2
A
7.8
B
4.9
C
4.1
D
3.1
E
2.2
F
Suspect not all pH 7 buffer was rinsed from electrode before
Table 11. Expressions for Rate Constants Based o n Plausible Mechanismsa Mechanism 1: Basic Molecule + NH,Cl+ (a) amino acid + NH,C1: anion + NH,Cl+ h(obsd)[Kat+ + (H+)l[f+(H+)+ Ka(2)1 k = -------(H+)Ka(2If+ti ( b ) amines + NH,Cl: neutral molecule + NH,Cl+ k = h(obsd)[Katk + (H+)I[(H+)+ KaNf+1
______.__________
!SEW:1.13 x
1O5mol/L
T:2&
pH 0
100
200
300
:
7.06
400
500
660
SECONDS
Flgure 1. Serine plus "&I.
Run SE13.
The features or characteristics of the reactions between and the zwitterion/anion formers can be accounted for on a molecular level by at least three mechanisms which are equally consistent with the data. The mechanism involving NH&l and the zwitterion, +NH3CH(R)COO-,is formally also consistent with the data, but is rationally less plausible and therefore rejected because no free electron pair is left on the NH3+ group to receive the transferred C1+. The two plausible mechanisms for the reaction of NHzCl with nitrogenous compounds are presented in Table I1 as are the corresponding kinetic expressions. A third mechanism involving reaction of the neutral species of the nitrogenous compounds with neutral NH&1 results in rate constants that are internally consistent for the amino acids, but which are not consistent for the amines, and, therefore, this mechanism is considered not to be plausible. Thus, the two plausible mechanisms, which are presented in Table 11, involve the common basic forms of the nitrogenous substrates, which are anions of amino acids and neutral molecules of amines. Both the first mechanism, which involves NH3C1+,and the second, general acid catalysis, were discussed previously by Isaac (6). The former
"$1
( H+)KaNtit; Mechanism 2: Basic Molecule + NH,C1 + H' (a) amino acid + NH,Cl: anion + NH,Cl (catalysis by H') k = k(obsd)l(H+)fi + Ka(2)I
Ka( 2)(H+It* (b) amine + NH,Cl: neutral molecule + NH,C1 (catalysis by H ') h(obsd)[(H') + KaNti I f * + k=--
KaNfi
a h = fundamental rate constant; k(obsd) = observed
experimental rate constant; Ka = acid dissociation constant of NH,Cl+; Ka(2) = second acid ionization constant for amino acids (Le., for the formation of the anion); KaN = acid ionization constant for amines; ft = activity coefficient for singly charged species; f + += activity coefficient for singly charged activated complex. Acid catalysis.
mechanism can be considered a specific pathway for acid catalysis, but it does lead to different values of the rate constants. The values of the rate constants based on each plausible mechanism are summarized in Table 111. There is no definitive method for establishing or verifying either of these mechanisms as the actual pathway through kinetic data alone. The mechanism involving NH3Cl+is the one employed by Margerum et al. (7) and Snyder and Margerum (8)to explain the results of similar experiments conducted at Environ. Scl. Technol., Vol. 19, No. 9, 1985
811
Table 111. NH&l plus Organic Nitrogenous Compound Rate Constants for Direct Transfer
(I
compound
PKb
glycine serine alanine glycylglycine sarcosine methylamine dimethylamine morpholine glycine ethyl ester
4.23 4.79 4.13 5.75 3.80 3.40 3.25 5.61 6.36
Anion of amino acids; neutral molecule of amines.
mechanism 1 (basic moleculen + NH3Clt) kb x lo+ f x 10-8c 9.36 4.18 6.94 1.10 7.40 7.62 7.98 2.30 0.297
Mean; k in (mol/L)-' s-l.
Ql 65-
T+'
4-
,,h
i E =: 3n v
Y
2-
k(obs), Synder & Margerum* T:25 OC
,oo
0.78 0.29 0.50 0.10 0.27 0.94 1.05 0.53 0.033
Data are presented in Table I The solid line is k(obs) calculated from the estimate of the reaction rate constant (k) presented in Table111
mechanism 2 (basic moleculeD+ NH&l + H') (acid catalysis) kb X A x 10-9c 9.36 4.15 6.94 1.10 7.40 7.62 7.98 2.30 0.297
0.78 0.29 0.50 0.10 0.27 0.94 1.05 0.53 0.033
195% confidence limits.
of the Debye-Huckel equation (expression 3). Values of rate constants for reactions between NHzCl and glycine, glycylglycine, and &alanine were also reported by Margerum et al. (7). More recently, Snyder and Margerum (8) reported a revised value for glycylglycine as well as ones for methylamine, L-threonine, and glycylglycine ethyl ester. For the three compounds common to both studies (methylamine, glycine, and glycylglycine),the values for the fundamental rate constants reported here are about 3 times higher than those reported by Snyder and Margerum. However, the average observed rate constants reported by Snyder and Margerum are within the variation of the values reported earlier by Isaac and Morris (1) for comparable values of pH as depicted in Figure 2. Snyder and Margerum collected their data at I = 0.5 mol/L while I = 0.0259-0.143 mol/L for this study. These latter ionic strengths are more nearly representative at least for domestic wastewater and fresh water. Given that the observed second-order rate constants agree very well, most of the difference appears to be from data analysis as opposed to the experimental data. In fact, uncertainty in the thermodynamic value of K, could account for the entire difference. Weil and Morris' estimated value of K , was intuitive, and therefore this estimate should be considered only as an order of magnitude approximation. Because Huffman measured a value of 0.0357 mol/L at I = 1.0 mol/L and extrapolated this to a value of 0.1 mol/L at I = 0, it makes sense to assess this range of values for K , on the calculated rate constants. As can be seen from the equation for pH value at which experiments were conducted, calculating the rate constants in accordance with mechanism 1 (Table 11),KJ* >> H+. Therefore, k varies directly as K,, Using K , = 0.1 mol/L will produce values of the rate constant k, which is approximately 3 times greater than if K, = 0.0357 mol/L. Thus, the value chosen for K , could account for much, if not all, of the difference between the absolute values of k presented for mechanism 1 in this report and those reported by Margerum and Snyder for the same mechanism.
Discussion Since the actual mechanism cannot be deduced from the kinetic data, one is free to choose among those that are in accord with the data. Also, since it is easier to discuss the data in the context of a particular mechanism, one has been selected for discussion purposes: the acid- (Le., H+) catalyzed reaction. Schematic representation of this mechanism is provided in Figure 3. With the selection of a plausible mechanism, estimation of the rate constants can be made. These estimates in turn can be used to calculate what k(obsd) should be for each reaction at each pH, and these calculations appear as the
Table IV. Differences in Rate Constant Estimates: Constants Derived under Pseudo-First-Order Conditions vs. Those Derived under Second-Order Conditions a t 26 OC (Estimate of Rate Constant) pseudo-first-order runs
compound glycine
second-order runs
Gl-Gl8 G31-G39 G43-G62 G19-G24 G40-G42
serine
SE1-SE12 SE16-SE21 SE13-SE15
glycylglycine
GG1-GG15 GG19-GG27 GG16-GG18
ji x 10-9, (mol/L)-l s-l
SD,"
8.35
1.84
13.31
3.52
3.92
0.47
4.37
0.47
1.10
0.28
1.10
0.03
tb
df"
sigd
4.12e
4"
Yes
1.43
19
no
0
25
no
aStandard deviation of sample. bTest statistic t. CDegreesof freedom. dSignificance at 95% confidence. 'Modified t test was employed to account for unequal variances (12). This modification results in a smaller value of t and many fewer degrees of freedom than would be the case otherwise. I
II
111
H
H
H
H
H
H
H
H
H
H
H
H
- OOCCH (R):N:+CI:N:-
- OOCCH(R):N:Cl:N:+Hh-OOCCH (R):N:CI:NH' H H !! H - OOCCH (R):N:Ct:N:H'+ - OOCCH (R) *:N:CI+:N:H H
IV
H H -0OCCH (R):N:Cl:N: H H
H
H
H
H' -OOCCH(R):N:Cl+-OOCCH(R):N:CI+H' H H
Figure 3. Schematic of acid catalysis mechanism.
solid line in Figure 2. It should be noted that while the absolute values of the rate constants do vary with mechanism, the calculated value of k(obsd) will remain the same. In addition, the entire data set can now be used to
assess the influence of various factors on estimates of the rate constants. Since the largest differences in the fundamental rate constant, k, occurred between those based on pseudo-first-order conditions and those based on equimolar concentrations of reactants, the estimates of the fundamental rate constant for the two different sets of conditions were compared through t tests. The three sets of reactions that involved both pseudo-first-order and equimolar conditions were glycine plus NH,Cl, serine plus NHzC1, and glycylglycine plus NHzC1. In the cases of serine and glycylglycine,the differences in estimates of the rate constant were not significant at the 95% confidence level when a two-sided t test was used. The difference was significant, however, in the case of glycine as shown in Table IV. A modification of the standard t test (12)was employed to account for the unequal variances of the rate constant estimates for the glycine-NH,CI reactions under equimolar vs. pseudo-first-order conditions. The contribution of NHzCl hydrolysis to all of these reactions is
10.5
Based on Acid Catalysis involving anion of amino acids and neutral molecule of amines.
of amino acids and
Envlron. Scl. Technol., Vol. 19, No. 9, 1985
813
minimal, and no other explanation has been thought of that would account for the difference. While this difference requires substantiation, the practical value of the rate constant remains unimpaired if all of the data are used to calculate an average value, and this is what has been done. A similar comparison of estimates of the rate constant based on pseudo-first-order glycine experiments buffered with KH2P0,-NaOH solutions (pH 6-8) and those buffered with borax-NaOH solutions (pH 8-9) was made. The analysis resulted in a t value of 0.718 with 28 degrees of freedom which fails to disprove that the mean values are the same at the 95% confidence level. In examining these data, it is interesting to note that the absolute rates based on C1+ acid-catalyzed transfer from NH2Cl to amino acids or amines are about 100-150 times faster than those reported for the same compounds with HOCl as the chlorinating agent. Furthermore, the rates of transfer can be related to the basicity of the acceptor compound, as not.ed by Friend (13) and more recently by Gray (4)for the HOCI-nitrogen compound system and also by Wajon (14) for the HOBr-nitrogen compound system. The relationship is qualitative with a high degree of scatter but, nonetheless, is evident as depicted in Figure 4. For the eight compounds (glycine, serine, alanine, glycylglycine, sarcosine, dimethylamine, morpholine, and glycine ethyl ester) common to both Friend’s study and this investigation, the correlation coefficient and slope between log k and -log Kb for the HOCl reactions are -0.919 and -0.45, respectively. These values are -0.916 and -0.45 for the NHzCl reactions, with the same compounds. For all nine compounds (i.e,, including methylamine) for which reactions with NH2C1were investigated, the correlation coefficient between log k and p&, is -0.911 with a slope of -0.42. An interesting observation can be made from a comparison of the rates of the acid-catalyzed reactions of NH2Cl with glycine and with glycine ethyl ester. The difference between these compounds is the presence of a negative charge on the COO- group in glycine and an undissociable CzH, group on the carboxyl in glycine ethyl ester. The rate of the acid-catalyzed reaction of NH2C1 with the anion of glycine is about 20 times as fast as the reaction with the neutral molecule of glycine ethyl ester. Friend (13) found the ratio for HOCl reactions for these two compounds to be about 13:l at 25 OC (7.4 X lo9 and 0.59 X lo9 (mol/L)-l m i d for the glycine and glycine ethyl ester reactions, respectively). This means that the presence of a negative charge at least on the smaller molecules has a substantial impact on the rate of reaction. This presumably is due to an increase in nucleophilicity. It also means that NH2Cl is likely to react with the neutral form of the anion formers, but at a slower rate. This slower rate combined with the fact that the anion concentration is much greater than that of the neutral molecule at pH >4 means that nearly all of the reaction results from the acid-catalyzed interaction of NH2Cl and NH,CH(R)COOaccording to this mechanism. A further confirmation might be obtained by measurement of comparative rates at pH
