Aikaline Hydrolysis of Ethyl Acetate (21) (22) (23) (24) (25)
See paragraph at end of paper regarding miniprinted material. D. R. Lide, Jr., and 5.E. Mann, J. Chem. Phys., 28,572 (1958). W. E. Palke, J. Chem. Pliys., 56, 5308 (1972). J. Timmermans, J. Phys. Chem. Solids, 18, 1 (1961). R. Thomas, Diss. Absfr., 26,3069 (1965).
1509 (26) R . C. Taylor and T. C. Bissot, J. Chem. Phys., 25, 780 (1956). (27) J. D. Odom, S. Riethmiller, J. D. Witt, and J. R. Durig, Inorg. Chem., 12, 1123 (1973). (28) J. N. Gayles, Spectrochim. Acta, Sect. A, 23, 1521 (1967). (29) P. H. Clippard and R. C. Taylor, J. Chem. Phys., 50, 1472 (1969).
Transition State Enthalpies of Transfer in Aqueous Dimethyl Sulfoxide Solutions. The Alkaline Hydrolysis of Ethyl Acetate Richard Fuchs,*l C. Patrick Hagan, and Randolph F. Rodewald Department of Chemistry, University of Houston, Houston, Texas 77004 (Received October 31, 1973; Revised Manuscript Received April 29, 1974)
Enthalpies of solution of ethyl acetate in water, DMSO, and nine aqueous DMSO mixtures, and of tetraphenylphosphonium bromide, sodium tetraphenylborate, sodium bromide, and sodium hydroxide in two aqueous DMSO mixtures have been measured. Using the extrathermodynamic assumption AAH,(Ph4P+) = AAH,(Ph.&) enthalpies of transfer of hydroxide ion from water to aqueous DMSO mixtures have been calculated. AAH, values for ethyl acetate have also been obtained. Transition state enthalpies of transfer for the alkaline hydrolysis reaction have been derived from the AAH, values and experimental enthalpies of activation. Above 15 mol 70DMSO AAH, (transition state) is more positive than AAH, (reactants). Thus, the increasing reaction rates observed with increasing DMSO concentration do not result from the large enthalpy of desolvation of hydroxide ion which is compensated by desolvation of the transition state, but, rather, by an entropy effect.
Introduction A remarkable increase in the nucleophilicity and basicity of anions is observed upon transfer from protonic solvents to dipolar aprotic solvents.2 The effect is particularly large for anions having negative charge localized on a small atom.2 ,3 Frequently, ionic compounds containing small anions (F-? HO-, CH30-) have low solubility in aprotic solvents, and it is fortunate that the high reactivity of these anions is retained to a considerable degree in mixtures of protonic and aprotic solvents in which the appropriate salts are soluble.2,4 Haberfield4 has compared the alkaline ester hydrolysis reaction in two aqueous ethanol and two aqueous dimethyl sulfoxide (DMSO) mixtures. Using the relationship5 AAHt = AAH, AAH*, where AAH, is the enthalpy of transfer of the reactants from one solvent to another, and AAH* is the difference in the activation enthalpies of the reaction in the two solvents, the enthalpy of transfer of the transition state (A.AHt) was determined. Haberfield made the reasonable assumption that the enthalpy of activation for the reaction is essentially equal to that of the first step, the addition of hydroxide ion to the carbonyl carbon atom giving a tetrahedral intermediate. The very large desolvation of hydroxide ion on transfer from aqueous ethanol to aqueous DMSO is only partially reflected in the enthalpy of activation. The transition states, like hydroxide ion, were demonstrated to undergo substantial desolvation upon solvent transfer. Although such discussions of ion and transition state solvation are based only OM the enthalpy contribution to solvation, and ignore the entropy contribution, it was nevertheless surprising that
+
hydroxide was reported4 to be 3.5 kcal/mol loss solvated upon transfer from a less to a more highly aqueous DMSO mixture (mole fraction 0.70- 0.60). We have recently examined single ion enthalpies of transfer among DMSO-water mixtures to 50 mol 70 DMSO.6 Hydroxide ion is increasingly desolvated as DMSO is increased beyond 10 mol %. Rates of alkaline hydrolysis of ethyl acetate have been measured and the activation enthalpies reported7,8 for mixtures of DMSO and water up to 60 mol 70DMSO. If we assume that the molar heat of vaporization is an approximate indication of the hole energy of a solvent, then it appears that large ions6 and neutral moleculesg of low polarizability have a tendency to be desolvated with increasing hole energyy. Since water and DMSO mix highly exothermally, it is likely that the total of all solvent-solvent, forces i s greater in the mixture than in either pure solvent, and that the hole energy of the mixture is also greater. There is, however, no general agreement on which experimental property of a solvent, if any, can be taken as an accurate measure of the hole energy. The enthalpy of transfer of a solute from one solvent to another depends on changes in hole energy, the strength of individual solvent-solute interactions, and the number of these interactions, as well as other factors. From the enthalpy of transfer one sees the change in the sum of these effects. Our surmises concerning hole energies thus represent only one possible model. The transition state for the alkaline hydrolysis of ethyl acetate (which presumably resembles the tetrahedral intermediate) is an anion of considerable size and rather low The Journal o f Physicai Chemistry. Voi. 78. No. 15. 1974
1510
R. Fuchs, C. P. Hagan, and R. F. Rodewald
TABLE I: Enthalpies of Solutiona in Aqueous DMSO at 25 %DMSDb
WZOf 15f 301
50f 60f 651 701 80' 85 90 DMSOf
Mol %"
PhaPBrd
0 5 11 20 27 32 37 50 59 70 11)O
1 . 9 i 0.18 4.0 i0.1 4.6 i O . 1 5.2 f O . l 4.7 i O . l 3.7 i O . 1 3.8 f O . l 2.5 i0.1 2 . 0 i.0 . 1 1.6 f O . l 0.34 f 0 . 0 7
O
NaBPhad
-4.5 -1.3 -0.46 -1,3 -4.2 -6.0 -7.5 -10.8 -12.3 -13.0 -14.3
NaOH
NaBr
i0 . 1 i0.1 i0.03 i0.1 f0.1 i0 . 1 f0.1 i0.2 f0.1 f0.4 f0.1
-0.12 i 0 . 0 1 -0.51 i 0 . 0 3 -0.74 i0.01 -1.1 f 0.1 - 1 . 6 It 0 . 1 -1.9 i0.1 -2.6 i0.1 -3.7 i0.1 -4.3 i 0 . 1 -4.9 i0 . 1 -6.2 i O . l
*
-10.5 -10.5 -10.4 -7.3 -6.4 -5.7 -4.1 -3.4 -2.8 -2.1 0 .O
EtOAc8
10.2 i0.1 i 0.3 i0.1 f0.1 f0.1 f0.1 i0 . 1 i0.1 i O . l
-2.3 i o . 1 -1.1 i0 . 1 0.19 f 0.06 1.6 f0.1 1.9 i0.1 2.0 i0.1 1.8 i O . l 1.5 i 0.1 1.2 i0 . 1 0.93 i 0 . 1 0.62 f 0.04
'
a Values of A H s in kcal/mole are averages of 3-12 determinations on samples a t 10 - 4 t o 10 - 3 M . Per cent by volume. X % DMSO i s made from X volumes of DMSO (100 - X ) volumes of water. Mole per cent DMSO. Ph = phenyl. e Ethyl acetate. Values other than EtOAc in these solvents from ref 6. Average deviation.
+
polarizability. It would therefore be of considerable interest to examine the transition state solvation in the various aqueous DMSO solutions.
TABLE II: Enthalpies of Transfer. from Water to Aqueous DMSO at 25 O
~
% DMSOb
Results
Some newiy measured enthalpies of solution (AH,) are listed in Table I. These, together with previously measured values,6 were used to calculate the enthalpies of transfer in Table 11. Single ion AAH, values were calculated using the assumption6 AAHS[(C6H5)4P+] = AAH,/(C6&)&% -1. The enthalpies of transfer for the reactants, hydroxide ion, and ethyl acetate, together with enthalpy of activation data were used to calculate the transition state enthalpies of transfer in Table 111. From the enthalpies of transfer and previously published free energies of transfer,1° entropies of transfer of ethyl acetate from water to aqueous DMSO mixtures have been calculated, and are presented in Table IV. iscussion T h e Tedruphenylphosphonium Tetraphenylborate Assumption. This assumption for estimating single ion enthalpies of transfer has previously6 been discussed and recommended. In transfers from water to DMSO the PLPBPh4 assumption gives single ion values6 in better agreement with Arnett's valuesll based on the PhrAsBPh4 assumption than does another set of values12 also based on the Ph4AsBPhe assumption. In transfers12 from methanol to DMF, methanol to DMSO, propylene carbonate to DMF, and propylene carbonate to methanol, single ion values for Ph&+ and Ph4P' differ by 0.2 kcal/mol or less, which is less than the combined experimental error limits in measurement. There appears to be no reason to believe that the two assumptions do not give equivalent results. If the two assumptions were not exactly equivalent, or if ANB(Ph4PBr) or AN,(NaBPh4) were somewhat in error, the calculated values of AAH, of all the anions would be incorrect by the same amount.6 Our conclusions in the next section regarding the relative (enthalpy of) solvation of hydroxide ion and an anionic transition state would, therefore, be unaffected. Transition State Enthalpies of Transfer. Over the solvent composition range examined in Table I11 (0-58 mol % DMSO) the rate o f alkaline ethyl acetate hydrolysis7 increases with increasing DMSO concentration, by a factor of about 6. The free energy of activation meanwhile decreases by about l kcal/mol. The enthalpy of activation (or on the other hand, increases by 1.1 kcal/mol, and T A P (at 25") increases by 2.1 kcal/mol. The minimum in The Journal o f Physical Chemistry. Vol, 78. No. 75. 1974
15 30 50 60 65 70 80 85 90
DMSO
AAHs:
PhrPTC
2.8 4.1 3.7 2.3 1.0 0.7 -1.0 -1.7 -2.0 -2.6
Na
0.4 0.9 -0.5 -2.0 -2.5 -3.7 -5.3 -6.1 -6.5 -7.2
Br -
-0.7 - I .4 -0.5 0.5 0.8 1.2 1.6 1.8(1. 2 ) d 1 . 7 ( 1.3)d 1.O
OH-
EtOAc
-0.4 -0.8 3.7 6.1 7.3 10.1 12.4 13.8 14.9 17.7
1.2 2.5 3.9 4.2 4.3 4.1 3.8 3.5 3.2 2.9
a Values of A A H s in kcal/mole. Estimated error 0.2-0.3 kcal/mol. Per cent by volume. AAHs(Ph4B-) = AAH,(PhiP-r). Values in parentheses from ref 16.
TABLE 111: Transition State Enthalpies of transfer^^^ in the Base-Catalyzed Hydrolysis a6 Ethyl Acetate at 25" %
Mol
HgO 10 20 40 60 80 84.5
AAH,-
Eabse
DMSOC
0 3 6 15 28 51 58
(OH-)'
11.4 11.2 11.0 11.2 11.7 12.3 12.5
-0.2 -0.4 -0.2 0.3 0.9 1.1
-0 3 -0.8 1.0 6 0 12.2 13.4
AM€,-
(EtOAc)b
AAHt
0 9 1.7 3.3 4 2 3.8 3.6
0.4 0.5 4.1 10.5 16.9 18.1
*
a Reference solvent: water. Kcal/mole. Solvent is 100 - (% DMSO) mi water per 100 nil total volume. Mole per cent DMSO. e Energy of activation from ref 7.
the plot of AH* us. solvent composition occurs at about 10 mol % DMSO. Tommila7 discussed the form of the curve in terms of changes in solvation of the reactants and transition state, and attributed the AN* minimum to a minimum in transition state s01vation.l~The present results (Table ID) indicate that the principal enthalpy factor operative at this composition is desolvation of ethyl acetate by 1.7 kcal/mol, relative to the solvent water. Hydroxide ion reaches a solvation maximum at this point (in terms of enthalpy). The transition state AAH, becomes increasingly positive with DMSO addition throughout the entire solvent composition range, but this becomes more pronounced at DMSO concentrations greater than about 10 mol %. It has been recognized that the free energies of transfer of neutral moleculesll and of ions14 from water to organic solvents are determined by the T A A S term as much as or more than by AAfi,. Unfortunately, comparable A A G values for hydroxide ion are not presently avail-
Alkaline Hydrolysis of Ethyl Acetate
1511
TABLE I V Free Energy, Enthalpy, and Entropy of Transfer of Ethyl Acetate from Water t o Aqueous DMSO at 25’
__
I -
70
Mol
DMSO“
%b
30.5 50 63
1:I 20 30
0 .088 0 . I05 0 ,1324
72.5 80
40
--0.134
50 5!3 6’7 80
-0.270 -0.421 -0.567 -0.675 -0.755
85 89
94 97.5
9’1
Log
”rsc
A A G ~ . ~
0.12 0.14
0.03 -0.18 -0.37 -0.57 -0.77 -0.92 -1.0
A A H ~ ~TAAS~J
2.58 3.9 4.28 4.08 3.8 3.5 3.3e 3.10 3.08
2.4 3.8 4.2 4.2 4.2 4.1 4.1 4.0 4.0
Per cent by volume. Mole per cent DMSO. Solvent activity coefficients of EtOAc from ref 10 Mca1,mole. e A A G = 2.303RT log wys. T A A S = IAN. - A A G . g Values estimaled graphically.
’
able.15 Whereas AAH, values for ethyl acetate suggest desolvation in all aqueous DMSO mixtures relative to water, the more appropriate criterion of solvation, AAG, indicates slight desolvation at low DMSO concentrations, and then increased solvation with increasing DMSO. Both AAH, and TAAS reacE maxima at about the concentration of maxirrium solvent structure, DMSO-BHzO (33 mol %). However, T A A S varies only slightly above 20 mol % DMSO, whereas AAW, decreases continuously, resulting in the observed decrease in A A G The fact that AG* decreases by about 1 kcal/mol over the range of 0-58070 DMSO, while AH* increases by 1.1 kcal/r~lol,~ indicates that the principal effect of solvent change in this system is a nonenthalpy (entropy) one. In view of the large pos tive AAH, values for OH- and EtOAe on transfer from water to the mixed solvents (Table a), it is surprising that AH* does not diminish. However, at geater than 15 mol % DMSO AAHt (for the transition state) is more positive that those of the reactants combined (Table In). Assuming that the transition state rather closely resembles the tetrahedral intermediale 0-
I
CE&-C-OCH,
I
OH
one would expect substantial desolvation with increasing DMSO Concentration, first, on the basis of the inability of DMSO to strongly solvate a negatively charged oxygen atom (as with HO-)> and, second, because of the large “hole” energy which must be expended to accommodate this large ion within the highly structured mixed solvents, which is greater than that required to make the “small hole” for the hydroxide ion. It is not immediately obvious whether AAHt should be smaller than AAHs (reactants), of equal size, or slightly !arger, as is observed.
A previous report4 that hydroxide ion has a positive enthalpy of transfer from 70 to 60 mol % aqueous DMSO is in disagreement with the present results (Table 11), which show a value of -1.1 kcal/mol (90 85 vol %; 70 59 mol %). Haberfield4 derived AAH,(OH-) from the AAH, value for aqueous tetrabutylammonium hydroxide, and AAHS(Bu4N+). Since the latter value is in agreement with a previous transfer value for tetrabutylammonium ion,la it is probable that an erroneous value of AAH,(OH-) resulted from the procedure of working with small differences between very large values of AH,(Bu4NOH.130.9HzO) and AHs(130.9H20). Values of AAH,(OH-) in Table I11 are based on direct AHs measurements using anhydrous sodium hydroxide. Corrections for large amounts of water were, therefore, unnecessary.
-
-
Experimental Section Ethyl acetate (Baker Analyzed reagent grade) samples were introduced into the calorimeter using a 10-@1syringe. The purification of solvents and other reagents and the calorimetric procedure have been previously described.6 Sample concentrations were to M . No dependence of AH, on concentration was noted in this range. References and Notes (1) Support of this research by the Robert A. Welch Foundation (Grant No. E-136) is gratefully acknowledged. The authors also wish to thank Professor A. J. Parker for helpful discussions. (2) A. J. Parker, Chem. Rev., 69, 1 (1969). (3) R . Fuchsand L. L. Cole, J. Amer. Chem. SOC.,95,3194 (1973). (4) P. Haberfield, J. Friedman, and M . F. Pinkston, J , Amer. Chem. SOC.,94, 71 (1972). (5) E. M. Arnett, W. G. Bentrude, J. J. Burke, and P. M. Duggleby, J. Amer. Chem. SOC.,87, 1541 (1965). (6) R. Fuchsand C. P. Hagan, J. Phys. Chem., 77, 1797 (1973). (7) E. Tomrnila and M. L. Murto, Acta Chem. Scand., 20, 923 (1966). (8) D. D. Roberts, J. Org. Chem., 30, 3516 (1965). (9) R. Fuchs and R. F. Rodewald, J. Amer. Ghem. SOC., 95, 5897 (1973). (10) E.G. Coxand P. T. McTigue,Aust. J. Chem., 20, 1815 (1967). (11) E. M. Arnett and D. R. McKelvey, J. Amer. Chem. SOC., 88, 2598 (1966). (12) C. V. Krishnan and H. L. Friedman, J. Phys. Chem., 73, 3934 (1969); 75, 3606 (1971). 13) It should be recognized that Professor Tornmila wrote this discussion more than a decade ago, when no experimental values of AAH, or AAG were available for the reactants or transition state. In spite of this the major points of his discussion are remarkably correct. 14) B. G. Cox and A. J. Parker, J. Amer. Chem. SOC, 95,402 (1973). 15) Values of AAG for hydroxide ion from water to aqueous DMSO mixtures have been reported [A. K. Das and K. K. Kundu, J. Chem. SOC., Faraday Trans. 7 , 69, 730 (1973)l. These are based on an extrathermodynamic assumption [D. Feakins and P. Watson, J , Chem. Soc., 4734 (1963)]. which, for the halide ions [K.H. Khoo, J. Chem. SOC. A, 2932 (1972)] gives values surprisingly similar to one another, and for chloride ion gives a value far larger than that based on the PhaMBPha type of assumption.2 We velieve that the use of these A A G ( 0 H - ) values with the present L3Hs values would not provide meaningful conclusions. (16) R. Fuchs, D. S. Plumbee, Jr., and R. F. Rodewald, Thermochim. Acfa, 2, 515 (1971).
The Journal of Physfcai Chemistry Voi. 76 No 15 7974
