Ind. Eng. Chem. Res. 1994,33, 2073-2083
2073
Kinetics of the Chlorination of Acetic Acid with Chlorine in the Presence of Chlorosulfonic Acid and Thionyl Chloride Paivi Maki-Arvela and Tapio Salmi' Laboratory of Industrial Chemistry, Abo Akademi, FIN-20500 Turku, Finland
Erkki Paatero Laboratory of Industrial Chemistry, Lappeenranta University of Technology, FIN-53851 Lappeenranta, Finland
The chlorination of acetic acid with molecular chlorine in the presence of chlorosulfonic acid and thionyl chloride as catalytic agents was studied in a laboratory-scale semibatch reactor operating a t atmospheric pressure. Monochloroacetic acid was the main product, and dichloroacetic acid was formed in parallel as a byproduct. Chlorosulfonic acid was a more active catalytic agent than thionyl chloride owing to its acid catalytic effect in enolization of the real catalytic intermediate, acetyl chloride. When the whole amount of the catalytic agent was introduced in the beginning of the reaction decreasing chlorination rates were observed, whereas autocatalytic kinetics appeared when stepwise addition of the catalytic agents was applied. In the former case the decreasing reaction rates were explained by the decomposition of the catalytic agents following first-order kinetics. The autocatalytic effects were explained by a kinetic model involving acid-catalyzed enolization of acetyl chloride and chlorination of the enol as rate-determining steps. The kinetic model provided a good description of the experimental results.
Introduction Monochloroacetic acid is synthesized through chlorination of acetic acid. Monochloroacetic acid is usually the dominating product, but dichloroacetic acid is formed as a byproduct:
+ C1, + CH,ClCOOH + HC1 CH,COOH + 2C1, s CHC1,COOH + 2HC1 CH,COOH
(1)
(2)
In the chlorination of acetic acid under pressure and without any catalyst, considerable amounts of di- and trichloroacetic acids are formed (Richarz and MathBy, 1967, Herold, 1968). In the absence of a catalyst the reaction proceeds via a radical mechanism, dichloroacetic acid being formed consecutively from monochloroacetic acid. The presence of a catalytic agent, however, changes the mechanism totally (Herold, 1968): the monochloroacetic acid formation is enhanced and the dichloroacetic acid formation is suppressed; also the consecutive mechanism becomes questionable (Herold, 1968). The chlorination can be selectively directed to monochloroacetic acid by using suitable inorganic or organic catalytic agents. Several inorganic compounds are known to be catalytically active in chlorination of acetic acid, e.g., fuming HzS04, SCl2, SOC12, FeCl3, PC15, and PC13. Also phosphorus trihalides other than Pc13 can be used. The chlorination in the presence of phosphorus trihalides is known as the Hell-Volhard-Zelinskii reaction, which is known to strongly favor a-chlorinated products in chlorination of organic compounds. Sulfur-containing agents like fumingsulfuric acid and chlorosulfonic acid have been used as catalytic agents to obtain high yields and high selectivities of longer chain a-chlorinated carboxylic acids (Ogata and Matsuyama, 1970;Ogataetal.,1979;PaateroetaE., 1992;Salmi et al., 1993). Beg and Singh (1968) used thionyl chloride as the chlorine source in the chlorination of acetic acid and propanoic acid. In the chlorination of acetic acid the organic catalytic agents are, however, dominating. Acetyl chloride acts as
an effective catalyst in chlorination favoring the formation of monochloroacetic acid. Acetyl chloride can also be created in s i t u in the chlorination process by allowing acetic anhydride to react with HC1. Today it is generally believed that acetyl chloride is the real chlorination intermediate also when inorganic catalytic agents are used the inorganic agents react with acetic acid-and sometimes also with Cl2 or HC1-forming acetyl chloride, which is enolized and chlorinated to monochloroacetyl chloride. Monochloroacetyl chloride then reacts with acetic acid giving monochloroacetic acid and regenerating acetyl chloride (Sioli e t al., 1979; Giuffre e t al., 1981).We have previously presented a systematic study about the chlorination kinetics of acetic acid in the presence of acetyl chloride and observed autocatalytic formation of monochloroacetic acid (Martikainen et al., 1987). The results were quantitatively explained, and a rate equation for the generation of monochloroacetic acid was derived (Salmi et al., 1988). The rate equation was based on the mechanism of Sioli et al. (1979) complemented with the acid catalyzed enolization of acetyl chloride as a ratedetermining step. Later we extended the kinetic studies to the chlorination of straight chain C4-Clp carboxylic acids (Paatero et al., 1992). Chlorosulfonic acid was used as a catalytic agent. The formation kinetics of a-mono- and a ,a-dichlorocarboxylic acids were parallel and autocatalytic. Rate equations for the formation of both mono- and dichlorocarboxylic acids were obtained (Paatero e t al., 1992; Salmi et al., 1993). The aim of the current work is to study the catalytic effects of chlorosulfonic acid and thionyl chloride in chlorination of acetic acid and to explain the reaction kinetics with rate equations based on a reaction mechanism.
Experimental Section Equipment Setup. The chlorination experimentswere carried out in a 100 mL glass reactor equipped with an magnetic stirring bar and a electrical heater with tem-
OS8S-58S5/94/2633-2073$04.5Q/Q0 1994 American Chemical Society
2074 Ind. Eng. Chem. Res., Vol. 33, No. 9, 1994
perature control. The chlorine gas was dispersed into the reaction mixture through separate glass sinters. A reflux condenser was placed on top of the reactor in order to prevent the escape of volatile liquid components. The gaseous components (including Cl2 and HC1) which passed through the condenser were bubbled through aqueous NaOH. The chlorine feed was metered and controlled with a Wallace & Tiernan dry feed chlorinator (Type BA 057) with a capacity of 600 g/h. Experimental Procedure. Acetic acid (Merck 63, 100%)to be chlorinated was placed in the reactor. The reactor contents were heated to the given reaction temperature after which a given aliquot of chlorosulfonic acid (ClSOsH, Fluka 26388, >98%) or thionyl chloride (SOCl2, Fluka 88952, >99%) was added using a 1 mL syringe. The chlorine (Nokia Chemicals Oy, >99.9%) feed (30 g/h) was started, and usually an immediate reaction was observed as a temperature elevation in the liquid phase. In some experiments HCl was also fed into the reactor. HC1 gas was prepared in situ by dropping a concentrated HC1 solution into a flask containing concentrated HzS04. After about 10 min the temperature was usually maintained within 1-2 "C. In case of stepwise catalyst addition the following aliquots of ClS03H or SOClz were manually introduced with a syringe at given time intervals. Samples of the liquid phase for analysis were withdrawn with a Pasteur pipette during the course of the reaction and placed in glass ampules. Chemical Analysis. The gas chromatograph (Varian 3300) was equipped with a single injector and dual capillary column system: a 30 m DB-5 (J.W. Scientific, USA) connected to a flame ionization detector (FID), and a 30 m DB-1 (J. W. Scientific, USA) connected to an electron capture detector (ECD) sensitive to chlorine-containing compounds. The carrier gas was helium with a velocity of 38 cm/s and a split ratio of 28. The temperatures of the injector and detector were 210 and 300 "C, respectively. The following temperature programs were applied 60 "C + 5 "C/min, 75 "C (10 min) + 5 "Omin, and 160 "C. The gas chromatographic data were computed with a Baseline 810 (Dynamic Solutions, USA) integration program. The carboxylic acids were analyzed as trimethylsilane (TMS) derivatives using N,O-bis(trimethylsily1)trifluoroacetamide (BSTFA) as silylation agent and trimethylchlorosilane as catalyst. The acid chlorides were analyzed as esters using 2-pentanol (Merck 807501, >98% ) as esterification agent. Propanoic acid (Fluka 81910, p.a.) and 2,3-dichloropropanoicacid (Fluka 36340,>98 % ) were used as internal standards for FI and EC detectors, respectively. All components were analyzed simultaneously; 100 mg of the sample from the reactor was mixed with 30 mg of 2-pentanol and 100 mg of standard solution, which contained 95 wt % propanoic acid and 5 wt % 2,3dichloropropanoic acid. A 20-mg quantity of this solution was mixed with 300 pL of BSTFA and 100 pL of TMCS. The silylation was continued for 1h at 60 "C, after which 500 p L of diethyl ether (Merck, p.a.1 was added. The solution was diluted (1:lO) with diethyl ether, and 1p L of the diluted sample was injected into the gas chromatograph. The peaks in the gas chromatogram were identified by injection of standard samples prepared from commercial acetic acid, mono- and dichloroacetic acids, and acetyl chloride. Also, separate runs on a VG Micromass 7070E gas chromatograph-mass spectrometer were performed to identify the FID-GC peaks. The sulfur contents of some samples of the product were determined using a sulfur analyzer (Leco Corp.).
Y 1 L
0.8 .-
0.6 .-
0.4 .-
0.2 .-
Wmin
0
50
100
150
200
250
300
350
Figure 1. Formation kinetics of mono- (0) and dichloroacetic acid (0) at 85 "C in the presence of chlorosulfonicacid (ClSOsH).Addition of C l S O a in aliquota of 1.0 mol % ClSOaH/h. Y
'1
0.8
Vmin
0
50
100
150
200
250
300
350
Figure 2. Formation kinetics of mono- (0) and dichloroacetic acid (0) at 85 OC in the presence of CISO3H. Addition of all ClSO3H in the beginning of the experiment yO,Cm3H = 5.0 mol % .
Results and Discussion Chlorination in the Presence of Chlorosulfonic Acid. The chlorination kinetics of acetic acid (CA) in the presence of chlorosulfonic acid (ClS03H) a t 85 "C is shown in Figures 1and 2, where the yields of the main product, monochloroacetic acid (MCA), and the byproduct, dichloroacetic acid (DCA) are displayed as a function of the reaction time. The results show that mono- and dichloroacetic acids are formed in parallel and that the formation rate of dichloroacetic acid is clearly lower than that of monochloroacetic acid. A higher initial rate was obtained in the chlorination, if all of the catalyst was introduced to the reaction mixture in the beginning of the experiment, but the reaction stagnated after about 4 h with ClS03H catalyst (Figure 2). The reason was the decomposition of the catalyst, which was confirmed by the sulfur analysis. On the other hand, if the same amount of the catalyst (5 mol % ) was added to the reaction mixture in five equal aliquots (1mol 7%) with 1h intervals (Figure l),the initial chlorination rate was low, but the reaction became autocatalytic and higher yields of mono- and dichloroacetic acids were obtained than with the sudden catalyst addition. When the catalyst was added as aliquota the yield of monochloroacetic acid exceeded 80% after 6 h of reaction (Figure 11,whereas the maximally attainable yield of monochloroacetic acid was 77 % in the instantaneous addition of the catalyst (Figure
' I
Ind. Eng. Chem. Res., Vol. 33, No. 9, 1994 2075
y DCA
Y
1
0.5
5.0E-02 4.OE-02
3.OE-02
t
0.4
2.OE-02
y MCA 0
0.2
0.4
0.6
0.8
Figure 3. Interdependence of the yields (y) of mono- and dichloroacetic acids. Conditions: catalyst ClSOsH, temperature 85 O C , addition of CISOsH in aliquota (0)and in the beginning of the experiment (0).
2). The addition of the catalyst in aliquots also favors the formation of dichloroacetic acid: the yield of dichloroacetic acid was about 8% after 6 h when the catalyst was added in aliquots, whereas the corresponding yield of dichloroacetic acid was only 2% with the instantaneous addition of the catalyst. Despite the big difference in the generation rates of mono- and dichloroacetic acids the principal forms of the formation kinetics of the acids are remarkably similar, as can be seen in Figures 1and 2. The formation of mono- and dichloroacetic acids can be illustrated by plotting the yield of monochloroacetic acid us the yield of dichloroacetic acid. The plot is shown in Figure 3. For low and intermediate conversions of acetic acid the yield of dichloroacetic acid is proportional to that of monochloroacetic acid, but the yield of dichloroacetic acid increases more at high conversions of acetic acid. Qualitatively similar results concerningthe autocatalysis have previously been observed by Paatero et al. (1992) and Salmi et al. (1993), who studied the chlorination kinetics of longer chain (C&Ip) carboxylic acids, especially that of butanoic acid in the presence of ClS03H: a-mono- and a,adichlorobutanoic acids were generated in parallel and their generation kinetics was autocatalytic, if chlorosulfonicacid was added in aliquots during the reaction. Hence, we conclude that the behavior of acetic acid resembles that of the longer chain carboxylic acids in the chlorination with molecular chlorine. Chlorination in the Presence of Thionyl Chloride. The chlorination kinetics of acetic acid in the presence of thionyl chloride (SOClz) at 85 "C is shown in Figures 4 and 5. Monochloroacetic acid is formed as a main product, and dichloroacetic acid appears as a byproduct. The catalytic activity of thionyl chloride is much lower than that of chlorosulfonic acid, as can be seen by comparison of Figures 1 and 4 the yields of monochloroacetic acid were maximally 20% and 80% after 6 h with thionyl chloride and chlorosulfonic acid, respectively. Mono- and dichloroacetic acids were formed in parallel, and thionyl chloride favored the formation of dichloroacetic acid: in the case of a sudden addition of the catalyst the amount of dichloroacetic acid was about 2% after 6 h for both ClS03H and SOClz (Figures 2 and 5) even though SOClz was much less active in the formation of monochloroacetic acid. The highest initial chlorination rate was achieved if all thionyl chloride (5 mol % ) was added to the reaction mixture in the beginning of the experiment, but the reaction stagnated-probably because of the loss of the catalyst-after 2 h (Figure 5). If the catalyst was added
0
50
100
150
200
300
250
350
Figure 4. Formation kinetics of mono- (0) and dichloroacetic acid (0) at 85 "C in the presence of thionyl chloride (SOC12). Addition of SOClZ in aliquota of 1.0 mol % SOCldh. Y
t
0.41 OS
0.3
0
*
- . * 0
50
100
,
, 150
_
, 200
.
, 250
.
, 300
.
Wmin
350
Figure 5. Formation kinetics of mono- (0) and dichloroacetic acid (0) at 85 O C in the presence of SOClz. Addition of all SOClz in the beginning of the experiment y o , s ~ =~ l5~mol % .
in five aliquots (1mol %) with 1-h intervals, the chlorination activity was maintained, and autocatalytic formation kinetics of mono- and dichloroacetic acids became observable (Figure 4). The formation of mono- and dichloroacetic acids can again be illustrated by plotting the yield of monochloroacetic acid us the yield of dichloroacetic acid. The plot is shown in Figure 6. We conclude that the principal behavior of SOClz is rather similar to that of ClSOsH, even though SOClz is a much less active catalytic agent in the chlorination of acetic acid. Reaction Mechanism. The key points in amechanistic interpretation of the kinetic data in the chlorination of acetic acid are: the parallel formation of mono- and dichloroacetic acids, the autocatalytic kinetics, whether the catalyst concentration is maintained constant, and the stagnation of the reaction caused by the catalyst decomposition and volatilization. Any consistent kinetic model has to provide an explanation to these dominating effects. In the earlier studies of the halogenation of carboxylic acids in the presence of chlorosulfonic acid (Ogata and Watanabe, 1979; Ogata et al., 1979) the decrease of the reaction rate was observed and some of the results-like the iodination of propanoic acid were interpreted by second order kinetics. The second-order rate constant, however, decreased during the progress of the reaction (Ogata and Watanabe, 1979). Accordingto our opinion the decreasing
2076 Ind. Eng. Chem. Res., Vol. 33, No. 9, 1994 yDCA 2.OE-02
1.5E-02
t 0
5 OE-02
01
0 15
02
YMCA 0 25
Figure 6. Interdependence of the yields (y) of mono- and dichloroacetic acids. Conditions: catalyst SOClz, temperature 85 O C ; addition of SOCl2 in aliquota (0)and in the beginning of the experiment (0).
reaction rate is caused by the compensating effects of autocatalysis and catalyst decomposition. Also, the chemical form of the active intermediate in the halogenation of carboxylic acids has been the subject of discussion. In the use of chlorosulfonic acid Ogata and co-workers (Ogataet al., 1977;Ogataand Adachi, 1982)have suggested that the active species is either a ketene (RCH=C=O) (Ogata et al., 1977) or a monoacyl sulfate (Ogata and Adachi, 1982), which is formed from the carboxylic acid and chlorosulfonic acid. The intermediate is then chlorinated to a monochloroacid chloride (RCHClOCl), which reacts further with the unchlorinated carboxylic acid via a chlorine exchange reaction to generate the chlorinated carboxylic acid (RCHClCOOH) and acid chloride (RCH2COC1). It has also been suggested that the external acid catalyst is able to catalyze the enolization of the carboxylic acid and the enol (RCH=C(OH)2) is chlorinated to monochlorocarboxylicacid (Ogata and Matsuyama, 1970). For the chlorination of acetic acid it is now generally agreed that the active intermediate is acetyl chloride (GiuffrB et al., 1981), which can be formed from the reactions between acetic acid and many pseudocatalytic agents like PC13, SOC12,fuming Hfi04, ClS03H, and acetic anhydride. Accordingto Sioli et al. (19791,acetyl chloride is enolized to CHyCOHCl. The enol is chlorinated to monochloroacetyl chloride, which forms an intermediate anhydride with acetic acid. The constituents of the anhydride undergo a rapid chlorine exchange reaction giving monochloroacetic acid and regenerating acetyl chloride. This mechanism of Sioli et al. (1979) as such is, however, not sufficient to explain the autocatalytic effect observed by Martikainen et al. (1987) in the chlorination of acetic acid in the presence of acetyl chloride. Therefore, the original mechanism of Sioli et al. (1979)was completed (Salmi et al., 1988)by the assumption that the enolization of acetyl chloride is acid catalyzed. The acidity of the system increases during the reaction, since the product, monochloroacetic acid, is a much stronger acid than the reagent, acetic acid the pK values of acetic and monochloroacetic acids are as follows in aqueous solutions: p K c ~ = 4.76 and PKMCA= 2.86 (Ringbom, 1963). Thus monochloroacetic acid acts more efficiently than acetic acid in enolization of acetyl chloride. This is according to our opinion the basic source of the autocatalytic effect. The formation mechanism of dichlorinated acids has not been discussed much previously. In the pioneering studies of the chlorination of acetic acid it was regarded that the formation of dichloroacetic acid might happen
via homogeneous chlorination of monochloroacetic acid. This is certainly the case, if acetic acid is chlorinated under pressure in the absence of any catalyst as shown, for example, by Herold (1968). This kind of mechanism can, however, not explain our kinetic data, and the presence of an ionizing agent completely changes the conditions in the liquid phase. It could be assumed that the intermediate in the formation of dichloroacetic acid is dichloroacetyl chloride formed through enolization and chlorination of monochloroacetyl chloride. However, the experiments of GiuffrtS et aZ. (1981) showed that acetyl chloride can be chlorinated directly in liquid SO2 to dichloroacetyl chloride. It was shown by Mlki-Arvela et al. (1994)that monoand dichloroacetyl chlorides are formed in parallel in the chlorination of acetyl chloride at 50.8 "C in the presence of chlorosulfonic acid as a catalyst. Based on the qualitative reasoning above the progress of the chlorination of acetic acid in the presence of chlorosulfonic acid and thionyl chloride can be described as follows. The catalytically active agent, acetyl chloride, is generated in a reaction between acetic acid and ClSOsH or soc12:
Acetyl chloride undergoes an acid (HAi) catalyzed enolization to form the double bond:
Any acid (HAi) present in the system (CH&OOH, CH2ClCOOH, CHClZCOOH, and ClS03H) can in principle contribute to step 4. The double bond of the enol is chlorinated in parallel to form mono- and dichloroacetyl chloride
It should be pointed out that eqs 5 and 6 represent more the overall stoichiometry of the formation of mono- and dichloroacetyl chlorides than the molecular mechanism. Especially the formation of dichloroacetyl chloride, eq 6, presumably proceeds via several steps. One possible route for the formation of mono- and dichloroacetyl chlorides is shown in Figure 7. The reaction path goes via a common intermediate generated from the enolic form of acetyl chloride and chlorine. This intermediate can decompose to monochloroacetyl chloride and to an enol, which is further chlorinated to dichloroacetyl chloride. For the consideration of the overall kinetics steps 5 and 6 are, however, sufficient. Mono- and dichloroacetyl chlorides form intermediate anhydrides with the carboxylic acids. For the net production of the new compounds the following reactions are assumed:
CH2CCco CI
__
+ CH Cfio O ‘H
Ind. Eng. Chem. Res., Vol. 33, No. 9, 1994 2077 CH&ICO 00 ‘OH’CT CH3COH’CICH3C*0
O ‘H
(k+4,Ki*[CH3COC11 - k,i[CH2COHC11)[HAil
r4 =
(8)
where Ki* denotes the equilibrium constant of the first step in reaction 4. The rates of the chlorination reactions 5 and 6 are given by
B CHCIzCCo CI
+
CH2CIC.’/o O ‘H
CHClzC
