Hydrolysis of Haloacetonitriles: Linear Free Energy Relationship, Kinetics and Products

Home , Trichloroacetic acid, Trichloroacetonitrile

HYDROLYSIS OF HALOACETONITRILES: LINEAR FREE ENERGY RELATIONSHIP, KINETICS AND PRODUCTS

VICTOR GLEZER*, BATSHEVA HARRIS, NELLY TAL, BERTA IOSEFZON and OVADIA LEV{*M Division of Environmental Sciences, Fredy and Nadine Herrmann School of Applied Science, The Hebrew University of Jerusalem, 91904, Jerusalem, Israel

(First received September 1997; accepted in revised form August 1998)

AbstractÐThe hydrolysis rates of mono-, di- and trihaloacetonitriles were studied in aqueous buer solutions at dierent pH. The stability of haloacetonitriles decreases and the hydrolysis rate increases with increasing pH and number of halogen atoms in the molecule: The monochloroacetonitriles are the most stable and are also less aected by pH-changes, while the trihaloacetonitriles are the least stable and most sensitive to pH changes. The stability of haloacetonitriles also increases by substitution of chlorine atoms with bromine atoms. The hydrolysis rates in dierent buer solutions follow ®rst order kinetics with a minimum hydrolysis rate at intermediate pH. Thus, haloacetonitriles have to be preserved in weakly acid solutions between sampling and analysis. The corresponding haloacetamides are formed during hydrolysis and in basic solutions they can hydrolyze further to give haloacetic acids. Linear free energy relationship can be used for prediction of degradation of haloacetonitriles during hydrolysis in water solutions. # 1999 Elsevier Science Ltd. All rights reserved

Key wordsÐhaloacetonitriles, hydrolysis, LFER, chlorination products

INTRODUCTION included 4 of the HANs, namely, trichloroacetoni- Water chlorination yields halogenated byproducts trile, dichloroacetonitrile, bromochloroacetonitrile due to the reaction of halogens with naturally and dibromoacetonitrile in the periodic monitoring occurring organics, such as humic acids (Bellar and requirements of the proposed United States' Lichtenberg, 1974; Rook, 1974; Johnson and Information Collection Rule (U.S. EPA, 1994). Randtke, 1983; Oliver, 1983; Ami et al., 1990). The HANs are less frequently studied compared to three most dominant families of chlorination by- THMs and HAAs and the occurrence of trihaloace- products, in order of abundance, are the trihalo- tonitriles in water is rarely reported. The most methanes (THMs), halogenated acetic acids (HAAs) abundant HANs after water chlorination are and haloacetonitriles (HANs) (Bellar et al., 1974; dichloroacetonitrile and its brominated analogs, U.S. EPA, 1979; Oliver and Shindler, 1980; Boyce bromochloroacetonitrile and dibromoacetonitrile and Hornig, 1983; Urano et al., 1983; Coleman et (Oliver and Shindler, 1980; Coleman et al., 1984; al., 1984; de Leer et al., 1985; Alouni and Seux, Reckhow and Singer, 1984; Trehy et al., 1986; 1987; Singer et al., 1995). The latter is a product of Reckhow et al., 1990; Peters et al., 1990a,b) and tri- the chlorination of aminoacides, proteins and other chloroacetonitrile (Coleman et al., 1984; Smith et nitrogen containing species. The toxicology of al., 1987; Koch et al., 1988; Singer et al., 1995). HANs is less documented as compared to the more Recently, Richardson et al. reported dibromochlor- abundant THMs and HAAs. For all HAN com- oacetonitrile in chlorine dioxide disinfected water, pounds there are no data appropriate for develop- though identi®cation was based on mass spectral ing acceptable limits for lifetime exposure to the match and was not con®rmed by identi®cation stan- chemicals though in some cases mutagenic (dibro- dards (Richardson et al., 1994, 1996). moacetonitrile and bromochloroacetonitrile against Several factors complicate the investigation of the Salmonella) and teratogenic action (trichloroaceto- HANs and particularly trisubstituted HANs in nitrile against Long±Evans rats) were reported (Bull chlorinated drinking water: (1) The HANs are inter- and Kop¯er, 1991). Nevertheless, the USEPA mediate compounds, susceptible to further conversion to their corresponding haloacetic acids. (2) Even in disinfectant-free water the HANs undergo *Present address: Public Health Laboratory of Ministry of further hydrolysis and the hydrolysis rates of most Health, Abu Kabir, Tel Aviv 61082, Israel. HANs are still not documented. (3) Some of the {Author to whom all correspondence should be addressed. [Tel: +972-2-6585558; Fax: +972-2-6586155; E-mail: HANs are still commercially unavailable, thus [email protected]]. quanti®cation and identi®cation require synthetic

1938 Hydrolysis of haloacetonitriles 1939 capabilities. In fact, the mass spectra of some Table 1. GC retention times of haloacetonitriles HANs, like bromochloroacetonitrile, bromodichlor- Compound Abbreviated Retention Relative oacetonitrile, dibromochloroacetonitrile and tribro- name time retention moacetonitrile are not reported in the MS libraries. (min) time This manuscript describes systematic studies of 1. ClCH2CN Cl1 4.47 0.23 the hydrolysis products and the rate of the hydroly- 2. Cl3CCN Cl3 4.88 0.26 sis of all 9 possible HANs as a function of pH. We 3. Cl2CHCN Cl2 5.79 0.30 demonstrate that linear free energy relationships 4. BrCH2CN Br1 8.05 0.42 5. BrCl2CCN BrCl2 10.69 0.56 can be used to predict the hydrolysis and oxidation 6. BrClCHCN BrCl 11.59 0.61 kinetics of the various HANs. 7. Br2CHCN Br2 17.81 0.94 8. Br2ClCCN Br2Cl 18.01 0.95 9. Br3CCN Br3 23.26 1.22 IS-CH2ClCHClCH2Cl 19.03 1.00 MATERIALS AND METHODS

Equipment and instrumentation trile exhibited shorter retention time than the bromochlor- Hewlett-Packard GC-MS system using GC 5890 and oacetonitrile. These changes can be explained by the 5971 Mass Selective Detector operated in EI (electron ion- dierences in the polarity of the haloacetonitriles. Using ization) mode equipped with Altech Heli¯ex AT-1 capil- these GC-conditions all 9 acetonitrile compounds could be resolved by a single run using extracting ion chromato- lary column (30 m long, 0.32 mm i.d., 0.25 mm ®lm thickness) was used. The mass detector temperature was grams. The 6 main MS-peaks and 3 MS-peaks, which were selected for EI-MSD-SIM detection, are presented in 2808C, the injection port was operated at 1808C, the GC Table 2. The standard deviation for analysis of the HANs temperature program was: initial temperature 358C, 9 min (for 20 ppm solutions) was <10% for consecutive analy- hold time; 28C/min ramp to 428C; a second ramp at 58C/ sis. The relative change of the retention time vs. the in- min to 1608C; third ramp at 308/min to 2208C and ®nally ternal standard (1,2,3-trichloropropane) was always less 4 min hold at 2208C. than 0.1 min. Chemicals Haloacetic acids were quantitated by a conventional standard methods procedure (APHA, 1995). The pro- Analytical reagents were used unless otherwise speci®ed. cedure includes sample acidi®cation till pH 1, extraction Commercial standards including chloroacetonitrile, bro- with MTBE and further methylation with diazomethane. moacetonitrile, dichloroacetonitrile, dibromoacetonitrile, Acidi®cation without methylation resulted in formation of trichloroacetonitrile and 2,2,2-trichloroacetoamide were the corresponding haloform by thermal decarboxylation in purchased from Aldrich. Bromochloroacetonitrile, bromo- the injector. This was con®rmed by injection of the dichloroacetonitrile, dibromochloroacetonitrile and tribro- extracts of model trichloro- and tribromoacetic acids moacetonitrile were synthesized by bromination of under the same chromatographic conditions, which gave chloroacetonitrile and dichloroacetonitriles according to chloroform and bromoform artifact peaks, respectively. reported procedures (Hechenbleikner, 1946). Bromochloroacetonitrile and bromodichloroacetonitrile were isolated as individual compounds, puri®ed by distilla- Application of linear free energy relationship (LFER) for tion under reduced pressure and used as standards. In the hydrolysis of HANs case of dibromochloroacetonitrile and tribromoacetonitrile Quantitative description of the in¯uence of substituents rich fractions of these compounds were obtained by frac- in organic molecules on their reactivity was for ®rst tional distillation of the bromochloroacetonitrile synthesis demonstrated by Hammett for the dissociation of substi- product. Pure compounds were not obtained and these tuted benzoic acids in 1937. Later, this approach was suc- compounds could only be used as reference materials but cessfully developed for dierent classes of organic not for accurate quantitation. Quantitation of these com- compounds and now it is well known as linear free energy pounds is reported in this article relative to trichloroaceto- relationship (LFER) (Taft, 1956; Lowry and Schueller nitrile MSD response. Richardson, 1987; Hansch et al., 1991). LFER allows one to explain the in¯uence of molecular structure on the ther- Chromatographic analysis modynamic and kinetic parameters of chemical reactions, Acetonitrile (Fluka) was used as solvent for the prep- to interpret IR-, UV-, NMR-spectra and also to predict aration of stock solutions instead of acetone that is tra- the structure-activity relationships in medical chemistry ditionally the recommended solvent for HANs studies and electrochemistry. The LFER approach penetrates (U.S. EPA, 1988). Quenching of free chlorine with NH4Cl slowly also into water and environmental chemistry dechlorinator forms chloramine. The latter interacts with (Schwarzenbach et al., 1993). LFER is manifested in the acetone, forming 2-chloroiminopropane, which yields an Hammett equation for aromatic compounds and in the additional chromatographic peak. Subsequently, phos- phate buer solutions Ð pH 5.4, 7.2, 8.7 and 0.1 N HCl Table 2. Main MS Ð peaks and relative abundances of haloace- Ð were used for preparation of the HANs water sol- tonitriles utions. HANs ®nal concentration ranged between 20± 50 ppm. Solutions were stored in a thermostat at 208C and 1. ClCH2CN 77(23), 75(100), 50(22), 48(67), 47(10), 40(25) analyzed after 1, 3, 24, 48 and 96 h. Methyl-tert-buthyl 2. BrCH2CN 121(97), 119(100), 94(11), 81(50), 79(24), 40(75) ether (MTBE) (Sigma, HPLC grade) was used for liquid 3. Cl2CHCN 84(44), 82(65), 76(33), 74(100), 48(9), 47(25) extraction, 50 ml of aqueous phase was extracted with 4. BrClCHCN 155(27), 153(21), 81(12), 79(12), 76(33), 74(100) 3 ml of organic phase in the presence of 10 g of NaCl 5. Br2CHCN 201(23), 199(46), 197(26), 120(100), 118(100), 81(22) 6. Cl3CCN 112(11), 110(68), 108(100), 82(11), 73(14), 47(15) (Frutarom, Israel). Extracts were dried over sodium sul- 7. BrCl CCN 154(26), 152(21), 112(11), 110(67), 108(100), 73(17). fate. Retention times of all 9 haloacetonitriles, under the 2 8. Br2ClCCN 198(10), 156(22), 154(100), 152(76), 81(10), 79(12). above-mentioned conditions, are presented in Table 1. The 9. Br3CCN 200(48), 198(100), 196(52), 119(11), 117(12), 79(12). retention times followed the molecular weight order with two exceptions: trichloroacetonitrile had shorter retention Italic values were used for SIM analysis. Values in brackets rep- time than dichloroacetonitrile and bromodichloroacetoni- resent relative abundance. 1940 Victor Glezer et al.

Taft equation for aliphatic compounds. The Taft equation Table 3. Taft's polar and steric constants and the corresponding divides the substituent eects to polar and steric com- rate constant for hydrolysis of haloacetonitriles at pH 8.7 ponents. In this work Taft equation is used to describe the hydrolysis rate of HANs according to equation 1: Compound s* ES k log k

a a 6 R±CN H2O 4R±CONH2, 1 ClCH2CN 1.05 0.24 1.2 10À 5.92 À a À a Â 6 À BrCH2CN 1.00 0.27 2.7 10À 5.57 where R = X X X C and X , X , X are chlorine, bromine b À a Â 6 À 1 2 3 1 2 3 Br2CHCN 1.86 1.86 4.0 10À 5.40 b À b Â 6 À or hydrogen atoms. BrClCHCN 1.90 1.70 5.3 10À 5.27 a À a Â 6 À The Taft's polar (s*) and steric (Es) substituent con- Cl2CHCN 1.94 1.54 5.6 10À 5.25 b À a Â 5 À stants can be used to predict the rate constants of hydroly- Br3CCN 2.55 2.43 4.5 10À 4.35 b À b Â 5 À sis reaction (equation 1) according to equation 2: Br2ClCCN 2.59 2.31 8.0 10À 4.10 b À b Â 4 À BrCl2CCN 2.62 2.19 2.2 10À 3.66 a À a Â 4 À log kR=k0 rs* dES, 2 Cl3CCN 2.65 2.06 3.9 10À 3.41 À Â À where k0 is the hydrolysis rate constant for unsubstituted aFrom /29/.bCalculated as described in text. compound (in this case acetonitrile); kR the hydrolysis rate constant for compound with substituent R; s* and ES are, Taft's polar and steric constants for all haloacetonitriles respectively, Taft polar and steric constants; and r and d are presented in Table 3. are empirical parameters representing the sensitivity of the hydrolysis rate to the polar and steric factors. The Taft polar constants (s*) for ClCH2, Cl2CH, Cl3C and BrCH2 substituents have been reported by Taft (1956) RESULTS AND DISCUSSION and Hansch et al. (1991). These constants for mixed bro- mochlorosubstituents and di- and tri-bromosubstituents Conversion of haloacetonitriles: structure and pH- were not available, but they could be evaluated based on dependence correlations describing the relationship between the inductive component of the Hammett constant, sI, and Taft The hydrolysis of all 9 HANs was studied at pH polar constant, s*. 8.7, 7.2, 5.4 and in 0.1 N HCl. The stability of Thus, the Taft polar constant for CH2X substituent can be calculated by equation 3 (Lowry and Schueller HANs depends on their chemical structure and on Richardson, 1987; Hansch et al., 1991). pH and can be divided into two groups, one for neutral and basic conditions and the second for =0:45, 3 sÃCH2X sI X acidic conditions. Examples of HANs disappear- where sI(X) is the inductive component of the Hammett ance at pH 8.7 and pH 5.4 are presented on constant and 0.45 is an empirical constant. Fig. 1(a) and (b). The HAN's are most stable in sI(X) of bromine and chlorine atoms were reported to be weak acidic media Ð less than 50% were degraded 0.47 and 0.45, respectively. Thus, using this approach =1.0 and =1.05 which coincide with the after 4 d at pH 5.4, while at pH 8.7 (the pH of the sÃCH2Br sÃCH2Cl published value of s * (Lowry and Schueller Richardson, Israel National Water Carrier) some of the com- 1987; Schwarzenbach et al., 1993). Extrapolation of this pounds, e.g. trihaloacetonitriles could not be approach can be used to extend equation 3 to multisubsti- detected after 24 h. Faster hydrolysis of dichloroa- tuent groups. cetonitrile in basic media in comparison to acidic A =0:45, 4 sÃCHX1X2 sI X1 sI X2 media was observed also by Oliver (1983) and now where A is a correction factor for the disubstituted com- it is clear that this conclusion can be extended for pound. The constant A can be evaluated based on the all tri- and di-haloacetonitriles. The trihalo-substi- known value of Taft's s*-constant for CHCl2 substituent tuted compounds were the least stable and the most reported to be 1.94 (Taft, 1956). So, A can be calculated sensitive to pH changes. Under basic conditions from the expression A=sÃ /(2sI(Cl)/0.45). Thus, CH2Cl decrease of trihaloacetonitriles can be observed A = 1.94/(2*0.47/0.45) = 0.93. Similarly, the constant for trisubstituted radical can be represented by, already after 1 h, while in acid media there was no signi®cant change in trihaloacetonitriles concen- B =0:45: 5 sÃCX1X2X3 sI X1 sI X2 sI X3 tration even after 24 h. Figure 2 compares the rela- The correction coecient, B, can be evaluated by the ratio tive stability of the HANs at the various pH levels. of Taft's *-experimental value, reported for to be s sÃCCl3 The percent of each of the HANs that remain after 2.65 and the constant's value calculated based on the ex- 96 h hydrolysis is described as a function of pH. pression for 3 chlorine atoms: sÃ =3sI(Cl)/0.45. This CCl3 For most compounds optimal stability of the triha- approach yields B = 2.65/(0.47*3/0.45) = 0.85. The values of the correction coecients A and B show that the contri- loacetonitriles is obtained at pH 5.4. Much faster butions of the halogen substituents are almost additive. hydrolysis occurs at higher pH with exception for The steric constants, Es, for ClCH2, BrCH2 , Cl2CH, chloroacetonitrile which remains in high concen- Br2CH, Cl3C and Br3C substituents were reported (Taft, tration also after 96 h. Comparison of hydrolysis 1956; Hansch et al., 1991). E values for other substituents s rates at pH 5.4 and pH 1 shows that the dierences were not reported, but they can be calculated considering that the constants for di- and tri-halosubstituents are com- in degradation rate vs. pH are less pronounced in prised of the sum of the speci®c contributions of the corre- acidic media as compared to basic solutions. sponding halogen substituents. The contribution of one Generally the stability of HANs is on the same chlorine substituent equals 1.54:2 = 0.77 for dihalo- level or even slightly improved at pH 5.4, except for compounds and 2.06:3 = À0.69 for trihalo-compounds.À The contributionÀ of each bromineÀ radical can be calcu- trichloroacetonitrile which was more stable in 0.1 N lated in a similar way to give 0.93 and 0.81 for the HCl. Mono- and di-substituted HANs are very double and triple halosubstitutedÀ compounds,À respectively. stable in acid media and only after 48 h some Hydrolysis of haloacetonitriles 1941

Fig. 1. Hydrolysis of haloacetonitriles at pH 8.7 (a) and at pH 5.4 (b).

decrease in their concentrations could be increases for the series Cl3CCN±BrCl2CCN± observed, while the stability of the trisubstituted Br2ClCCN±Br3CCN. compounds was much inferior. The nature of the halogen atom in substituent in¯uences also the Hydrolytic pathways of trihaloacetonitriles HANs hydrolysis rate. Subsequent substitution of Under all pH conditions, the decrease in concen- chlorine with bromine atoms in trichloroacetoni- tration of trihaloacetonitriles is accompanied by trile increases HANs stability. The in¯uence appearance of new chromatographic peaks. These 1942 Victor Glezer et al.

Fig. 2. The in¯uence of pH on stability of haloacetonitriles. Remaining percentage after 96 h. were identi®ed as the corresponding trihaloaceta- (2.1%)/M+/; 205 (1.4%), 207 (2.8%), 209 (1.8%)/ + mides. The commercial standard of trichloroaceta- CBr2Cl/ ; 170 (2.0%), 172 (3.5%), 174 (1.0%)/ + mide was available and the compound was CBr2/ ; 154 (1.1%), 156 (1.4%), 158 (0.2%)/ identi®ed both by matching chromatographic reten- CBrClCO/+; 142 (2.5%), 144 (3.5%), 146 (0.7%)/ + tion time and by spectral matching. For other triha- CBrClNH2/ ; 126 (5.0%), 128 (6.7%), 130 (1.0%)/ loacetamides the library mass-spectra were not CBrCl/+; 91(2.8%), 93 (2.8%)/CBr/+; 79 (3.5%), available and they were identi®ed based on their 81 (3.9%)/Br/+; 47 (3.5%), 49 (1.1%)/CCl/+; 44 + MS-spectra fragmentation. In the case of dibromo- (100%)/CONH2/ . Small relative abundances of all chloroacetamide the following peaks correspond to mass-spectra peaks in comparison with the base + the EI fragments, their relevant chemical structure peak, 44 corresponding to the ion /CONH2/ was is given in brackets: 249 (0.7%), 251 (2.1%), 253 typical also for trichloroacetamide.

Fig. 3. Products formation by the hydrolysis of trichloroacetonitrile at pH 8.7: (1) trichloroacetonitrile, (2) trichloroacetamide, (3) trichloroacetic acid (detected in ester form) and (4) sum of 1 + 2 + 3. Hydrolysis of haloacetonitriles 1943

Hydrolysis of the HANs under basic conditions Table 4. Retention time of the hydrolysis products of trihaloaceto- yielded also the corresponding haloacetic acids as nitriles demonstrated for trichloroacetonitrile (Fig. 3). The Compound Retention time (min) haloacetic acids were determined as the correspond- 1. Cl3CCOOCH3 19.71 ing methylester by MTBE extraction in acidic sol- 2. BrCl2CCOOCH3 23.70 3. Br ClCCOOCH 27.21 ution and subsequent methylation with 2 3 4. Cl3CCONH2 29.10 diazomethane. The chromatographic retention times 5. BrCl2CCONH2 32.29 for all trihaloacetonitriles hydrolysis products are 6. Br2ClCCONH2 35.02 7. Br CCONH 37.10 presented in Table 4. In basic solution, the molar 3 2 sum of haloacetonitrile + haloacetamides + haloacetic acids was time invariant (Fig. 3). In acid and Table 3 also depicts the values of the hydrolysis neutral solutions the sum of haloacetamide and rate constants of the HANs at pH 8.7. The hydroly- haloacetonitrile was conserved demonstrating that sis rate constants depend on the number of halogen these are indeed the only, or at least the dominant atoms in the HAN molecule. The trihaloacetoni- hydrolysis products. triles are less stable than mono- and di-substituted compounds. Thus, the expected quantitative depen- Taft equation for haloacetonitriles hydrolysis dence between the chemical structure and the reac- The hydrolysis of the trihaloacetonitriles obeyed tivity of the HAN molecules can be described by ®rst order kinetics as demonstrated for the hydroly- linear free energy relationship (LFER), demonstrat- sis of trichloroacetonitrile. The kinetic coecient ing the in¯uence of structural changes in the or- can be obtained from the linear ln(C) vs. t depen- ganic molecule on its chemical characteristics, using dence (Fig. 4). The C and 1/C vs. t dependencies the constants of Table 3. were not linear showing that zero order and second For pH 8.7 the LFER equation is best ®tted by order kinetics can be ruled out. The hydrolysis rate equation 6: constants of the trihaloacetonitile compounds at log kR 8:4420:26 2:9120:50 sÃ dierent pH-values are presented in Table 5. The À qualitative conclusions that were derived earlier 1:3620:40 ES: 6 based on Figs 1 and 2, that rate constants (k) increase with pH and depend on the nature of the The standard deviations of the coecients are given substituents can now be con®rmed based on the in the regular brackets in equation 6. The linear quantitative kinetic data of Table 5. For the triha- correlation coecient was r = 0.97 and the number loacetonitrile molecules, consecutive substitution of of degrees of freedom was n = 6. Graphic presenta- bromine atoms by chlorine increases the stability of tions of equation 6 are given in Fig. 5(a), (b) for ES the trihaloacetonitriles. and s* parameters, respectively. Comparison of

Fig. 4. ln(C) vs. time dependence for the hydrolysis of trichloroacetonitrile at pH 8.7 (correlation coecient, R = 0.99, degrees of freedom, n = 5). 1944 Victor Glezer et al.

1 Table 5. Hydrolysis rate constants (sÀ ) of trihaloacetonitriles constant s* increases the hydrolysis rate constants, while increasing the absolute value of the steric con- pH 5.4 pH 7.2 pH 8.7 stant, Es, decreases the hydrolysis rate constant. 6 4 4 Cl3CCN 2.0 10À 1.5 10À 3.9 10À Thus, the increase of amount of halogen atoms in a Â 6 Â 5 Â 4 BrCl2CCN 1.3 10À 1.2 10À 2.2 10À Â 6 Â 5 Â 4 HAN molecule increases log k due to larger rs* Br2ClCCN 1.0 10À 0.9 10À 0.8 10À Â 6 Â 5 Â 4 Br CCN 0.7 10À 1.6 10À 0.4 10À contribution which is more signi®cant than the 3 Â Â Â change in dEs. Inside a group of the same number of halogen atoms the substitution of chlorine by Taft's sensitivity constants r and d in equation 6 bromine atoms decreases the hydrolysis rate shows that the hydrolysis rate is more sensitive to because the polar constant, s* decreases and the the polar than to the steric constant. These con- absolute values of the steric constant, Es, increases. stants have opposite signs and in¯uence the log k- The hydrolysis rate constants also depend on the values in opposite directions. Increasing the polar pH of the media Ð lowering the pH diminishes the

Fig. 5. Projections of the linear free energy relationship for haloacetonitriles hydrolysis at pH 8.7: (a) Dependence of Taft steric constants ES, (b) Dependence of Taft polar constants s*. Hydrolysis of haloacetonitriles 1945 hydrolysis rates (Fig. 2) and decreases the value of presented by equation 10: the coecients in equation 2. This is demonstrated for pH 7.2 (equation 7) and pH 5.4 (equation 8):

log kR 8:6520:28 2:7320:53 sÃ À

1:3120:41 ES, 7 r 0:97, n 6 ,

log kR 8:4720:14 1:7020:27 sÃ À

0:8120:21 ES, 8 The hydrolysis of haloacetonitrile compounds r 0:97, n 6 : under basic conditions yields the corresponding acetamides, which in basic media can be further Comparison of equations 6)±(8) suggests that the hydrolyzed to the corresponding acid. During the log k values be twice more sensitive to substi- R extraction with MTBE the trihaloacetic acid anions tuent's polar constants as compared to the steric remain in the aqueous phase and can not be ident- constants. i®ed by GC-MS analysis, whereas acidi®cation Mechanism of trihaloacetonitrile hydrolysis yields the acidic form, which can be extracted to the organic phase and further methylated. The hy- The hydrolysis of trihaloacetonitriles can be drolysis of trihaloacetamides is a much slower pro- either acid or base catalyzed (Schaefer, 1970). The cess as compared to the hydrolysis of the starting mechanism of basic hydrolysis involves nucleophilic compound (see Fig. 3) and it is carried out only addition of OHÀ (Schaefer, 1970): under basic solutions. In acid solutions the hydrolysis process is terminated by formation acetamides RCN OH À which are stable under acidic conditions. Based on

H2O these observations it can be concluded that preser- 4RC OH .N À 4OH RC OH .NH À À À À vation of HAN samples between sampling and analysis, can be best accomplished by adjusting the 4RCONH2, 9 À pH to weakly. where R = CH3 nXn, n = 1,2,3 and X = Cl, Br or À Chlorination of haloacetonitriles their combination. The main products of this process are the aceta- The mechanism of HANs chlorination was mides and further basic hydrolysis of these forms suggested by Peters et al. (1990b), equation 11. It the corresponding carboxylic acids. The acid-cata- may followed either direct chlorination by HOCl, lyzed process starts with protonation of the nitro- forming the corresponding N-chloroamides (route gen atom in the nitrile group (Schaefer, 1970) and A) or an indirect route through hypochlorite cata- is slower than in basic media. The dierences lyzed hydrolysis of the cyano group producing an between pH 1 and pH 5 can be explained by amide that reacts with HOCl to give the corre- increase of hydrolysis rate in more acidic media. sponding N-chloramide (route B). The proposed The general scheme of haloacetonitriles hydrolysis mechanisms of HAN chlorination are analogous to followed by determination of the corresponding HAN hydrolysis, which allows use of the LFER esters after methylation by diazomethane can be approach also for this case. 1946 Victor Glezer et al.

Fig. 6. Disappearance of haloacetonitriles in the presence of 30 ppm chlorine at pH 7.2.

Degradation of the HANs by chlorination is much reaction as well. The stability increases with increas- faster compared to their hydrolysis. For example, at ing number of bromine atoms in the molecule. pH 7.2 only 2 of the 4 starting trihaloacetonitriles The successful application of LFER in describing remained in solution after 1 h (Fig. 6) and after the hydrolysis of the HANs stimulated the appli- 24 h only the monohaloacetonitriles remained unde- cation of the LFER and the Taft's parameters to graded. The stability ranking observed for the hy- describe the chlorination of the HANs as well. This drolysis step was preserved for the chlorination is of course relevant as an end in itself and as a

Fig. 7. Stability of haloacetonitriles at pH 7.2 in the presence of chloramine (see text). Hydrolysis of haloacetonitriles 1947 further supports for the linear approach taken to can be adequately described by LFER approach. derive the missing Taft's constants, though unlike The stability of haloacetonitriles for both hydrolysis hydrolysis chlorination is a second order process: and chlorination reactions decreased for larger number of halogen substituents and increased upon log k 1:6920:16 1:0520:47 sÃ À replacing chlorine with bromine. Under acid conditions the hydrolysis of haloacetonitriles gave 0:4820:33 ES, 12 haloacetoamides and in basic media further hy- r 0:93 : drolysis to the corresponding trihaloacetic acid was observed. Equation 12 describes the best ®t second order reac- A practical conclusion that arise from the studies tion kinetic coecient of chlorination at pH 7.2. is that HAN samples are best preserved (between The comparison of equation 12 and equation 7, sampling and analysis) under weak acid conditions describing the kinetic coecients of hydrolysis due to their superior stability under such con- under the same pH, reveal that the independent ditions. Weak acid conditions are also optimal for constant is much larger for chlorination as com- preservation of chlorinated samples after am- pared to hydrolysis while the steric and polar monium chloride quenching of the oxidant. dependencies are somewhat less signi®cant for chlorination as compared to the hydrolysis. Thus, AcknowledgementsÐThis research was supported by the the overall stability of unsubstituted acetonitrile is 6 Avicennia initiative of the European Community; German orders of magnitude larger for hydrolysis as com- Federal Ministry of Education, Science, Research and Technology (BMFT) and the Israeli Ministry of Science pared to its stability in the presence of the speci®ed (MOS) and U.S. EPA. chlorination conditions. Interestingly, the polar constant was found to be twice larger than the steric constants for all the hydrolysis and chlorination test REFERENCES cases. This again point on the mechanistic similarity Alouni Z. and Seux R. (1987) Cinetiques et mechanismes of hydrolysis and chlorination. de l'action oxydative de l'hypochlorite sur les acides a- amines lors de la desinfection des eaux. (Kinetics and Termination of the chlorination reaction by chlorami- mechanisms of hypochlorite oxidation of a-amino acids nation at the time of water chlorination). 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