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Kinetics of Cyanuric Chloride Hydrolysis in Aqueous Solution Zhi Yan, Wei-Lan Xue,* Zuo-Xiang Zeng, and Mei-Rong Gu Institute of Chemical Engineering, East China UniVersity of Science and Technology, 200237 Shanghai, China
The hydrolysis of cyanuric chloride is investigated as a function of temperature, pH, and cyanuric chloride concentration in liquid phase. The hydrolysis reaction is carried out in water or acetone/water system in the temperature range of 283.15-303.15 K. The hydrolysis rates of cyanuric chloride are monitored at different temperatures and at various pHs. The experiment data show that the hydrolysis rates are independent of pH value when pH e 6, and increase with the pH value when pH g 7. On the basis of the results, it is supposed that the hydrolysis reaction takes place by the unimolecular nucleophilic substitution (SN1) mechanism when pH e 6 and by the bimolecular nucleophilic substitution (SN2) mechanism when pH g 7. Kinetics models corresponding to the mechanisms are proposed as follow: -d(nA/V)/dt ) 7.625 × 109e-69298/RTcA for the acidic situation and -d(nA/V)/dt ) 7.112 × 1011e-59267/RTcAcB0.5 for the alkaline situation. Experiment data show that the models work well with the average deviations of 2.01% and 9.88%, respectively. 1. Introduction Cyanuric chloride, also named tricyanogen chloride, cyanuric trichloride, and cyanuryl chloride, is a white, easy hydrolytic degradation monoclinic crystal of pungent odor. The compound is soluble in acetonitrile, ether, ketones, and chlorinated hydrocarbons, but nearly insoluble in water.1 As a useful organic intermediate, cyanuric chloride is most used to produce triazine derivation which has been widely used in the dye industry,2 agriculture chemistry,3 plastic and rubber industry,4 etc. Cyanuric chloride has a molecular structure with three chlorines which can be substituted by nucleophilic substance step-by-step, such as hydroxybenzene, sulfide, amine etc., in the presence of a hydrochloride scavenger. The stepwise manner can be controlled at a well-defined temperature. However, the substitution pattern also depends on the structure of the nucleophile, its basic strength and steric factors, the substituent already present in the s-triazine ring and the nature of solvent used. For example, Menicagli5,6 achieved nearly quantitave yields of both symmetric and nonsymmetric mono-, di-, and trisubstituted alkoxy and amino 1,3,5-triazines by nucleophilic substitution of cyanuric chloride in one pot in the presence of a catalytic amount of 18-crown-6. Cyanuric chloride has a tendency to hydrolyze with water in the process of the triazine derivatives production,7,8 which will generate 2,4-dichloro-6-hydroxy-s-triazine or 2-chloro-4,6-dihydroxy-s-triazine even 2,4,6-dihydroxy-s-triazine(cyanuric acid). So, controling the amount of the byproduct becomes a serious problem. Fierz-David and Matter9 have studied the cyanuric chloride hydrolysis in a suspension with calcium carbonate as the acid scavenger, but they did not detail the kinetics of cyanuric chloride hydrolysis. The intention of this paper is to discuss the hydrolysis kinetics of cyanuric chloride suspension in water or organic/water system and provide a consultant in the process of 1,3,5-trizine derivations production. 2. Experimental Details 2.1. Materials. Cyanuric chloride, acetone, hydrochloric acid, sodium hydroxide, and sodium carbonate (purchased from Shanghai chemistry Reagent Company, China) used for the * To whom correspondence should be addressed. E-mail: wlxue@ ecust.edu.cn.
experiment are analytical reagent grade. The mass fraction purities of these reagents are higher than 98%. Bidistilled water is used. 2.2. Reaction System. The cyanuric chloride hydrolysis is studied at atmospheric pressure in a 250 mL four-neck-flask with a pH meter, a thermometer, and a stirrer. Temperature is kept by an isothermal water bath, and pH is adjusted to the requested value with hydrochloric acid or sodium hydroxide (sodium carbonate). The concentration of cyanuric chloride in liquid is changed by adding different amount of acetone to the water. Cyanuric chloride is feed to flask in which the temperature and pH of water or water/acetone are all controlled at the desired values, and then, the contents are stirred thoroughly. The effect of the agitation speed on the reaction rate is studied at the beginning. The results indicated that the rate of reaction is found to be practically independent of the speed of agitation over 800 rpm. Therefore, in this work, the agitation speed is higher than 800 rpm. Additional sodium hydroxide or sodium carbonate is used to make the pH steady. Samples are taken from the flask at each time interval, and additional acetone is put into the samples to make them clear. The contents of cyanuric chloride and 2,4-dichloro-6-hydroxy-s-triazine in the diluted samples are determined by high performance liquid chromatograph (HPLC) at 220 nm. The reaction temperature is controlled at a specified value changing from 283.15 to 303.15 K, and samples are taken at each of the specified temperature and pH, and the previous operations are repeated. In this way, the relationship between the conversion of cyanuric chloride and time is obtained. 3. Reaction Mechanism and Kinetic Model When cyanuric chloride is placed in water, hydrolysis reaction will occur, and the three chlorines on cyanuric chloride will be substituted by OH- in turn. An empirical rule, based upon observation, is that monosubstitution of chlorine occurs below or at 273.15 K, disubstitution at room temperature and trisubstitution above 333.15 K.9–12 The monosubstitution of cyanuric chloride is shown in Figure 1. Being similar to benzene, cyanuric chloride also has a cyclic system in which there are three nitrogen atoms. Because the electronegativities of both nitrogen atom and chlorine atom are stronger than that of carbon atom, the electron cloud density of
10.1021/ie071289x CCC: $40.75 2008 American Chemical Society Published on Web 07/11/2008
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Figure 1. Monosubstitution of cyanuric chloride.
Figure 2. Charge distribution of cyanuric chloride.
attack the electrophilic carbon atoms directly, which means that cyanuric chloride hydrolysis in protic solvent is likely to undergo SN1 mechanism but not SN2 mechanism. In protic solvent, as shown in Figure 5, cyanuric chloride is solvated by the solvent and then ionized into a carbocation and a chlorine ion. A carbocation may form hydrogen bonds with the -OH hydrogen atoms, and a chlorine ion may complex with the nonbonding electrons of the -OH oxygen atom, which is beneficial to the SN1 reaction. Once the carbocation formed, it is attacked by the nucleophiles and the monosubstitution of cyanuric chloride is generated rapidly. So, the first step is the rate-determining one, and the rate of the SN1 reaction is proportional to the concentration of cyanuric chloride and independent of the nucleophile concentration. 3.3. Kinetic Model. According to the above analysis, the relationship of the hydrolysis rate of cyanuric chloride with the concentrations of cyanuric chloride and OH- can be expressed as follows. Hydrolysis rate rA1 at acidic situation rA1 ) k1cAγ
(1)
hydrolysis rate rA2 at alkaline situation rA2 ) k2cAβ cRB Figure 3. Reaction equation of cyanuric chloride hydrolysis at alkaline liquid.
carbon atom is the lowest one, which makes carbon atoms present partial positive charge and somewhat electrophilic. Figure 2 depicts the scheme for the charge distribution of cyanuric chloride. So, when cyanuric chloride is exposed to a nucleophile, nucleophilic substitution may be undergone. 3.1. Hydrolysis Mechanism at Alkaline Situation. For the hydrolysis of cyanuric chloride at alkaline liquid (pH > 7), the reaction may undergo SN2 mechanism due to the existence of the nucleophile OH-. The reaction equation is shown in Figure 3. This is a concerted reaction, taking place in a single step with bonds breaking and forming at the same time. As shown in Figure 4, the middle structure is a transition state, the bond to the nucleophile(hydroxide) is partially formed, and the bond to the leaving group(cholorine ion) is partially broken. The transition is not a discrete molecular that can be isolated; it exists for only an instant.13 The rate of SN2 reaction is proportional to the concentration of both the cyanuric chloride and the hydroxide ions. 3.2. Hydrolysis Mechanism at Acidic Situation. When cyanuric chloride is hydrolyzed in proton solvent, for example water at pH < 7, the O-H group forms hydrogen bond to negatively charged nucleophile(OH-). Therefore, only when some of the hydrogen bonds are broken and some of the solvent molecular (H2O) are “stripped off”, can the nucleophile OHattack the electrophilic carbon atoms. It is difficult for OH- to
Figure 4. Mechanism of cyanuric chloride hydrolysis at alkaline liquid.
(2)
where cA is the concentration of cyanuric chloride in liquid (mol · L-1), cB is the concentration of OH- (mol · L-1). Let nA represent the mole numbers of cyanuric chloride at the moment t (s) and V represent the volume of the mixture (L). Proposing the hydrolysis rate is first-order to cA, rA1, and rA2 can be expressed as eqs 3 and 4, respectively. d(nA/V) ) k1cA dt
(3)
d(nA/V) ) k2cAcRB dt
(4)
r1 ) rA2 ) -
where k1 and k2 are reaction rate constants. The temperature dependence of these rate constants can be represented with Arrhenius equation. k1 ) A1e-E1/RT
(5)
k2 ) A2e-E2/RT
(6)
Substituting eqs 5 and 6 into eqs 3 and 4 respectively, we yield d(nA/V) ) A1e-E1/RTcA dt
(7)
d(nA/V) ) A2e-E2/RTcAcRB dt
(8)
-
where E1 and E2 are activation energies (J · mol-1), R is the ideal gas constant (8.314 J · mol-1 · K-1), T is absolute temperature (K), A1 and A2 are collision frequency factors.
5320 Ind. Eng. Chem. Res., Vol. 47, No. 15, 2008
Figure 5. Ionization of cyanuric chloride in protic solvent.
The term cA is determined just by the solubility of cyanuric chloride in the liquid, and it is a constant when the temperature and the solvent are fixed. The volume V can be regarded as a constant in the process of cyanuric chloride hydrolysis. Integrating eqs 7 and 8, respectively, we obtain nA0 - nAt ) A1e-E1/RTcAt V
(9)
nA0 - nAt ) A2e-E2/RTcAcRBt V
(10)
Figure 6. Effect of acetone on the solubility of cyanuric chloride in water.
where nA0 and nAt are mole numbers of cyanuric chloride at the initial and the moment t, respectively. Fractional conversion, xA, is defined as xA ) 1 -
nAt nA0
(11)
Equations 9 and 10 can be written as xA1 )
A1e-E1/RTcAV t nA0 -E2/RT
xA2 )
A2e
cAcRBV
nA0
(12) Figure 7. Effect of cA on the cyanuric chloride hydrolysis at 298.15 K and pH 7.
t
(13)
Equations 12 and 13 will be used to describe the variation of cA and cB with time and temperature. Hereinafter, we will examine their applicabilities by experimental data. The value of A1, A2, R, E1, and E2 can be obtained by least-squares method with the data obtained experimentally. 4. Results and Discussions 4.1. Effect of the Cyanuric Chloride Concentration. The solubility of cyanuric chloride in water is tiny and constant at a fixed temperature, for example, 49 mg at 298.15 K. A cosolvent is added to water to change the concentration of cyanuric chloride (cA) in liquid. In this work, using acetone as cosolvent, the concentrations of cyanuric chloride in the water/acetone mixtures are determined at 298.15 K by the analytical method.14,15 See Figure 6; the concentration of cyanuric chloride ranges from 0.00206-0.0105 mol/L with the content of acetone changes from 0-250 g/L. It is found from the figure that the cyanuric chloride concentration is proportional to the content of acetone. Figure 7 shows the effect of cA on the rate of cyanuric chloride hydrolysis at 298.15 K and pH 7. It is found that xA vs t for all cA can be represented by a straight line. 4.2. Effect of Temperature. Temperature is an important factor in affecting the reaction rate. Fierz-David9 mentioned that a suspension of cyanuric chloride in ice water (273.15 K) is fairly stable for nearly 12 h and the hydrolysis rate is very slow till the temperature rises to 283.15 K. But when the temperature grows to 303.15 K or higher, the hydrolysis is speedy. In this paper, the effect of temperature on the rate of cyanuric chloride hydrolysis is studied in the range of 283.15-303.15
Figure 8. Plot of x-t lines of cyanuric chloride hydrolysis at pH range from 1 to 6: (a) 283.15, (b) 293.15, (c) 303.15 K.
K. The results are shown in Figures 8–10. From the figures, it can be found that the hydrolysis rate at 303.15 K is obviously faster than that at 283.15 K. For example, the conversion (pH 5 and 1 h reaction time) is just 3.3% at 283.15 K and increases with an increase in temperature to 34.9% at 303.15 K. A similar result is found in the solution with pH 8; the cyanuric chloride conversion (25 min reaction time) increases from 8.8% at 283.15 K to 81.2% at 303.15 K. From the figures, we can also find that cyanuric chloride conversions increase linearly with time. The results show that when temperature and pH are fixed, the hydrolysis of cyanuric chloride in water can be regarded as a pseudo-zero-order reaction. 4.3. Effect of pH. In the process of cyanuric chloride hydrolysis, sodium carbonate, and sodium hydroxide are used as acid receptor to neutralize the product hydrochloric acid. The
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The experiment data shown in Figures 7, 9, and 10 are related with eq 13, and the model parameters are obtained by leastsquares method. The kinetic model in alkaline situation can be written as d(nA/V) ) 7.112 × 1011e-59267/RTcAcB0.5 (15) dt The correlation coefficient (R2) of the model for the experimental data is 0.971; the comparison between the values predicted with eq 15 and experiment ones are shown in Figures 7, 9, and 10, respectively. The average deviation of the model is 9.88%. The above results indicate that the kinetic models provide a reasonable fit of the experiment data and can be used to describe the variation of the cyanuric chloride conversion with time, temperature, and pH. From eqs 14 and 15, it can be found that the activation energy in the alkaline situation (59 267 J · mol-1) is lower than the one at acidic situation (69 298 J · mol-1), which further proves that cyanuric chloride hydrolysis follows two different mechanisms at acidic and alkaline situation respectively. -
Figure 9. Plot of x-t lines of cyanuric chloride hydrolysis at pH 7, 8, and 9 and 283.15 K.
6. Conclusions
Figure 10. Plot of x-t lines of cyanuric chloride hydrolysis: (a) pH 7, 293.15 K; (b) pH 7, 303.15 K; (c) pH 8, 293.15 K; (d) pH 8, 303.15 K.
pH is controlled by 0.5 mol/L sodium carbonate and 1.5 mol/L sodium hydroxide at pH < 7 and pH > 7, respectively. The effect of pH under acidic conditions is examined in a solution with a pH value from 1 to 6, and the results are shown in Figure 8. It can be found that all x vs t at the same temperature follow the same equation when pH changes from 1 to 6, which means that the cyanuric chloride conversion is independent of pH, i.e., the rate of cyanuric chloride hydrolysis is zero order in OH- concentration over this range. This suggests that cyanuric chloride hydrolysis follows SN1 mechanism in acidic solution. Effect of pH on the rate of cyanuric chloride hydrolysis is also investigated at alkaline situation. Figures 9 and 10 show the x vs t of cyanuric chloride hydrolysis when pH changes from 7 to 9. From the figures, it can be seen that the reaction rate increases with an increase in OH- concentration, for example, cyanuric chloride conversion (1 h reaction time), increases from 6.5% at pH 7 to 69.4% at pH 9, which means that the cyanuric chloride hydrolysis may follow the SN2 mechanism. 5. Correlation and Examination of Kinetics Model Relating the experimental data shown in Figure 8 with eq 12, the model parameters are obtained by least-squares method, and the kinetic model of cyanuric chloride hydrolysis in acidic situation may be written as -
d(nA/V) ) 7.625 × 109e-69298/RTcA dt
In this work, the hydrolysis of cyanuric chloride is investigated experimentally and theoretically. The results show that the hydrolysis reaction of cyanuric chloride follows two different mechanisms at acidic and alkaline situation respectively. In acidic situations, cyanuric chloride conversion is independent of pH, which means cyanuric chloride hydrolysis follows the SN1 mechanism. At alkaline situation, the hydrolysis rate is affected by pH and the concentration of cyanuric chloride, which means cyanuric chloride hydrolysis follows the SN2 mechanism. Kinetic models, eqs 14 and 15, are proposed to describe the kinetic data of cyanuric chloride hydrolysis at acidic situation and alkaline situation with the average deviations of 2.01% and 9.88%, respectively. Nomenclature A1 and A2 ) collision frequency factor cA ) concentration of cyanuric chloride in liquid, mol · L-1 cB ) concentration of OH-, mol · L-1 E1 and E2 ) activation energies, J · mol-1 k1 and k2 ) reaction rate constants nA ) the mole numbers of cyanuric chloride at the moment t, mol rA1 ) hydrolysis rate at acidic situation rA2 ) hydrolysis rate at alkaline situation R ) the ideal gas constant, 8.314 J · mol-1 · K-1 t ) reaction time, s T ) absolute temperature, K V ) represent the volume of the mixture, L xA ) fractional conversion, defined by eq 11 Greek Letters R ) reaction order to OH- at alkaline situation β ) reaction order to cyanuric chloride concentration at acidic situation γ ) reaction order to cyanuric chloride concentration at alkaline situation
(14)
The correlation coefficient (R2) of the model for the experimental data is 0.997. The comparison between the values predicted with eq 14 and experiment ones are shown in Figure 8. The average deviation of the model is 2.01%.
Literature Cited (1) Huthmacher, K.; Most, D. Cyanuric Acid and Cyanuric Chloride. In Ullmann’s Encylopedia of Industrial Chemistry; Wiley-VCH:Weinheim, Germany, 2002; online version.
5322 Ind. Eng. Chem. Res., Vol. 47, No. 15, 2008 (2) Irikura, T.; Morrinaga, F. Triazine DeriVatiVes. US Patent 759,911, 1973. (3) Bester, K.; Huhnerfuss, H. Triazines in the Baltic and North Seas. Mar. Pollut. Bull. 1993, 26, 423–427. (4) Blotny, G. Recent Application of 2,4,6-Trichloride-1,3,5-Triazine and its Derivatives in Organic Synthesis. Tetrahedron 2006, 62, 9507– 9522. (5) Menicagli, R.; Malanga, C.; Peluso, P. Selective Mono- or Dialkoxylation of 2,4,6-Trichloro-1,3,5-Triazine in Solid-Liquid Phase Transfer Conditions. Synth. Commun. 1994, 24, 2153–2158. (6) Samaritani, S.; Peluso, P.; Malanga, C.; Menicagli, R. Selective Amination of Cyanuric Chloride in the Presence of 18-Crown-6. Eur. J. Org. Chem. 2002, 155, 1–1555. (7) Michaud, H.; Trostberg von Seyerl, J. Cyanuric Chloride HaVing ImproVed Shelf Life. US Patent 4,492,679, 1985. (8) Vollbrecht, H. R.; Wagner, F. Cyanuric Chloride Mixtures and Process for Producing same. US Patent 4,329,325, 1982. (9) Fierz-David, H.; Matter, M. Azo and Anthraquinonoid Dyes containing the Cyanuric Ring. J. Soc. Dyers Colourists 1937, 53, 424–436. (10) Haschke, H.; Schreyer, G.; Schwarze, W.; Suchsland, H. Proscess for the Substitution of Chlorine Atoms of Cyanuric Chloride. US Patent 4,054,739, 1977. (11) Brewer, S. A.; Burnell, H. T.; Holden, I.; Jones, B. G.; Willis, C. R. Synthesis of a Series of Dichloroamino- and Dihalosulfonamido-1,3,5-
Triazines and Investigation of Their Hindered Rotation and Stereodynamic Behaviour by NMR Spectroscopy. J. Chem. Soc., Perkin Trans. 1999, 2, 1231–1234. (12) Jan, J. Z.; Huang, B. H.; Lin, J. J. Facile Preparation of Amphiphilic Oxyethylene- Oxypropylene Block Copolymers by Selective Triazine Coupling. Polymer 2003, 44, 1003–1011. (13) Wade, L. G. Organic Chemistry, 5th ed.; Pearson Education. Inc. International Prentice Hall: Upper River, NJ, 2003; pp 229-246. (14) Tsavas, P.; Polydorou, S.; Faflia, I.; Voutsas, E. C.; Tassios, D.; Flores, M. V.; Naraghi, K.; Halling, P. J.; Chamouleau, F.; Ghoul, M.; et al. Solubility of Glucose in Mixtures Containing 2-Methyl-2-butanol, Dimethyl Sulfoxide, Acids, Esters, and Water. J. Chem. Eng. Data 2002, 47, 807–810. (15) Chen, X.-H.; Zeng, Z.-x.; Xue, W.-L. Solubility of 2,6-Diaminopyridine in Toluene, o-Xylene, Ethylbenzene, Methanol, Ethanol, 2-Propanol, and Sodium Hydroxide Solutions. J. Chem. Eng. Data 2007, 52, 1911– 1915.
ReceiVed for reView September 26, 2007 ReVised manuscript receiVed November 19, 2007 Accepted May 24, 2008 IE071289X
