Solvolysis rates in aqueous-organic mixed solvents. IX. Solvolysis of dichloroacetate ion in water-methanol solutions

EL-HUSSIENY M. DIEFALLAH,' MOHAMED A. ASHY, AND AHMED 0. BAGHLAF Department of Chemistry, King Abdulaziz University, Jeddah, Saudi Arabia

Received August 19, 1980

EL-HUSSIENY M. DIEFALLAH, MOHAMED A. ASHY, and AHMED 0. BAGHLAF. Can. J. Chem. 59, 1208 (1981). The kinetics of the alkaline solvolysis of dichloroacetate ion in water-methanol solutions have been studied in the temperature

range of 50.0 to 65.0°C and the influence of solvent variation on reaction rate has been examined in terms of changes in the activation parameters. The activation parameters AH* and AS+ for the solvolysis reaction showed a minimum at about 0.8 water mole fraction. The significance of the results was discussed in view of the electrostatic theory and the changing of solvent structure.

EL-HUSSIENY M. DIEFALLAH, MOHAMED A. ASHY et AHMED 0. BAGHLAF. Can. J. Chem. 59, 1208 (1981). On a etudie la cinetique de la solvolyse alcaline de I'ion dichloroacetate dans des solutions de methanol-eau dans I'intervalle de

temperature allant de 50,O a 65,0°C et on a examine I'effet de la variation du solvant sur la vitesse de reaction en fonction des variations des parambres d'activation. Les paramktres d'activation AH+ et AS* de la reaction de solvolyse atteignent un minimum lorsque la fraction molaire de I'eau est d'environ 0,8. On discute de la signification de ce resultat par rapport a la theorie electrostatique et au changement de la structure du solvant.

[Traduit par le journal]

Introduction Mixed solvents, in which water was frequently

one component, provided a graded series of solvo- lytic media in which to explore the relation between rates, mechanism, and the effect of changing sol- vent properties (1-4). In alcohol-water binary solvent systems, an extremum in the activation parameters towards the pure water end of the composition scale has been observed (5-13). The extremum behaviour seems to reflect a common solvent behaviour (12-14) and has been related to the accompanying solvent reorganization attending the activation process in binary alcohol-water systems (13). In the present investigation, we report the results of the study of the kinetics of the alkaline solvolysis of dichloroacetate ion in water- methanol solutions. Studies of the alkaline sol- volysis of mono- and trichloroacetate ions in binary alcohol-water systems were recently published (5-8). Comparisons between the mono-, di-, and trichloroacetates have shown that there are large differences in order, specific rate constant, and mechanisms of the solvolysis reactions of these ions.

mixture, the concentration of the base was always less than about twice that of dichloroacetate ion in the reaction solution.

Results and Discussion Dilute alkaline solutions of sodium dichloroace-

tate turn acidic on standing due to the hydrolysis of dichloroacetate ion to yield glyoxylate and hydro- chloric acid (15). The rate of the reaction follows the second-order rate law:

where A, is the initial concentration of dichloroac- etate ion, B, is the initial concentration of base, A and B are the concentrations of dichloroacetate ion and hydroxide ion at a time t during the reaction, and k is the specific reaction rate constant. The rate constants were evaluated from the slopes of the straight lines obtained when In (ABo/AoB) is plotted against t . Figure 1 shows typical plots in water- methanol solutions at 55.0°C. The mechanism of the reaction would thus involve the initial formation of the unstable chlorohydroxyacetate intermediate which, in turn, eliminates hydrochloric acid to yield glyoxylate:

Experimental OH

The rate of reaction was determined in the temperature range -C1- I 50.0 to 65.0°C, using a titrimetric method by following the [2] CHC12'C00- f OH- - H-y-COO- (slow) release of chloride ions according to Volhard's technique (6-8). The initial concentrations of dichloroacetic acid were in the range 0.2 to 0.5 mol/L, while the initial concentrations of the base were in the range 0.05 to 0.4mol/L. In each reaction OH

I -HCI 1 I 'Author to whom all communications should be addressed. [31 H-C-COO- - H-C-COO- (fast)

Present address: Department of Chemistry, Assiut University, Assiut, Egypt.

I C1

0008-404218 1/08 1208-04$01.00/0 01981 National Research Council of CanadaIConseil national de recherches du Canada

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DIEFALLAH ET AL. 1209

Time ( h )

FIG. 1 . Second-order plots at 55.0°C in water-methanol solu- tions. Water mole fraction: @, 1.00; 0, 0.941; x , 0.806; A, 0.727; 8, 0.542; 0, 0.308.

Table 1 lists the values of the rate constants obtained at the specified temperatures and the medium dielectric constant (D) of the various solvent mixtures investigated. The rate constants represent the average of at least three determina- tions and the reported errors are average errors. In each solvent mixture, D was taken from the experimental data of Akerlof (16) or calculated from these values by interpolation between differ- ent temperatures or between different solvent mixtures. The results show that the rate of reaction first increases as the methanol content of the solution increases up to a maximum at about 0.8 water mole fraction, and then decreases. The behaviour of the solvolysis rates shows similarity to that of monochloroacetate ion in the same medium (6) but is different from those of the decarboxylation reaction of trichloroacetate ion (8). This would be due to the fact that the rate of the alkaline solvolysis in the mono- and dichloroace- tates is determined by the attack of the hydroxide ion on the respective ion, whereas the rate of the alkaline decarboxylation of the trichloroacetate ion is independent of the concentration of the hydrox- ide ion and the rate-determining step is the breaking of the carbon-carbon bond with the concomitant release of C 0 2 and the formation of trichloromethyl

carbanion intermediate (7, 8). In solvent mixtures with compositions in the range 0 to 30% by weight methanol (1.0 to 0.8 water mole fraction), the results are not in the same order as that predicted by the simple electrostatic theory (17) and indicate that there is no simple correlation between the dielectric constant and the rate constant (18).

The energy of activation of the reaction was calculated from the temperature coefficient of the reaction rates by the least-squares method. The enthalpy and entropy of activations were calculated from the rate constants using absolute rate theory equations (19) and the results (calculated at 50.O"C) are tabulated in Table 2. Entropy of activation values are all negative in all solvent mixtures investigated indicating that in all of these mixtures the activated complex structure is relatively more ordered than the structure of the separated reac- tants. It is seen that when the concentration of methanol in the solvent mixture is increased, the enthalpy and entropy of activation first decrease to a minimum at about 0.8 water mole fraction, and then increase. The positions of the AH* and A S * minima appear to coincide, which is characteristic of binary solvent mixtures in which both compo- nents are hydroxylic. The depth of the A H * mini- mum relative to pure water is about 4 kcallmol and that for AS* is 12cal deg-I mol-I. These values are the same as those observed for the solvolysis of monochloroacetate ion (6) and the trichloroacetate ion (7) in the same media. The calculated values of the free energy of activation, AG* (listed in Table 2), show that they do not change much with solvent composition. This must be due to the linear com- pensation between A H * and AS*. The plot of AH* versus A S * gave rise to a straight line (Fig. 2), the slope of which, known as the isokinetic or isoequi- librium temperature, equals 320 K. Linear AH*- AS* relationships were also observed for the sol- volysis of monochloroacetate (5, 6) and trichloro- acetate ions (7, 8) and for other systems (12, 19). This behaviour is expected ifthe solvents in a series perform closely similar roles in the reaction (18).

The solvolysis of dichloroacetate ion in water- methanol solutions showed that for solvent mix- tures with compositions in the range 1.0 to 0.8 water mole fraction, the principal contribution toward increasing rate is from the decrease in AH* which outweighs the decrease in rate caused by a decrease in AS*. However, at water mole fractions less than about 0.8, A S * increases by about 15 cal deg-I mol-I between 0.8 and 0.3 water mole fraction, but its effect on the rate of reaction is far outweighed by the increase in AH* (which is about

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CAN. J. CHEM. VOL. 59, 1981

TABLE 1. Effect of temperature and solvent composition on the rate constants of the alkaline solvolysis of dichloroacetate ion in water-methanol solutions

Methanol Water mole Temperature Dielectric k x lo7 (%by wt.) fraction "C constant (L mol-' s-I)

TABLE 2. Activation parameters (at 50.0°C) for the alkaline solvolysis of dichloroacetate ion in water-methanol solutions

Water mole E, AH' -AS' AG' fraction (kcallmol) (kcallmol) (cal deg-I mol-') (kcallmol)

1.00 23.5 22.8 16.4 28.1 0.941 21.7 21.1 21.2 27.9 0.877 19.6 18.9 27.5 27.8 0.806 18.9 18.3 29.4 27.8

5 kcal mol-' in the same composition range), which . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . causes a net decrease in reaction rate for solvent

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . mixtures with less than about 0.8 water mole fraction. Comparison of the results obtained for the solvolysis of mono-, di-, and trichloroacetates indicated that in the same solvent system the activation parameters may show similar variations irrespective of the reaction substrates, order, and mechanism. The results reflect a common solvent behaviour and emphasize the previously stated

necessity of taking structural properties of the solvent into consideration.

The behaviour of the activation enthalpy and entropy towards solvent change observed here is similar to that observed for many solvolytic reac- tions in aqueous-organic solvent mixtures under a variety of conditions (2-6, 10). Arnett et al. (14) have shown that the appearance of the minimum in aqueous-organic solvent mixtures is readily ex- plained on the basis of the behaviour of heats of

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DIEFALLAH ET AL. 121 1

FIG. 2. A plot of AH* versus AS* for the alkaline solvolysis ' of dichloroacetate ion in a series of water-methanol solutions.

solution in aqueous alcohols. It was concluded that both heats of solution and the enthalpy of activa- tion were related to the same kind of changes in solvent structure with changing composition. The results reported here indicate that the activation parameters AH* and AS* pass through a minimum at about the same composition range where other tests showed extreme behaviour, which could be attributed to a change in structural stability with composition (12-14). Hence, the behaviour of AH* and AS* is interpreted on the basis that the break-

down of solvent structure, as a consequence of charge separation in the activation process, paral- lels the sensitivity to changes in the solvent compo- sition (13, 14).

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