Transition state volumes and solvolysis mechanisms

Size: px

Start display at page:

Download "Transition state volumes and solvolysis mechanisms"

Clarence Garrison
5 years ago
Views:

1 Transition state volumes and solvolysis mechanisms MOYRA J. MACKINNON, A. B. LATEEF,' AND J. B. HYNE Department of Chemistry, The University of Calgary, Calgary, Alberta Received January 29, 1970 The transition state partial molal volume behavior, rt, as a function of binary solvent composition was obtained for three reactjons bl dissection of the activation volume, AV*, into initial and transition state components: AV* = V, - Vg. The solvolyses of t-butyl chloride, benzylchloride, andp-chlorobenzyl chloride represented a gradation of reaction type between SNI and SN2 and the transition state partial molal volume behavior was found to be distinctly different in each case and in agreement with the mechanistic classification of these reactions. Canadian Journal of Chemistry, 48, 2025 (1970) Introduction de-ionizing resin and then redistilled in a Pyrex apparatus from which carbon dioxide was excluded. The con- Golinkin, Lee, and Hyne (1) have reported a ductivity of the water was about 8 x lo-' a-' cm-' method for obtaining partial molal volumes of at 25 "C. solutes in binary solvent mixtures, vg, and have p-chlorobenzyl Chloride used these volumes in conjuction with pressure- P-Chlorobenz~l chloride supplied by Eastman Organic Chemicals was analyzed by gas-liquid chromatography rate data, ~vx, to obtain the partial molal and found to contain about 2% impurity. After two "lumes the state, K, in solvo1ysis recrystallizations from ether, samples obtained from reactions by use of the equation A V* = vt - vg. liquor and crystal fractions varied in impurity from almost A "roller+oaster" type curve was obtained for none to about 15%. Kinetic runs with these different the transition state behavior in aqueous ethanol samples yielded perfect straight-line first-order plots and the rate constants obtained were identical to within of benzyl which is generally accepted as experimental error (+ 0.5 %). It was concluded that the solvol~zing by an SN~ mechanism close to the impurity did not affect the kinetic results. The crystals SN1 limit (2). The present investigation was with negligible impurity, obtained from two crystallizaundertaken to compare this type of behavior with tions from ether, were used throughout the investigation. that for the transition states for solvolysis reac- t-butyl Chloride t-butyl chloride supplied by British Drug Houses was more 'N' and S~2 in character' found to about 3% impurity when analyzed by A reaction, the solvolysis Of t-butyl gas-liquid chromatography. After two distillations under chloride in aqueous ethanol, was chosen since vacuum the impurity was reduced to about 0.5 %. Partial pressure-rate data were already available (3). molal volumes determined with samples containing 0.5 Also chosen was the solvolysis ofp-chlorobenzyl and 3 % impurity were identical; it was therefore conchloride since it reacted at an experimentally cluded that the impurity did not seriously affect the partial molal volume determinations. suitable rate and represented a reaction more Benzyl Fluoride 'N2 in character than benz~l (2). Laidler Benzyl fluoride supplied by Pierce Chemicals was used and Martin (4) have reported a similar dissection without further purification for partial molal volume for p-methylbenzyl chloride in acetone-water determinations. mixtures. However, since the solvolyses con- Ethanol sidered in our analysis were carried out in Absolute alcohol from Reliance Chemicals was dried ethanol-water mixtures, a direct comparison over magnesium turnings and then distilled (5). Binary mixtures with water were prepared by weight and stored cannot be made with the acetone-water results. in glass-stoppered bottles. These reactions were therefore studied to determine if there existed some regular variation in the Kiy;f sates of hydrolysis ofpihlorobenzyl chloride and transition state partial molal volume behavior in of t-butyl chloride were studied in ethanol-water mixbinary solvents with change in mechanism. tures over a ~ressure ran~e from 1 to atm. The rate of of HCI resulting from the hydrolysis was Experimental followed conductimetrically. The experimental details Materials have been previously reported (6) except that in this work an automatic balancing a.c. conductance bridge with Water digital print-out (General Radio type 1680A) was used. House distilled water was first passed through Illco-Way Pseudo first-order rate were obtained by 'Present address: Chemistry Department, Youngstown graphical analysis of the data by the Guggenheim method State University, Youngstown, Ohio, U.S.A. (7). The high pressure systems and controls have also

2 CANADIAN JOURNAL OF CHEMISTRY. VOL. 48, 1970 mww 000N-N I +I +I +I +I +I ooma -m--on '?'=????" mw-a I +I +I +I +I amom -0---

3 MACKINNON ET AL.: TRANSITION STATE VOLUMES AND SOLVOLYSIS MECHANISMS 2027 TABLE 2 Rate constants for the solvolysis of t-butyl chloride at 0.2 "C in aqueous ethanol at various pressures k x lo5 (s-') for mole fraction of ethanol = Pressure (atm) been previously described (6). The temperature in a thermostatted oil bath was maintained at k0.01 "C in the case of the p-chlorobenzyl chloride and at 0.20 k0.01 "C in the case of the t-butyl chloride. Partial Molal Vol~imes The method for determination of partial molal volumes by injection from a microsyringe into a dilatometer as previously reported (1) was used unchanged. Calibration of the dilatometer was done by using the known molal volumes of water and ethanol to obtain apparatus constants when pure water was injected into water, and pure ethanol into ethanol at the temperature of the thermostatted oil bath. Partial molal volumes were obtained for four solutes in ethanol-water mixtures which were degassed before use by boiling in a flask fitted with a Dry Icelacetone condenser. Determinations of densities before and after degassing showed no change in composition of the aqueous binary solvents (changes in density were no more than 1 part in ). Partial molal volumes of benzyl chloride, p-chlorobenzyl chloride, and benzyl fluoride were measured at "C in a thermostatted oil bath. A glycol-water bath at "C was used for t-butyl chloride volume determinations. Calculation of the partial molal volumes required the densities of the solutes at the temperature of injection. The densities of benzyl chloride and t-butyl chloride were taken from refs. 8 and 9. The densities of p-chlorobenzyl chloride and benzyl fluoride were determined with a micropycnometer calibrated with distilled water. Density of benzyl chloride at 32.5 "C, 1.02, g/ml; p-chlorobenzyl chloride at 31.0 "C, g/ml. Results Rates The conductance data yielded the first-order rate constants for p-chlorobenzyl chloride in ethanol-water mixtures at "C as a function of pressure. These rate constants, presented in Table 1, are the average of at least two, and in many cases four or more, runs with the indicated errors being average deviations. The fourth figure in some of these rates is not significant, but merely a guard figure when the error is quoted to this degree of accuracy. The rate constants for t-butyl chloride at 0.2 "C are presented in Table 2. Although there is still considerable discussion as to the most appropriate form of the rate-pressure relationship (3), we believe (10) that for solvolytic systems of this type the quadratic yields a best fit and the solvent dependence of the extracted AV* values differs little from that obtained by use of other forms of the rate-pressure relationship. Whalley and Baliga (3) have obtained some values of AV* for the solvolysis of t-butyl chloride at 0 "C and these are presented in Table 3, in addition to our values of A V* for t-butyl chloride and p-chlorobenzyl chloride. The possible errors given in our AV* values are the standard deviations from the quadratic analysis, although more realistic experimental uncertainties might be to 1.0 ml/mole. Also included in Table 3 are values of AV* for benzyl chloride as previously reported by Golinkin, Lee, and Hyne (1). The value in pure water for benzyl chloride is a revised value supplied by Mr. S. J. Dickson of this laboratory. Partial Molal Volunles The partial molal volumes for benzyl chloride, p-chlorobenzyl chloride, t-butyl chloride, and benzyl fluoride in ethanol-water mixtures are presented in Table 4. These values are the average of at least four determinations with their average deviations as indicated. The volumes obtained by Golinkin, Lee, and Hyne (1) for benzyl chloride were in reasonable agreement (about 1 %) with the present results except for their value at 0.3 mole fraction ethanol of ml/mole as compared to our value of ml/mole. From the activation volume, AV*, and the partial molal volume of the initial state, Vg, the partial molal volume of the transition state, F, can be calculated, AV* = I/, - V,. Figure 1 represents the dissection of the volume parameters for p-chlorobenzyl chloride, benzyl

4 CANADIAN JOURNAL OF CHEMISTRY. VOL. 48, 1970 TABLE 3 Activation volumes for the solvolysis of p-chlorobenzyl chloride, benzyl chloride, and t-butyl chloride in ethanol-water mixtures A V* (ml/mole) Mole fraction ethanol p-chlorobenzyl chloride* Benzyl chloride? t-butyl chloride "Present work 'C. thyne, ~olinkin, and Laidlaw O C (6). bbaliga and Whalley, O C (3). The authors wish to thank Drs. Baliga and Whalley for the t-butyl chloride AV* data prior to publication. Present work, 0.20 'C. TABLE 4 Partial molal volumes in aqueous ethanol mixtures of benzyl chloride, benzyl fluoride, and p-chlorobenzyl chloride at "C, t-butyl chloride at 1.35 "C Partial molal volume (ml/mole) Mole - fraction Benzyl Benzyl p-chlorobenzyl t-butyl ethanol chloride fluoride chloride chloride chloride, and t-butyl chloride using the data in Tables 1-4. Figure 2 is a plot of our partial molal volumes of benzyl fluoride and those of anilinium chloride, tetraethylammonium chloride, and tetramethylammonium chloride, as reported previously by Lee and Hyne (1 1, 12), whose figures are included for comparison with our results. Although we are comparing Vt curves obtained at different temperatures for different compounds (p-chlorobenzyl chloride and benzyl chloride at "C, t-butyl chloride at 1.35 "C) the temperature variation should not invalidate a comparative analysis. Millero and Drost-Hansen (13) report that the apparent molal expansibility of tetramethylammonium chloride (0.053 m) over "C is about ml mole-' deg-'. Dickson (14) has found a maximum temperature dependence of A V* for benzyl chloride in aqueous t-butanol of ml mole-' deg-' and Lateef and Hyne (15) found AV* changed by about ml mole-' deg-' for the solvolysis of ally1 chloride in aqueous ethanol. These small temperature effects of comparable magnitude on both sides of the equation AV* = Vt - Vg should cancel within the accuracy of our experimental results in the calculation of Vt.

5 MACKINNON ET AL.: TRANSITION STATE VOLUMES AND SOLVOLYSIS MECHANISMS 2029 p-chlorobenzyl Chloride Benzyl Chloride t-butyl Chloride 1240 I I I I 112 I I I I 1020 I I I I Mole Fraction EtOH FIG. 1. Volume parameters for the solvolysis of p-chlorobenzyl chloride, benzyl chloride, and t-butyl chloride in aqueous ethanol. Mole Fraction EtOH FIG. 2. Partial molal volumes as a function of mole fraction ethanol of benzyl fluoride, tetraethylammonium chloride, tetramethylammonium chloride, and anilinium chloride. Discussion The dissection of the observed volume of activation behavior into initial and transition state componentsfor the three solvolysis reactions is presented in Fig. 1. In all these cases AV* passes through a minimum as a function of solvent composition and the initial state partial molal volume passes through a maximum. The transition state volume behavior, however, is distinctly - different in each case. A maximum in Vt is observed in the more SN2 type p-chlorobenzyl chloride solvolysis while a very distinct minimum characterizes the Vt behavior for the S,1 t-butyl chloride solvolysis. The Vt behavior for benzyl chloride is clearly intermediate between these two extremes. It would therefore appear that the behavior of the partial molal volume of the transition state as a function of solvent composition is directly related to the position occupied by the reaction in question on the mechanistic scale from SN2 to S,1. The observed maximum in the volume of the transition stateforp-chlorobenzylchloride should be compared with the similar behavior for all the initial states, Vg, andfor benzyl fluoride (Fig. 2). This maximum behavior appears to be characteristic of all non-ionic substrates. The very pronounced minimum in volume observed for the transition state of t-butyl chloride, however, is characteristic of the behavior of ionic species, as was demonstrated for tetramethylammonium and anilinium chlorides by Lee and Hyne (11, 12) (Fig. 2). The transition state behavior of benzyl chloride was shown by Lee

6 2030 CANADIAN JOURNAL OF CHEMISTRY. VOL. 48, 1970 and Hyne (11) to be more similar to the partial molal volume curve for tetraethylammonium chloride, indicating less charge development in the transition state than is represented by tetramethylammonium chloride. - The behavior of the transition state volumes V, in the three cases, therefore, is in general agreement with the mechanistic classification of the three reactions. The transition state for p-chlorobenzyl chloride is more dipolar than ionic (SN2); the transition state for t-butyl chloride is well simulated by the ionic tetramethylammonium chloride (SN1); while that for benzyl chloride is intermediate between the two extremes as was previously suggested by the similarity of the V, behavior with that of tetraethylammonium chloride. These findings suggest that the solvent dependence of the partial molal volume of a solvolytic transition state obtained by dissection of the observed activation volume can serve as a useful yardstick in the characterization of reaction mechanism. The authors wish to thank the National Research Council of Canada for financial assistance in support of this work. 1. H. S. GOLINKIN, IKCHOON LEE, and J. B. HYNE. J. Amer. Chem. Soc. 89, 1307 (1967). 2. E. T. TOMMILA, E. PAAKKALA, U. K. V~RTANEU, A. ERVA, and S. VARILA. Ann. Acad. Sci. Fenn. All (gi), i (1959). 3. B. T. BALIGA and E. WHALLEY. Can. J. Chem K.. J. LAIDLER and R. MARTIN. Int. J. Chem. Kinetics, 1, 113 (1969). 5. A. I. VOGEL. A textbook of practical organic chemistry. 3rd ed. Longmans. Green and Co.. Ltd.. \-- -,- 6. J. B. HYNE, H.'S. GOLINKIN, and W. G. LAIDLAW. J. Amer. Chem. Soc. 88, 2104 (1966). 7. E. A. GUGGENHEIM. Phil. Mag. 2 (7), 538 (1926). 8. A. I. VOGEL. J. Chem. Soc. 644 (1948). 9. J. TIMMERMANS. Physico-chemical constants of pure organic compounds. Elsevier Publ. Co., Inc., Amsterdam H. S. GOLINKIN, W. G. LAIDLAW, and J. B. HYNE. Can. J. Chem. 44, 2193 (1966). 11. IKCHOON LEE and J. B. HYNE. Can. J. Chem. 47, 1437 (1969). 12. IKCHOON LEE and J. B. HYNE. Can. J. Chem. 46, 2333 (1968). 13. F. J. MILLERO and W. DROST-HANSEN. J. Phys. Chem. 72, 1758 (1968). 14. S. J. DICKSON. Ph.D. Thesis, University of Calgary Calgary, Alberta A. B. LATEEF and J. B. HYNE. Can. J. Chem. 47, 1369 (1969).

Enthalpy and entropy of activation for benzyl chloride solvolysis in various alcohol-water solvent mixtures

Enthalpy and entropy of activation for benzyl chloride solvolysis in various alcohol-water solvent mixtures H. S. GO LINK IN^ AND J. B. HYNE Departtnetlt of Clremistry, Vt2iversity of Calgcrry, Calgary,