J. Am. Chem. SOC. 1982, 104, 5419-5421
5419
Temperature Dependence of the Neutral Ester Hydrolysis of Chloromethyl Dichloroacet ate in 2-but ox yet hanol- W ater Mixtures in a Wide Temperature Range Liisa T. Kanerva* and Erkki K. Euranto Contribution from the Department of Chemistry a n d Biochemistry, University of Turku, SF-20500 Turku, Finland. Received January 20, 1982. Revised Manuscript Received April 1 , 1982
Abstract: Rates of the water-catalyzed hydrolysis of chloromethyl dichloroacetate in 2-butoxyethanol-water mixtures in the temperature range from 2.5 to 45 O C a t 2.5 “ C intervals were measured conductometrically when the mole fractions of water were 0.988, 0.984, 0.981, 0.980, and 0.975. The Arrhenius plots for the reactions are S shaped when the mole fraction of water is from 0.984 to 0.975. Thus the sign of the heat capacity of activation depends on temperature. At higher temperatures ACp* is positive, in agreement with the results obtained by Holterman and Engberts’ for other esters. The positive ACp*was shown not to be caused by a change in reaction mechanism. At lower temperatures the reaction gives a negative ACp*,in agreement with other aqueous solutions studied earlier. The estimated ACD*values at lowest temperatures are exceptionally negative.
Holterman and Engbertsl recently found a large positive value for ACp*= d@/dT in the neutral hydrolysis ofp-methoxyphenyl dichloroacetate (ACp*=+224 cal K-’ mol-’) and 2,2-dichloropropionate (ACp* = +342 cal K-l mol-’) in a 2-butoxyethanolwater system a t the mole fraction 0.98 of water. This is in disagreement with the negative values of ACp* found for different solvolytic reactions in water and aqueous solvent m i x t ~ r e s . ~ For -~ instance, in the neutral hydrolysis of chloromethyl dichloroacetate, the value of ACp*has been found to be -43.5 & 2.0 cal K-I mol-’ in water.5 In methanol-water mixtures there seems to be a minimum when the mole fraction of water is 0.95 (ACp*= -58.3 f 3.0 cal K-l Like methanol, 2-butoxyethanol is a “water structure-making” (TA) s ~ l v e n t . ~Usually ,~ the value of ACp* decreases when small amounts of such a solvent a r e added to ~ a t e r . ~Therefore, -~ the value of ACp* could be expected to be smaller in 2-butoxyethanol-water mixtures, rich in water, than in pure water. The aim of this work was to study the behavior of the neutral hydrolysis of CHC12COOCH2C1 in 2-butoxyethanol-water mixtures when the mole fraction of water is close to 0.98. The reaction is assumed to take place as a generalbase-catalyzed ester hydrolysis, BAc3, with a second water
Q-
R
II
R-C-G-CH2CI
+
-
& RHC$,-2CI II
5-H I
k2
k-1
R-C
nI
f
HO-CH$
+
(1)
Y
ClCHZOH (1)
fast
>> kl
CHiO
+ H+ + C1-
condition^.^ Experimental Section The ester was prepared by chlorinating methyl dichloroacetate with gaseous chlorine9 and purified by careful fractional distillation using a Todd apparatus, with all disturbing impurities being removed. The solvent mixtures were prepared by diluting a known weight of distilled water with 2-butoxyethanol (Fluka AG, purum), purified by ion exchange and redistillation, to a known volume in a volumetric flask. The initial ester concentrations were about 0.0001 M. The temperature was stable to about 0 . 0 1 K. The reaction was followed conductometricallywith a Beckman RC-18 conductometer, with concentrations of the acids being measured. In theory, the degree of reaction is not a precise linear function of conductance. It can be estimated, however, that if the concentration of the acids is such as in our kinetic experiments, the error introduced in rate coefficients is about 0.1% and is even partly compensated by Guggenheim’s calculation method. Further, the effect is quite similar at different temperatures and does not affect the calculated activation parameters. Guggenheim’s methodi0 was used, e.g., because the final value may be in error as adsorption or desorption may cause some drift in longer time periods. Final values were measured in some cases, and the obtained rate coefficients were essentially the same. The standard deviations of the rate coefficients were in general less than 0.05% but may sometimes be about 0.1%. The conductometric method and the accuracy of ACp* determinations will be described in greater detail elsewhere.’’
Results and Discussion B :
7: k2, k-1
molecule acting as a general base :Bas In the case of chloromethyl esters, the mechanism can be written in the form of eq 1 and 2. The possible concurrent rate-determining nucleophilic displacement of chlorine of the chloromethyl group is insignificant in our reaction
(2)
Holterman, H. A. J.; Engberts, J. B. F. N. J . Am. Chem. SOC.1980,
102, 4256-4257. (2) Robertson, R. E. Prog. Phys. Org. Chem. 1967, 4, 213. (3) Cleve, N. J. Ph.D. Thesis, University of Turku, 1973. (4) Cleve, N. J.; Euranto, E. K.; Kanerva, L. T. ‘Abstracts of
Papers”, 4th International Symposium on Physical Organic Chemistry, York, 1978; p 251. (5) Kanerva, L. T. Licentiate Thesis. Universitv of Turku. 1979. (6) Roux, G.; Perron, G.; Desnoyers, J. E. J . Solufion Chem. 1978, 7, 6 39-65 4. (7) Blandamer, M. J. Adv. Phys. Org. Chem. 1977, 14, 203.
0002-7863/82/ 1504-5419$01.25/0
The experimental rate coefficients for the neutral ester hydrolysis of CHC12COOCHzCl in 2-butoxyethanol-water mixtures are given in Table I, and their logarithms are plotted vs. 1/ T in Figure 1. The plots are S shaped in four different 2-butoxyethanol-water mixtures (x, 0.984, 0.981,0.980, and 0.975). Thus even the sign of ACp*depends on the reaction temperatures used: (8) Euranto, E. K. “The Chemistry of Carboxylic Acids and Esters”; Patai, S., Ed.; Interscience: London, 1969; Chapter 11, pp 529-535, 577-579. Euranto, E. K.; Cleve, N. J. Suom. Kemistil. B 1970, 43, 147-153. Euranto, E. K. Ann. Acad. Sci. Fenn. Ser. A2 1970, No. 152. (9) Euranto, E. K.; Noponen, A.; Kujanpaa, T. Acfa Chem. Scand. 1966,
20, 1273-1280. (10) Guggenheim, E. A. Philos. Mag. 1926, [7] 2, 538-543. (1 1) Kanerva, L. T.; Euranto, E. K.; Cleve, N. J. Acta Chem. Scand.,Ser. E , submitted for publication. (12) Winter, J. G.; Barron, J. P.; Scott, J. M . W. Can. J. Chem. 1975, 53, 1051-1055. (13) Cleve, N. J. Acta Chem. Scand. 1972, 26, 1326-1336.
0 1982 American Chemical Society
5420 J. Am. Chem. SOC.,Vol. 104, No. 20, 1982
Kanerua and Euranto
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-200-
-4001
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i
I A"
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-600 0 2.8
I
I
I
I
3.2
3.3
3.4
3.5
io? TIKY
Figure 1. Arrhenius plots of -log k vs. (l/T) for the neutral hydrolysis of chloromethyl dichloroacetate in 2-butoxyethanol-water. x, = 0.988 (A), 0.984 (B), 0.981 (C), 0.980 (D), and 0.975 (E).
10
I
I
I
20
30
40
Figure 3. Values of ACp* at different temperatures for the neutral hydrolysis of chloromethyl dichloroacetate in 2-butoxyethanol-water with x, 0.980, calculated by the method of Clarke and Glew,14with three (A), four (B), and five (C) parameters (eq 3), by the method of Blandamer et al.I5 (D), and from AH' values for narrow temperature ranges (triangles, see text). Standard deviations for some points indicated by bars.
suming that the reaction mechanism changes when the temperature is raised, in the former case from BA,3 to BM1, in the latter from BA,3 to a nucleophilic substitution of bromine, SN2. Also, Holterman and Engberts' considered a change in mechanism as a possible, although improbable, explanation for their results. Indeed, in their case a change in mechanism from BA,3 to BAI1 is possible. Because the electronegativities of the alkyl components of their and our esters are very different, it is highly improbable that for both esters the same or some other change in mechanism takes place a t the same conditions. The Arrhenius plots (Figure 1) indicate that AH*and even AC,* are temperature dependent. Their values can be estimated, e.g., by calculating AH* for short temperature intervals. The results obtained by the method of least squares for x, = 0.980 employing 5-K intervals with three temperatures are indicated in Figure 3. It is better, however, to employ eq 3 as proposed
3.1
3.2
3.3
3.4
3.5
3.6
Figure 2. Eyring plots of -In ( k / T ) vs. (1/T) for the neutral hydrolysis of esters in 2-butoxyethanol-water at x, = 0.98. Comparison of the present results ( C ) with those of Holterman and Engbertsl (A and B). (A) p-Methoxyphenyl 2,2-dichloropropionate (-In ( k / t ) - 3); (B) pmethoxyphenyl dichloroacetate (-In ( k / T ) - 2); (C) chloromethyl dichloroacetate.
at higher temperatures AC,* is positive, but at lower temperatures negative. Further, it is seen that, in the mole fraction range studied, the smaller the water content of the solution, the wider is the temperature range that gives a positive value for ACp*. Holterman and Engberts' did their experiments in the temperature range from 20 to 48 OC. So our results with a different ester confirm their positive ACp* (Figure 2) but also show that at lower temperatures the reaction gives a negative AC,*, which is in agreement with other aqueous solutions studied earlier. Positive values of ACp* have earlier been found for the hydrolyses of 4-methoxybenzyl trifluoroacetateI2 and bromomethyl ch10roacetate.l~ In both cases the result was explained by as-
by Clarke and Glew.14 Calculations up to seven parameters show that the five-parametric equation (3) is flexible enough to correctly represent the data. In general, the first five parameters differ from zero at the 98% level. The values of AH* and AC,* calculated from the five-parametric equation (3) are presented in Table I and AC,* values for the three to five parameter equations ( x , = 0.980) in Figure 3. Blandamer et al.ls proposed that a method, actually based on eq 3 with three parameters, can be used to estimate AC,*. The method was employed in the present case (Figure 3). It is seen, as stated by Blandamer et al., that the method underestimates the temperature effect on AC,,*. All three methods, however, give similar estimates for ACp*,which is highly (14) Clarke, E. C. W.; Glew, D. N. Trans. Faraday SOC.1966, 62, 539-541. (15) Blandamer, M. J.; Robertson, R. E.; Scott, J. M. W.; Vrielink, A. J . Am. Chem. SOC.1980, 102, 2585-2592.
Temperature Dependence of C12CHCOO(CICH2) Hydrolysis
J . Am. Chem. SOC.,Vol. 104, No. 20, 1982 5421
Table 1. Kinetic Data for the Neutral Hydrolysis of Chloromethyl Dichloroacetate in 2-Butoxyethanol-Water Mixtures xw
T,"c
0.988
0.07 2.60 4.99 7.41 10.05 12.53 15.02 17.59 20.00 22.70 24.95 27.65 30.14 32.67 35.05 37.71 0.984 2.50 5.03 7.50 7.51 10.05 12.51 15.06 17.45 20.02 22.67 24.91 27.64 30.09 32.63 35.01 37.69 37.69 40.04 42.54 42.54 45.06 0.981
2.61 4.99 7.46 7.55 9.96 12.55 15.02 15.10 17.37 20.03 22.62
AH*,c d kobsd,
s-'
2.3261 f 0.0003 2.7776 f 0.0006 3.2691 f 0.0006 3.8537 i 0.0010 4.5452 f 0.0014 5.3010 f 0.0012 6.1362 f 0.0016 7.1319 f 0.0014 8.209 f 0.003 9.492 f 0.005 10.667 f 0.003 12.152 i 0.009 13.689 f 0.006 15.338 f 0.012 17.041 f 0.015 19.086 f 0.015 2.471 f 0.003 2.937 f 0.003 3.484 f 0.003 3.467 f 0.004 4.042 f 0.002 4.673 c 0.002 5.358 i 0.003 6.108 c 0.003 6.924 i 0.003 7.845 f 0.003 8.673 f 0.004 9.769 f 0.004 10.708 f 0.004 11.690 f 0.006 12.705 f 0.007 13.577 f 0.009 13.634 f 0.018 14.912 f 0.015 16.237 f 0.015 16.213 f 0.009 17.600 f 0.009
9650 f 350 9570 f 200 9390 f 110 9390f 110 9130 f 70 8810 f 80 8430 f 80 8040 f 80 7610 f 60 7170 f 60 6820 f 60 6420 f 70 6120 f 80 5880 f 80 5730 f 80 5670 * 80 5670 f 80 5730 f 130 5930 f 240 5930 f 240 6270 f 390
-12 f 70 -54 f 52 -89 f 37 -89 f 37 -119 f 24 -140 i 15 -156 t 10 -165 * 10 -168 i 12 -163 f 13 -153 f 13 -135 f 11 -111 f 9 -80 i 11 -44f 17 31- 29 3 f 29 50 f 41 107 f 57 107 i 57 171 j: 75
2.2101 f 0.0006 10380 f 210 -631 f 44 2.599 f 0.002 9000 i 130 -530 f 33 3.002 f 0.003 7810 i 70 -433 f 24 7770 f 70 -430 f 23 3.017 f 0.003 3.3640 f 0.0018 6840 f 40 -343 f 15 3.7731 f 0.0019 6070 f 50 -257 f 9 4.1866 f 0.0018 5530 f 50 -182 f 6 4.204 f 0.003 5510 f 50 -180 f 6 4.5277 f 0.0013 5180 f 50 -118f 6 4.9988 f 0.0011 4950 f 40 -54 f 8 5.3823 f 0.0013 4880 i 30 159
temperature dependent, dAC,*/dT being 19 f 1 and 16 f 1 cal mol-' K-* for x, 0.981 and 0.980, respectively. Evidently this is the highest temperature dependence of AC,* ever found for solvolytic reactions. Albery and RobinsonI6 proposed a different mechanistic model to explain the deviations from the Arrhenius equation. Blandamer et al." recently applied it to other solvolytic reactions. It cannot, however, be used in the present case because it always gives a negative ACp*. (16) Albery, W. J.; Robinson, B. H. Trans. Faraday SOC.1969, 65, 980-991.
AH*,c d
24.95 5.8059 f 0.0019 27.66 6.2950 k 0.0019 30.12 6.807 f 0.003 32.68 7.458 f 0.002 35.02 8.005 f 0.003 37.74 8.900 i 0.002 40.08 9.735 f 0.012 42.55 10.680 f 0.011 45.04 11.745 f 0.009
Acp*, cal mol-' mol-' K-' 4930 f 40 43 f 8 5110 f 50 83 i 7 5350 f 50 112 f 6 5670 f 60 135 f 7 6000 ?: 50 1 4 8 f 11 6410 f 50 156 i 19 6780 f 90 155 f 27 7150 f 160 147 f 37 131 f 49 7500 f 260
0.980
2.52 2.1091 f 0.0005 5.04 2.4387 f 0.0006 7.34 2.7496 f 0.0007 10.01 3.0875 i 0.0004 12.47 3.4050 f 0.0005 15.00 3.7140 f 0.0006 17.50 4.0228 f 0.0011 20.04 4.3743 f 0.0005 20.04 4.3717 f 0.0007 22.67 4.754 f 0.002 24.97 5.0643 f 0.0019 27.59 5.521 f 0.003 30.10 5.976 f 0.005 32.66 6.544 f 0.005 35.03 7.167 f 0.004 37.71 7.991 t 0.002 37.75 7.974 f 0.004 40.06 8.621 f 0.008 4 2.54 9.459 f 0.009 45.08 10.69 f 0.03 45.08 10.76 f 0.02
9100 f 280 7800 f 170 6840 f 90 5980 f 60 5400 f 60 5000 i 70 4780 f 70 4710 f 60 4710 f 60 4780f 50 4940 i 50 5220 c 50 5560 f 6 0 5960 f 70 6360 c 60 6820 f 60 6820 f 60 7210f 90 7590f 160 7940 f 270 7940 i 270
0.975
2.48 4.98 7.43 10.06 12.49 15.01 17.59 20.02 22.56 24.98 27.64 30.11 32.63 35.02 37.73 40.05 42.55 45.09
4610 f 240 -103 k 50 4420 f 140 -51 i 37 -6 f 27 4350 f 80 3 7 r 17 4390 f 70 4520 f 60 7 0 f 10 97i 7 4730 f 60 120 f 7 5010 f 60 5320 f 50 134 f 9 144 f 10 5680 i 40 6030 f 40 147 f 10 6420 f 50 143 f 8 134 f 7 6760 f 60 118f 8 7080 i 60 98 ?: 12 7340 f 60 67 ?: 21 7570 f 60 7690 i 90 36 f 30 -4 f 41 7730 f 170 7660 f 280 -51 i 55
Acp*, cal
mol-' mol-' K-' 9980 f 150 -33 f 35 9880 f 80 -41 f 25 -47f 17 9780 f 40 -54 f 10 9660 f 30 9510 f 40 -60 f 5 -66 f 5 9350 f 40 9180f 30 -71 ?: 6 -76 C 7 8990 f 30 -8Ot 7 8800 f 30 8580 f 30 -84 f 6 8390 f 40 -87 f 5 8150 f 40 -90 * 6 7920 f 30 -93 f 11 7680 f 50 -95 f 18 -96 f 27 7460 f 90 7200 f 170 -97 i 38
xw
"c
kobsd, i o - 3 s-'
1.5408 f 0.0006 1.6818 f 0.0006 1.8369 f 0.0011 1.9593 f 0.0006 2.1283 f 0.0014 2.2888 t 0.0012 2.5007 f 0.0005 2.7256 i 0.0004 2.9825 f 0.0006 3.2595 f 0,0009 3.6141f 0.0012 3.9553 f 0.0012 4.412 f 0.003 4.8784 f 0.0018 5.463 f 0.004 6.015 f 0.004 6.669 f 0.003 7.482 f 0.005
-569 f 57 -462 f 43 -372 f 31 -277 f 20 -197 f 12 -122 f 7 -57 f 7 1f9 1f9 5 2 f 11 8 9 f 11 123 f 9 147f 8 164 f 8 171 i 12 171 f 20 171 f 20 163 f 29 147 f 41 122 f 55 122 f 55
The temperature dependence of AC,* will be discussed more precisely elsewhere, employing other esters and other cosolvents in water. At present, we want to warn against drawing too hasty conclusions on the basis of ACp*values calculated from results obtained when studying a limited temperature range or using different temperature intervals. (17) Blandamer, M. J.; Burgess, J.; Duce, P. P.; Robertson, R. E.; Scott, J. W. M.J.Chem.Soc.,Chem. Commun. 1981, 13-14. Blandamer, M. J.; Burgess, J.; Duce, P. P., Robertson, R. E.; Scott, J. W. M. J . Chem. SOC., Faraday Trans. I 1981, 77, 1999-2008.
