1704
AMI. Chem. 1001, 63,1794-1001
(21) Hmng, E. D.; Wachs, T.; Conboy, J. J.; Henbn, J. D. Anal. Chem. 1990, 62 (13), 713A-725A. (22) Duaakr, E. N.; Paul, I. C. I n p0l)lelher AntbMcs: Namw Occcxrtng AcM ImwhOres; Westley, J. W., Ed.; Marcel Dekker: New York, 1982; VOI. 11, pp 189-191. (23) Duesler. E. N.; Paul, I. C. In polLelher Antwotks: Occumng AcM Ionophores; Westley, J. W., Ed.; Marcel Dekker: New York, 1982; Vol. 11, p 87. (24) Schmldt, P. 0.;Wang, A. H. J.; Paul, I. C. J . Am. Chem. Soc. 1974, 96, 6189-6191.
(25) Patel, D. J.; Shen, C. Roc. Net/. Aced. Scl. U.S.A. 1976, 73 (6), 1786-1790. (26) Slesel, M. M.; Colthup. N. B. Appl. Spectrosc. 1987, 41 (7), 1227-123 1. (27) Thomson, B. A.; Irlbarne, J. V. J . Chem. Phys. 1979, 71, 445 1-4463.
RECEIVEDfor review January 18,1991. Accepted May 17,1991.
Direct Determination of Chlorination Products of Organic Amines Using Membrane Introduction Mass Spectrometry T.Kotiaho,’ M. J. Hayward,”and R. G. Cooks* Department of Chemistry, Purdue University, West Lafayette, Indiana 47907
Dlrect analyds of organlc chloramines In low concentratlons In aqueous solutlons Is achleved by membrane lntroductlon mass spectrometry. Tandem mass spectrometry allows structure8 of the chlorlnatbn products to be determined and shows that nltrogen chlorlnatlon occurs for allphatlc amlnes and rlng chlorkratlon tor anlllne. Momchlorlnatlon of anlllne occurs mdnly at the ortho posltkn, while d k h k h t h ylekls 20-35 % dlortho and 80-65 % ortho-para wbstltutlon products. Depending on the reactkn condltlons, the actual chlorlnatlon reagent can be chloramlne, hypochlorous acid, and POsdMy c h b r h . The high ortho chlorination ylekls of anlllne are explained by a mechanlsm In whlch chlorlnatlon occurs flrst at the nltrogen atom wlth subsequent Intramolecular rearrangement to the ortho pcmltbn In analogy to rearrangement of the nltro group In N-nltro aromatic amines. Uslng flow Injection analysis procedures, lt Is posdble to follow In an on-line fashlon reactions that yleld (or remove) organlc chloramlnes at sub parts per mllllon levels.
INTRODUCTION Chlorine is the most widely used agent for disinfection of potable water and wastewater. During the chlorination of water that contains ammonia or organic amines, inorganic and organic chloramines are formed (14). Inorganic chloramines themselves are also used as disinfection agents (5, 6). The most important reason to develop specific analytical methods to analyze organic chloramines is the possible toxicity and mutagenicity of these compounds (5,7,8). A secondary reason is that the formation of organic chloramines can cause overestimation of the disinfection level if conventional colorimetric methods are used to analyze free available chlorine ( 5 9 , 10) because these methods cannot distinguish between inorganic and organic chloramines. Potential precursors of organic chloramines found in wastewater include aniline (II), isobutylamine, 2-methylbutylamine,isoamylamine, pyrrolidine, and piperidine (12).The concentrations of organic amines in wastewater vary greatly as exemplified by the fact that aniline was present at levels of 650-2100 ppm in certain chemical plant wastewaters (11),whereas concentrations of ‘On leave from the Technical Research Center of Finland, Chemical Laboratory, Biologinkuja 7, 02150 Ea 00, Finland. Wurrent address: AmeriFan Cyanamid t o . , Agricultural Reaearch Division, P.O. Box 400, Princeton, NJ 08543-0400. 0003-2700/91/0363-1794$02.50/0
other amines varied between 4 and 26 ppb in municipal wastewaters (12). The analytical methods used to a n a l p organic chloramines include gas chromatographic analysis of nitrogen-halogenated methylamines using a Hall electrolytic conductivity detector and mass spectrometric detection after extraction with ether (13).Liquid chromatography has also been used to analyze organic chloramines as dansyl derivatives (14) or sulfenamide derivatives (15) by using UV or electrochemical detection, respectively. Recently, Scully and Nweke have emphasized the importance of the analysis of chloramines without prior derivatization (16).They used direct high-performanceliquid chromatography with UV or scintillation detection to study the chlorination products of the amino acid isoleucine. Although some of these methods give low detection limits for particular compounds (for example, about 10 ppb for N chloropiperidine (14)), they are time consuming, have relatively low selectivity, and, due to the labile nature of organic chloramines during concentration or isolation, can give unreliable results. Membrane introduction mass spectrometry (MIMS) addresses these disadvantages and is of particular interest since it has already been proven to be of value in the analysis of inorganic chloramines (17,18). Membrane introduction maas spectrometry is an analytical method in which a membrane is used as an interface between an aqueous phase and the mass spectrometer vacuum. The analyte is transferred from the aqueous phase to the vapor phase by pervaporation, which occurs at widely varying rates for different analytes and can therefore be used for analyte enrichment (19). This method was f i t used in 1963 by Hoch and Kok for analysis of gaseous products of photosynthesis (20). Since that time, MIMS has undergone several stages of development and is now being increasingly applied to trace analysis of organic compounds in aqueous solutions (21-25), environmental analysis (26-281,and on-line analysis of biochemical reactors (29,30). The introduction of the flowthrough mode of sampling (31,32)was rapidly followed by direct insertion membrane probes based on this principle (32, 33). In the flow-through method, the sample flows through or across the membrane surface, which is located in the ion source of the mass spectrometer. This arrangement minimizes memory effects and response times and gives very low detection limits and high specificity, especially when used together with tandem mass spectrometry. In this study, on-line membrane introduction mass spectrometry was employed to monitor the formation of organic chloramines and tandem mass spectrometry was used to 0 1991 American Chemical Soclety
ANALYTICAL CHEMISTRY, VOL. 63,NO. 17, SEPTEMBER 1, 1991
1795
Table I. Maximum Number of Hydrogens Substituted by Chlorine and Substitution Position
Flgurr 1. Schematic presentation of the membrane introductionapparatus used in thls study.
compound
max. no. of chlorine5
sub position
2-aminobutane 1,3-diaminopropane aniline
2 4
nitrogen nitrogen ring
A
3
2-aminobutane 0.2 ppm 100.0
confirm the structures of the reaction products. The measurement of organic chloramines represents a novel application of MIMS, and the direct analysis of organic chloramines from aqueous samples is demonstrated for the first time. The detection limits of MIMS for organic chloramine analysis were also determined.
EXPERIMENTAL SECTION The membrane introduction apparatus used is shown schematically in Figure 1. The membrane was mounted in a direct insertion membrane probe, the analyticalcharacteristicsof which have been previously reported (33). The membrane used was a dimethylvinylsiliconepolymer (ASTM. VMQ Dow Coming) with a thickness of 0.25 mm (0.01 in.). The temperature of the membrane probe (70 "C) was controlled independently of that of the ion source (190 "C). A sample solution was continuously supplied to the membrane interface via an Ismatec multichannel peristaltic pump (Model 7618-30) at a flow rate of 1 mL/min. The mass spectrometer employed in these experiments was a Finnigan triple-quadrupole mass spectrometer (TSQ 4500) equipped with an INCOS data system. The mass spectrometer was operated under 70 eV of electron ionization (EI) or isobutane chemical ionization (CI) conditions (ion source pressure 0.4 Torr). Data were collected continuously either by measuring full mass spectra or by using the selected ion-monitoring method. The latter method was used to establish detection limits. The structures of the reaction products were confirmed by recording the corresponding product (daughter) spectra after collision-activated dissociation and then comparing these spectra with those of ions of known structure when standard compounds were readily available. The collision energy used in these experiments was 20 eV, and the collision gas (argon)pressure was 1 mTorr, which corresponds to multiple collision conditions. Some off-line GC/MS measurements were also performed to further confirm product identity and to measure the yields of different chlorination ITS-40maas products. These measurements employed a F-an spectrometer equipped with a 3Gm X 0.25-rcm DB-l(0.25"-i.d.1 capillary column. Aqueous amine solutions (typically 100 ppm) were prepared by using commercial reagents (aniline, Mallinckrodt, Inc., St. Louis, MO; 2-aminobutane and 1,3-diaminopropane, Eastman Kodak Co., Rochester, NY). Detection limits were obtained by using solutions prepared by serial dilution of the stock solutions with distilled water. In a typical on-line reaction monitoring experiment, the water stream flowing through the membrane probe was first replaced by an amine solution (50 or 100 mL) and the following reaction sequences were then performed: (i) the pH of the amine solution was raised to 10-11 by addition of 1.5 M NHs solution; (ii) 1or 2 mL of calcium hypochlorite solution (1mg/mL) was added; (iii) the pH of the reaction mixture decreased to below l by successive additions of 0.1 M HCl or concentrated HCl. The pH of the reaction mixture was continuously monitored by using a Coming 130 pH meter and recorded by using a chart recorder. The recorded pH values show that the pH of the reaction mixture reached stable values after the reagent additions typically in less than 30 s. RESULTS The chlorination of aqueous solutions of 2-aminobutane, l,&diaminopropane, and aniline by chlorination reagents NHICl and HOC1 was studied on-line by using membrane introduction mass spectrometry. Table I gives the maximum number of hydrogens substituted by chlorine and the position
(C,H,-NH,+H)' I
,
47.6-
7.4-
(C.H,.N%'%I+
H)'
162
Y
c
I
I
2.7
I
I
GY%NY+H)'
IS
Figure 2. On-line monitoring of the chlorinatlon reactions of 0.2 ppm 2-aminobutane (A) and anHlne (8). The mass chromatograms are for the following ions for p-aminobutane: mlz 74 (C,H,-NH, H)', m l z 108 (C,9-NH3%I H)+, m l z 110 (C,H,-NHS7CI H)+, mlz 142 H)+, and m l z 144 (C,H,-N96C137CI H)+. In the (C,H,-N CI, anline experiment, mlz 94 (CeH5-NH, H)+, mlz 128 (C,,H,'CI-NH H)+, mlz 130 (C H~'CI-NH, HI+, mlz 162 ( C e ~ ~ I z - N H , HI+: m l z 164 (CeH335Cb7CI-NH, H)', m l z 196 (CeH, CI,-NH, HI+, H)+ are displayed. Replacement of and m l z 198 (CeH,3sCl-NH, water by the corresponding amine solution occurred at the time represented by the asterisk. Addition of calclum hypochlorite solution (small arrow) and HCI solution (heavy arrow) additions occurred at the times indicated. Note also that the pH of the reaction mixtures is presented at the bottom of the figures. Data were collected by using C I (isobutane) while monitoring only the ions indicated.
+
+
+
+ + +
+
+ +
+
+
+
of chlorination. As can be seen from the table, the maximum number of chlorine atoms is equal to the number of hydrogen atoms bonded to the amine nitrogen in the aliphatic amines, indicating that the chlorination occurs at the nitrogen atom. Aniline, however undergoes ring chlorination, which is consistent with the activating effect of the amino group in electrophilic substitution reactions. The site(s) of chlorination of 2-aminobutane and aniline was studied in detail, especially by the use of tandem mass spectrometry (see below). Typical results of on-line continuous monitoring of the reactions leading to chlorination of 2-aminobutane and aniline are presented in Figure 2. The reaction monitoring exper-
1786
ANALYTICAL CHEMISTRY, VOL. 63, NO. 17, SEPTEMBER 1, 1991
Table 11. Product Ion Spectra of Different Isotopic Forms of Protonated 2-(Dich1oroamino)Butane"
parent ion m / z (% RA) ion species
+ H)+ (C,Hg-NSCl3'C1 + H)+ (C,Hg-NaC12
142 (0.9) 144 (1.0)
(CIHg-N"C12
146 (1.1)
+ H)+
ions characteristic of chlorination position m / z (% RA) ion species 86 (0.1) 78 (0.6) 88 (0.1) 78 (0.3) 80 (0.3) 90 (0.1) 80 (0.5)
other significant ions m/z (% RA)
NH$C12+ C2H5NSCl+
57 (loo),55 (1.3),41 (40),39 (1.3),29 (73),27 (1.9)
NH2W13'C1+
57 (loo),55 (1.31,41 (381,39 (1.3),29 (66),27 (1.7)
C2HbNSC1+ CzH6N3'C1+ NH2"C12+
57 (loo),55 (1.6),41 (30),39 (3.2),29 (52),27 (1.8)
C2H6N3'C1+ a Collision energy 20 eV and collision gas (argon) pressure 1 mTorr. iments were done by continuously measuring either full mass spectra or ions characteristic of the reaction products (viz. selected ion monitoring). Both electron ionization and isobutane chemical ionization were used. The results presented in the Figure 2 used CI (isobutane) and measured the following selected ions: mlz 74 (C4Hg-NHz + HI+, mlz 108 (C4HgNH3%CI + H)+, mlz 110 (C4Hg-NH37Cl+ H)+, m / z 142 (C4Hg-NS6CIz NH)+, and m/z 144 (C4Hg-N3SC137C1 + H)+ for 2-aminobutane and, in the case of aniline, mlz 94 (C6H6-NH2+ HI+, m / z 128 (C6H4s6C1-NHz+ HI+, mlz 130 (C6H437C1-NHz+ H)+, m / z 162 (C6H336C1z-NHz+ H)', mlz 164 (C6H336C137C1-NH2 + HI', m / z 196 (C6Hz36C13-NHz+ H)+, and mlz 198 (C6Hz36C1237C1-NHz + H)+. The measurements were done according to the following sequence: (i) Water being pumped through the membrane probe was first replaced by an aqueous solution of 0.2 ppm amine (indicated in Figure 2 by the position of the asterisks). (ii) The pH of the amine solution was raised to 10-11 by addition of 1.5 M NH3 solution. (iii) One milliliter of calcium hypochlorite solution (1mg/mL) was added (indicated in Figure 2 by small arrows). (iv) Finally, the pH of the reaction mixture was lowered below 1 by addition of aliquots of 0.1 M HCl or concentrated HC1 (indicated by heavy arrows). Note that the simultaneous increase in ion abundance of both isotopic forms of the different protonated molecules is good evidence for the formation of the specific reaction products. Note also that 2-aminobutane is observed after the addition of hypochlorite because the associated change in pH liberates the free base, that crosses the membrane and therefore is measured. There are two important points to notice in the data presented in Figure 2. First, it is apparent that the monochloro derivative of 2-aminobutane is formed after addition of calcium hypochlorite solution (pH of the reaction mixture 10.3) in contrast to which the monochloro derivative of aniline is formed only after acidification of the reaction mixture (pH of the reaction mixture 2.0-2.6). This difference in reactivity of 2-aminobutane and aniline suggests that chlorination might occur by different mechanisms for these compounds. It is also apparent, since formation of all the possible reaction products can be seen, that the starting amine concentrations of 0.2 ppm represent the limits of concentration at which the reaction sequences can be followed when continuous monitoring membrane introduction mass spectrometry is used, whereas detection limits for individual compounds are much lower. As an example, a detection limit (SIN 3) of 25 ppb was measured for 2-chloroaniline. It was also observed that the response of 2-chloroaniline was linear at the range of 25 ppb to 1ppm. These measurements were done by using isobutane chemical ionization, by using a 2-mL sample size, and by measuring the two different protonated forms of the molecule a t m/z 128 and 130 together with a background ion at mlz 120. The structures of the reaction products were confirmed by recording product ion spectra of different isotopic forms of the protonated reaction products formed. The amine con-
A"ooi i 1
m/z 108
1
57
+
I I II 0
B
10
50
30
I1
108
70
90
110
29 1001
I
- n 1j
0 ,..... 10
II
I
.1!, . 30
m/z 110
110 II
. 7
50
70
90
.
'
""
' ' -1
110
m/z
Flgure 3. Product ion spectra of different isotopic forms of the protonated moiecuk (W)of the monochioro dwivative of 2aminobutane. A resents the spectrum measured for the isotope (C,H,-NH"CI + H)P at mlr 108 and B presents the spectrum measured for the isotope (C,b-Np7Ci + H)+ at m l z 110. Collision energy 20 eV and collision gas (argon) pressure 1 mTorr.
centrations used in these experiments were typically 100 ppm. The measured product ion spectra were compared with product ion spectra of standards of known structure when standard compounds were available. The product ion spectra of different isotopic forms of the protonated monochloro derivative of 2-aminobutane (2-(chloroamino)butane) are presented in Figure 3. The spectra of the W1 and isotopic forms are almost identical except for the shift of the fragment ion a t mlz 52 to mlz 54 when the chlorine-37 isotope is selected as the parent ion. This demonstrates that this fragment ion contains one chlorine atom and therefore the only reasonable structure is NH3CI+, an indication that chlorination occurs on the nitrogen. The other fragmentation routes of the MH+ ions of the monochloro derivative are typical of the hydrocarbon part of the molecule, and this also indicates that the chlorine atom is bonded to nitrogen in the neutral precursor. From the product ion spectra (Table E)of different isotopic forms of the dichloro derivative of 2-aminobutane (2-(di-
ANALYTICAL CHEMISTRY, VOL. 63,NO. 17, SEPTEMBER 1, 1991
1797
Table 111. Product Ion Spectrum of Different Iiotopic Forms of the Protonated Molecules of 2-Chloroaniline,4-Chloroaniline, 2,4-Dichloroaniline,2,6-Dichloroaniline,2,4,6-Trichloroaniline,and Reaction Products Formed during Chlorination of Anilinea ions characteristic of chlorination position, m / z
(90 RA)
Darent ion m/z (% RA)
2-chloroaniline 128 (67) 130 (64) 4-chloroaniline 128 (56) 130 (56) monochloro product 128 (66) 130 (59) 2,4-dichloroaniline 162 (100) 164 (100) 166 (100) 2,6-dichloroaniline 162 (91) 164 (92) 166 (86) dichloro product 162 (100) 164 (100) 166 (100) 2,4,6-trichloroaniline 196 (100) 198 (100) 200 (100) trichloro product 196 (loo) 198 (100) 200 (100) a
(MHHC1HCN)+
(MHHClHCl)+
126 (94) 128 (41) 126 (41) 128 (88)
99 (34) 101 (16) 99 (i5)' 101 (32)
90 (26) 90 (29) . .
91 (3.41, 63 (3.7) 91 (3.2). .. 63 (4.2) . .
90 (30)
91 (1.2), 63 (2.4)
127 (17) 129 (7.1)
126 (66) 128 (26)
99 (4.9) 101 (2.21
90 (100) 90 (100)
91 (18, 63 (lo), 39 (2.3) 91 (1.8).63 (12). 39 (2.1)
127 (29) 129 (12) 127 (13) 129 (28)
126 (80) 128 (34) 126 (37) 128 (88)
99 (14) 101 (7.4) 99 (5.8) 101 (15)
90 (84) 90 (89)
91 (2.6), 63 (lo), 39 (1.8) 91 ( 2 8 , 63 (121, 39 (1.4)
90 (99)
91 (1.9), 63 (lo), 39 (68)
161 (29) 163 (21) 161 (8.0) 165 (7.7) 163 (20)
160 (52) 162 (35) 160 (14) 164 (14) 162 (38)
133 (2.6) 135 (2.0) 133 (1.3) 137 (0.5) 135 (1.4)
124 (52) 126 (15) 124 (34) 126 (31) 124 (15)
97 (1.9) 99 (0.9), 97 (0.7)
161 (28) 163 (18) 161 (8.8) 165 (5.8) 163 (17)
160 (57) 162 (36) 160 (15) 164 (14) 162 (34)
133 (2.3) 135 (1.4) 133 (0.9) 137 (0) 135 (1.2)
124 (58) 126 (14) 124 (36) 126 (25) 124 (7.2)
97 (1.1) 99 (0.91, 97 (1.2)
(MHNH3)"
(MHCl)"
(MHHC1)"
(CaH$CI-NH2 + H)" (CeH,87Cl-NH2 + H)"
111 (0.4) 113 (0.2)
93 (16) 93 (12)
92 (100) 92 (100)
(CaH4Wl-NH2 + HI" (C6H437Cl-NH2 + H)"
111 (20) 113 (24)
93 (100) 93 (100)
92 (1.3) 92 (0.9)
111 (1.1) 113 (1.0)
93 (20) 93 (17)
92 (100) 92 (100)
127 (41) 129 (19) 127 (20j 129 (40)
ion species
other significant ions m / z (%
RA)
99 (LO), 97 (1.1)
99 (1.9), 97 (1.1)
Collision energy 20 eV and collision gas (argon) pressure 1 mTorr.
chloroamino)butane), three separate facta provide evidence for nitrogen chlorination. First, the product ion spectra show the presence of the m / z 86,88, or 90 ions corresponding to the three main isotopic forms of the parent ion. When the all-36c1parent ion is selected, this fragment ion occurs at m/z 86, for the parent ion with both chlorine-35 and -37, it occurs at m/z 88, and finally, for the alL3'Cl parent ion, it occurs at m/z 90. These mass shifts clearly show that the fragment ion must contain two chlorine atoms and most probably is NH2C12+,a fragment that can be readily formed only if both chlorine atoms are bonded to the nitrogen atom in the dichloro derivatives of 2-aminobutane. Second, in all the measured daughter ion spectra, peaks occur at m/z 78 or 80 (depending on the chlorine isotope). These ions contain one chlorine atom since the protonated molecule isotope that contains only chlorine-35 isotopes gives a peak at m / z 78, that which contains both chlorine35 and -37 atoms gives m/z 78 and 80, and finally, the all-chlorine-37 molecule yields only the fragment at m / z 80. Therefore, the probable formula of this fragment ion is C2H6NC1+. This ion can be formed from MH+ by sequential losses of neutral hydrochloric acid and ethene. Third the product ion spectra of different isotopic forms of the MH+ ions of the reaction products are dominated by the typical fragmentation of the hydrocarbon part of the molecule, which is only possible if chlorination is confined to the nitrogen. The product ion spectra of the chlorination reaction products of aniline were compared with those of 2-chloroaniline, Cchloroaniliie, 2,4dichloroaniline, 2,6-dichloroaniline,
and 2,4,64richloroaniline (Table 111)in order to establish the site of chlorination of aniline in solution. The spectra of protonated 2-chloroanilineand 4-chloroaniline (Figure 4) are easily distinguished. For 2-chloroaniline, the main fragmentation route is the loss of neutral HC1, giving the fragment ion at m / z 92, which subsequently dissociates by HCN loss to yield the m / z 65 ion. Other fragmentation routes to note are the loss of neutral ammonia (NH,) and loss of a chlorine radical (Cl), giving fragment ions at m / z 111 and 93, respectively. The two main fragmentation routes of the MH+ ions of 4-chloroaniline are competitive losses of neutral ammonia and a chlorine radical, yielding the ions at m/z 111and 93. The reason for these differences between the isomers is that in 2-chloroaniline the two substituents can interact through the well-known ortho effect (34). In this case, the ortho effect allows the loss of neutral HC1 as a main fragmentation route. The ion spectrum of MH+ ions of the monochloro derivative of the aniline is almost exactly identical with the spectrum of MH+ ions of 2-chloroaniline (Figure 4), establishing that chlorination occurs primarily at the ortho position The product ion spectra of the MH+ ions of 2,4-dichloroaniline and 2,&dichloroanilinealso can be distinguished easily from each other (Table 111)due to the strong influence of the double ortho effect on the spectrum of 2,6-dichloroaniline. The main fragmentation routes are common to both isomers, but the abundance5 of the fragment ions differ significantly. The main fragmentation of MH+ ions of 2,4-dichloroaniline occurs through losses of HCl and C1, giving ions at m / z 126
.
1798
ANALYTICAL CHEMISTRY, VOL. 63, NO. 17, SEPTEMBER 1, 1991 92
Table IV. Experimental Conditions for Chlorination Reactions of Aniline Studied by Using GC/MS with an ITS-40 Mass
I
SpectrometeP compd added in amine soln in order of addn ammonia Ca(OCl), HCl
expt 1
7.5 X lo-'; 9.3 7.5 X lo4; 9.4 ---; 4.8 (amine soln) 1.4 X lo6; 9.4 1.4 X lo-'; 10.6 1.4 X lod; 7.7 4 X IO-'; 2.8 5 X lo-'; 2.9 2 X lo-'; 2.5
a Aniline amount is 1.35 X in all the experiments.
1
2-chloroaniline 4-chloroaniline 2,4-dichloroaniline 2,6-dichloroaniline 2,4,6-trichloroaniline a Sample
0 30
50
70
90
110
30
X,
70
90
110
% isomer; % products expt 1 expt 2 expt 3
81; 76 19; 18 82; 5 18; 1 100, 98%) and only traces of a dichloro derivative. On the other hand, chlorination of 2,4,6-trimethylaniline was unsuccessful, showing only traces of a monochloro derivative. These latter results indicate that while some nitrogen chlorination cannot be excluded, the dominant sites of chlorination are the activated positions on the benzene ring. To confirm the identity and further characterize the yields of the different ring chlorination products of aniline, GC/MS measurements using a Finnigan ITS-40 mass spectrometer were performed (Tables IV and V). These measurements were done by directly injecting the reaction mixture (1 p L , split ratio about 301) into the GC/MS instrument after the HCl addition. To confirm that the GC/MS method gives correct product distributions for chloroanilines, standard solutions in the pH range 1.4-8.5 were analyzed. These standard solutions contained 2-chloroaniline,4-chloroaniline, 2,4-dichloroaniline, 2,6-dichloroaniline,and 2,4,6-trichloroaniline. For standard solutions above pH 2.5, the relative peak areas of different chloroaniline isomers stayed constant (within *lo%), but when the pH of the standard solution was 2 or below, the GC/MS method failed to give correct results. At
ANALYTICAL CHEMISTRY, VOL. 63, NO. 17, SEPTEMBER 1,
these lower pH's the relative peak area of Cchloroaniliie was greatly decreased (about 80%)and the peak shape was distorted. This behavior is believed to be due to the fact that the 4-chloroaniline (4-chloroaniline pK, = 4.2 and 2-chloroaniline pK, = 2.7 (35))is almost entirely in the ionic form at these pH's. It is not fully understood why thisionization effect shows up clearly only after the pH of the solution is more than 2 pH units lower than the pK, value of 4-chloroaniline, At these lower pH's, the peak area of 2,4-dichloroaniline was also slightly depressed due to the protonation of the analyte. The various experimental conditions presented in Table IV were used in the GC/MS studies. The main points to note concerning the experimental conditions used are the variations in the amount of calcium hypochlorite added, the fact that experiment 3 was done without ammonia addition, and, finally, the fact that the pH of the reaction mixture after the HCl addition is in the same range in each experiment. The data shown in Table V represent the observed isomer distribution and the distribution of all the products. These distributions were estimated based on the molecular ion abundance of the individual reaction products. The data obtained (Table V) show the preferential formation of 2-chloroaniline as the monochlorination product and therefore are in good agreement with the data measured earlier by membrane introduction mass spectrometry. The GC/MS data showed also that 2,4-dichloroaniline was the major dichlorination product and that the distribution of different dichlorination products was greatly effected by the reaction conditions. In addition, the data presented in Table V show interesting effects of the presence of ammonia in the chloroaniline isomer distribution. When ammonia is added (experiment l), the relative yields of dichloroanilinereaction products are higher than without ammonia addition (experiment 3). Note also that the greater amount of chlorine added in experiment 2 than in experiment 1 shows up clearly in increased production of dichloroanilines and trichloroaniline.
DISCUSSION The mechanism of chlorination of organic amines in aqueous solution by the chlorinating reagents NH2C1, HOC1, and C12 has been studied extensively (I-3,13, 36-39). The chlorination reactions with different chlorination reagents are the following: RNHz NHzCl- RNHCl + NHS (1)
+ RNHz + HOC1
RNH2
+ Cl2
+
-+
RNHCl
RNHCl
+ HzO
+ H+ + C1-
(3)
For reactions 1 and 2 two different detailed mechanisms have been proposed. According to results of Margerum and Snyder, reaction 1occurs in the pH range 2-10 by a mechanism in which protonated monochloramine (pK, = 1.5 (39)) reacts with neutral amine (39). Isaac and Morris have proposed that reaction 1o c m either by the mechanism proposed by Margerum and Snyder or by the acid-catalyzed reaction between neutral amine and chloramine (36,37). They studied the mechanism in the pH range 6-9. For reaction 2, Margerum et al. proposed a mechanism in which hypochlorous acid (pK, = 7.5 (40)) reacts with neutral amine (3). These studies were done in the p H range 610. Weil and Morris have proposed two different mechanisms for reaction 2 in the pH range 4-12 (I). One is the same as that proposed by Margerum et al. and the other one is a mechanism in which hypochlorite anion reacts with protonated amine. Margerum and Snyder have shown also that protonated chloramine is a more efficient chlorination reagent than hypochlorous acid in the pH range 7-11 (39), suggestive of preferential chlorination by reaction 1 rather than reaction 2. Once the chloramine derivative of an organic amine is formed by one of the
lWl
1799
reactions presented above, it can react further with the chlorinating reagent and form the corresponding dichloroamine (3, 13). In this study, ammonia was normally added to the reaction mixture in order to achieve efficient chlorination by reaction 1. The formation of monochloramine after calcium hypochlorite addition is evident from the MIMS measurements. This observation confirms that chlorination can occur by reaction 1. However, the fact that the monochloro derivative of 2-aminobutane (pK, = 10.6 (41)) was formed upon addition of calcium hypochlorite (Figure 2A, pH of the mixture 10.3) indicates that chlorination by reaction 2 also occurs. The formation of the dichloro derivative of the aliphatic amine was only observed after acidification of the sample mixture (pH down to 1.3), which is in agreement with reaction 1. However, at lower pH (
