1170
transition state for the cation-anion combination reactions is such that part of this outer shell solvation around the cation has been released into bulk solvent, we then expect a negative entropy contribution to AS*. Similarly, on going from the neutral product, Ar3CX, to the transition state, the solvation around the developing cation and anion is largely electrostrictive, and the full second-shell solvation disorder has not been attained. These postulates may ultimately help in understanding the relative reactivities of the cations, a topic which has caused some difficulties arising from the previously' postulated nature of the transition states. Acknowledgments. This work has been supported by grants from the National Science Foundation (GP-29164x), and National Institutes of Health-Public Health Service (GM-12832). Acknowledgment is also made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. We also express our appreciation to Mrs. S. v. McKinley for sending us samples of several of the carboniu m ions used in the present study. References and Notes (1) C . D. Ritchie and P. 0. I. Virtanen, J. Amer. Chem. SOC., 95, 1882 (1973); 94, 3275 (1972). and earlier references cited there for previous papers in this series. (2) R. A. Diffenbach, K. Sano, and R. W. Taft, J. Amer. Chem. Soc., 88, 4747 (1966). (3) J. W. Bunting and W. G. Meathrel, Can. J. Chem., 51, 1965 (1973); 52, 951, 962, 975, 981 (1974).
(4) C. A. Bunton and S. K. Huang, J. Amer. Chem. SOC.,94, 3536 (1972); 95, 2701 (1973); 96, 515 (1974). (5) J. H. Ride, P. A. H. Wyatt, and 2. M. Zochowski, J. Chem. SOC.,Perkin Trans. 2, 1188 (1974). (6) See S. V. McKinley, J. W. Rakshys, Jr., A. E. Young, and H. H. Freedman, J. Amer. Chern. SOC.,93, 4715 (1971). (7) R. H. Boyd in "Solute-Solvent Interactions." J. F. Coetzee and C. D. Ritchie, Ed., Marcel Dekker, New York, N.Y., 1969, Chapter 3. (8) J. W. Larson and L. G. Hepler in ref 7, Chapter 1. (9) H. H. Freedman in "Carbonium Ions," Vol. 4, G. A. Olah and P. v. R . Schleyer, Ed., Wiley, New York, N.Y., 1973, Chapter 28. (10) NG :. Belotserkovskaya and 0. F. Ginzburg, Dokl. Akad. Nauk SSSR, 155, 1098 (1964); frob. Acad. Sci. USSR, 155, 350 (1964). (1 1) Reference 9 quotes pKn = 5\24 for Malachite Green carbinol from E. Cigen, Ph.D. thesis, University of Lund, 1959. (12) C. D. Ritchie, G. A. Skinner, and V. G. Badding, J. Amer. Chem. SOC., 89, 2063 (1967). (13) D. T. Y. Chen and K. J. Laidler, Can. J. Chem.. 37, 599 (1959). (14) W. P.Jencks and N. Gravitz, J. Amer. Chem. SOC.,96, 489 (1974). (15) P. M. Laughton and R . E. Robertson in ref 7, Chapter 7, see particularly D 479ff. (16) H. L. Goering, R . G. Briody, and J. F. Levy, J. Amer. Chem. SOC.,85, 3059 (1963): H. L. Goering and H. Hopf, ibid., 93, 1224 (1971). (17) W. P. Jencks, Chem. Rev., 72, 705 (1972). (18) N. C. Deno, J. J. Jaruzelski, and A. Schriesheirn, J. Amer. Chem. SOC., 77, 3044 (1955). (19) E. F. Caldin, "Fast Reactions in Solution," Blackwell Scientific Publishers, Oxford, England, 1964, p 247. (20) E. M. Arnett, Progr. Phys. Org. Chem., 1, 222 (1963). (21) D. S. Noyce and C. A. Lane, J. Amer. Chem. SOC.,84, 1635 (1962). (22) N. C. Deno, P. T. Groves, J. J. Jaruzelski, and M. M. Lugasch, J. Amer. Chem. SOC.,82, 4719 (1960). (23) W. P. Jencks and M. Gilchrist, J. Amer. Chem. Soc., 90, 2622 (1968). (24) C. D. Ritchie, J. Amer. Chem. SOC.,97, 1170 (1975). (25) C. D. Ritchie, Accounts Chem. Res., 5, 348 (1972). (26) J. E. Dixon and T. C. Bruice, J. Amer. Chem. Soc., 93, 3248 (1971). (27) R . E. Robertson, Progr. Phys. Org. Chem., 4, 213 (1967). (28) W. J. LeNoble. Progr. Phys. Org. Chem., 5, 207 (1967). (29) H. S. Frank and M. W. Evans, J. Chem. Phys., 13, 507 (1945); H. S. Frank and W. Y. Wen, Discuss. Faraday SOC.,24, 133 (1957); see also E. F. Caldin and H. P. Bennetto. J. Solution Chem., 2, 217 (1973).
Cation-Anion Combination Reactions. XIII. Correlation of the Reactions of Nucleophiles with Esters Calvin D. Ritchie Contribution from the Department of Chemistry, State University of New York at Buffalo, Buffalo, New York 14214. Received September 6, 1974
Abstract: The N + correlation of nucleophilic reactivities, previously shown to apply to the reactions of nucleophiles with organic cations, is now shown to apply to the reactions of nucleophiles with esters when explicit consideration of a two-step mechanism is included. The consideration of this mechanism requires that leaving-group abilities of attacking nucleophiles relative to leaving-group abilities of the ester leaving group be evaluated. This evaluation is accomplished by empirical data fitting under the stringent restriction that the leaving-group ability of a given group is assumed to be independent of other groups present in the intermediate. The new correlations extend previous observations to include reactions of 26 electrophiles and 52 nucleophiles. Interpretations of the correlations considerably extend and slightly modify earlier conclusions regarding the mechanisms of nucleophilic reactions with esters. The range of applicability of the N+ equation documented in the present paper is far greater than could be anticipated on the basis of current theories of nucleophiiiweactivity. Our own earlier rationalizations of these correlations are, a t best, incomplete and need thorough reexamination.
The reactions of nucleophiles with acetate esters have been the subject of numerous experimental studies. The rate constants obtained in these studies, particularly those obtained by Jencks and his coworker^,^-^ probably furnish the most extensive body of data obtained under a standard set of conditions that is available for any class of reactions. These data furnish a unique opportunity for testing of hypotheses concerning the effects of structure on reactivity. Figure 1 shows plots of log k for reactions of nucleophiles with phenyl acetate (PA) vs. log k for reactions of the same nucleophiles with other acetate esters., Quite obviously, no simple general linear correlation is shown by these data. Previous interpretations of the rate data have relied on the Journal of the American Chemical Society
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97.5
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application of the Br4nsted catalysis r e l a t i ~ n s h i p , ~in- ~ which limited "structural families" of nucleophiles, such as primary amines, are examined individually. The complexity of the general picture indicated in Figure 1 almost certainly has its origin in mechanisms of the reactions, and interpretations of the Br4nsted relationships have centered on this point. Early discussions of the mechanisms of these reactions postulated the existence of a tetrahedral reaction intermediate. Somewhat later, strong arguments against the existence, of the tetrahedral intermediate were advanced.2 These arguments were based on the apparently reasonable assumption that such an intermediate would have to be a t equilibrium with respect to proton-transfer
March 5. 1975
1171
6l
reactions. Recently, it has been shown that this assumption is not valid for a t least some cases.s In the most recent ~ o r k , the ~ - reactions ~ of nucleophiles with aryl acetates are postulated to proceed by a simple two-step mechanism uncomplicated by proton-transfer reactions of the intermediate (eq I ) .
0 0 0
0
II
/c\ 'HX
+ Aro-
(I)
CH3
,
L -3
Application of the Bodenstein approximation to this reaction scheme gives:
-I
I
3
5
Log k Phenyl Acetate
(1) k0BSd = k , / [ l + k.,/k,l 61 0 for the observed second-order rate constant of the reaction. 0 In our own work, we have demonstrated that the'reacL 0 0 tions of a wide range of nucleophiles yith various types of *e 0 organic cations are well correlated by the equation: 8.'0 log k = log ko + N+ (2 1 8 *L where k is the observed second-order rate constant for the 0 0 0 reaction of a given nucleophile with a given cation in a ' L a given solvent, ko is dependent solely on the identity of the cation, and N+ is dependent on the identity of the nucleophile and on the solvent in which the rate is measured. We noted that eq 2 gives a very good correlation for the rates of , , , , , reactions of nucleophiles with N-acetoxy-4-methoxypyri-5 -3 -I I 3 5 dinium ion (AMPP) and with several substituted N-acetylL o g k Phenyl A c e t a t e pyridinium ions. These esters had been studied by J e n ~ k s ~ , ~ as examples of esters with extremely good leaving groups. If I the two-step mechanism applies to these reactions, the k , step should be rate determining a t least for the most basic 0 nucleophiles. The good correlation of data by eq 2 suggests, 0 0 in fact, the the k, step is rate determining for reactions of nearly all nucleophiles. When we attempted6 to apply eq 2 to reactions of nucleophiles with aryl acetates, we found reasonable correlation for reactions of 2,4-dinitrophenyI acetate (DNPA), but poor correlations for reactions of p-nitrophenyl acetate (PNPA) and of phenyl acetate (PA). The correlation for reactions of DNPA showed rather large deviations for weakly basic nucleophiles. At this point, it was not clear whether the deviations from eq 2 were due to mechanistic -5 -3 -I I 3 5 changes in the reactions or simply due to inapplicability of L o g k Phenyl Acetate the equation to reactions of neutral electrophiles. Kice's observation' that the reactions of nucleophiles Figure 1. Log-log plots of rate constants for reactions of nucleophiles with phenyl a-disulfone (PDS) correlated well with reacwith phenyl acetate vs. those for reactions of other acetate esters. Open circles are for reactions of anionic nucleophiles; closed circles are for tions of the same nucleophiles with AMPP, and, therefore, reactions of amines. DNPA is 2,4-dinitrophenyl acetate; AMPP is N must be correlated by eq 2, suggested strongly that the N + acetoxy-4-rnethoxypyridinium ion. equation might apply to reactions of nucleophiles with neutral electrophiles if the attack of nucleophile is rate deteradjustable quantities that even perfect data fits would not mining. be valid tests of our hypothesis. We hypothesize, then, that the rate constants, k,, in the The most restrictive condition imaginable for the evaluaabove mechanism for reactions of nucleophiles with esters tion of the k - , and k - , parameters is that relative values of are correlated by eq 2 . Substitution of eq 2 into eq 1 gives k - , be required to be independent of the identity of the ester (i.e., k-, is independent of k - y ) . This condition is quite drastic. It requires, for example, that k - , be indepenIn order to test the validity of the hypothesis and the resultdent of the attacking nucleophile. Thus, if the k - , step is ing eq 3, it is necessary to have some way of evaluating the rate determining, the observed rate constant will be directly proportional to the equilibrium constant for formation of quantities k - , / k - , for various nucleophiles and esters. The the tetrahedral intermediate. It also requires that the relaonly available methods for this purpose appear to involve tive value of log k - , for an attacking nucleophile be identidata fitting. If the k - , parameters were allowed to vary for different esters, the parameterization would involve enough cal with the relative value of log k-, when the nucleophile is
",:
Ritchie
/
Correlation of the Reactions of Nucleophiles with Esters
1172 Table I.
No.
Nucleophilic Parameters Nucleophile (solvent) H 2 0(water)
0
CH30H (methanol) DabcoH+ (water)* CF3CHzNH2(water) HINCONHNHz (water) PiperazineH+ (water) Imidazole (water) C6HaSOz- (methanol) CN- (water)
1.18 1.4 2.89 3.17 3.18 3.66 3.67 3.67
14' 15' 16' 17'
CH30NHz(water) H2NCHzCH2NHa+(water) p-OzNC&O- (water) Aniline (water) H?NCH2COZCzH5 (water) Cd-LNHNHz (water) HzNCHzCONHCHzC02- (water) OH- (water)
3.88 3.91 4.05 4.10 4.21 4.38 4.48 4.75
18 19' 20' 21' 22'
Pyridine (water) CHa0CH:CHzNH, (Rater) H2NOH (water) H2NCH2COO-(water) Morpholine (water)
5.00
23 24' 25' 26 27 28' 29 30'
Dabcob(water) CzHjNHz(water) H~NCH~CHZNH? (water) 4-Methylpyridine (water) Quinuclidinol (water) CH3CH2CHzNHz (water) C6H50- (water) HzNNH?(water)
5.3 5.31 5.37 5.40 5.5 5.55 5.6 5 66
31 32' 33'
34' 35'
p-CH3C6H,0- (water) CH3NHNH2(water) CN- (methanol) Piperazine (water) Piperidine (water)
5.8 5.78 5.94 5.94 6.11
36' 37' 38' 39' 40' 41' 42' 43' 44' 45' 46' 37' 48' 49 50 51 52
CF3CHz0-(water) CHaCONHO- (water) OC1- (water) CH30- (water) CH30- (methanol) SO3?-(water) HOO- (water) CN- (dimethyl sulfoxide) Nj- (methanol) CN- (dimethylformamide) Ns- (dimethyl sulfoxide) C6H,S- (methanol) C&S- (dimethyl sulfoxide) Nos- (water) F- (water) CH3COO- (water) N:- (water)
1
2' 3 4' 5'
6' 7 8' 9'
10' 11' 12 13
N,
5.05
5.05 5.22 5.25
6.42 6.94 7.13 7.28 7.68 7.90 8.08 8.60 8.85 9.33 10.07 10.51 12.83 3.04 >3.45 >2,95 7.6
Log k - , re1
Rmsa dev
Electrophiles studied.
5.20
0.40
1,2,3,4,11,12,13,14,15,18,19,20,
45 3 47 4 22 3 14 1 42
0 38 0 08 0 21 0 27 0 33 0 29 0 06 0 31
