N UTES
Vol. 73
sulfoxy group in the transition state against the dipole of the neighboring group. In the case of the complex neighboring groups such as acetoxy and p-bromobenzenesulfoxy, a priori calculation of the carbonium rates was not readily feasible. I t turned out2 that the cis- and trans-cyclohexane 1,2-di-p-bromobenzenesulfonates reacted a t slow Table I shows the results of trials on solutions and nearly equal rates. The cis-2-acetoxycyclohexyl p-bromobenzenesulfonate also reacted slowly, of 0.393 equivalent of selenious acid per 25 i d . but the trans-ester was several thousand times TABLE1 faster. This contrasting behavior suggested2 that Cunversion t u the trans-2-acetoxy group was furnishing a driving Equ1v I.? l - f o l d exceCb 1 HzSeOr To force which the cis-group, due to its geometry] 46 6 1.2 3 could not provide. To estimate this driving force, 38 0 2.8 it was assumed that the carbonium rate kc for the 59 8 3.0 7 3 tvans-compound was nearly equal to the observed 70.8 4.5 1l.j rate for the cis-compound. Although #is assump69 8 7.8 19 8 tion seemed reasonable in view of the reactivities During the fluorination the volume of the solu- of the cyclohexane 1,2-di-p-bromobenzenesulfotion was reduced considerably due to evaporation nates further evidence is desirable. and to reaction between water and fluorine. In all The cis- and trans-2-chloro- and 2-bromocycloconcentration ranges studied, a maximum con- hexyl p-bromobenzenesulfonates are excellent version limit was approached beyond which further models for further study of these point^.^ The fluorination had no effect. neighboring halo-groups are simple enough to The progressive addition of water during fluorina- permit a priori calculation of the carbonium rates tion, such that the volume of solution was held kc with some certainty even for the cis-isomers. constant, resulted in virtually complete conversion Thus, some insight may be gained into the acetolysis to selenic acid. In all cases a few tenths of a per mechanism of the cis-compounds. cent. of unoxidized selenious acid was detected, as The pertinent first-order acetolysis rate conhas been reported using hydrogen peroxide as stants a t 74.9" are summarized in Table I. The oxidizing agent. The X-fold excess fluorine new rate constants5 corroborate our previous needed to attain 99+% conversion did not vary conclusions2 based on data for the cis- and trans-2significantly with the original concentration of acetoxy- and 2-bromobenzenesulfoxycyclohexyl selenious acid. compounds. In the case of trans-2-chlorocyclohexyl p-bromobenzenesulfonate where the previous (1) L 1. Gilbutson and G. B. King, "Inorganic Syntheses," Vol 111 McGraw-Hill Book Co.,Inc , New York, N Y , 1930, p. 137 evidence2i6 had suggested a mechanism of the carbonium type, the rate agrees with that of the DEPARTMENT OF CHEMISTRY UNIVERSITY OF NORTH CAROLINA cis isomer within a factor of 4 (Table I). On the CHAPEL HILL,SORTH CAROLINARECEIVED JULS 16, 1951 other hand, the trans-%bromo ester reacts 800 times faster than the cis-2-bromo ester. This is again consistent with our previous judgment that the Acetolysis Rates of the cis- and transd-Chloro- trans-2-bromine atom is furnishing a nucleophilic and 2-Bromocyclohexyl pBromobenzenesulfo- driving force.2.6 The rates of the cis-2-chloronates and cis-2-bromocyclohexyl esters are nearly equal, consistent with the nearly equal inductive effects of BY ERSESTG R U ~ U W A L D ~ these two atoms. In the previous work on the acetolysis of various Beyond their low rates, little is known about the substituted cyclohexyl p-bromobenzenesulfonates, * solvolytic reactions of the 2-halocyclohexyl esters. it was important t o know the driving forces due to We have calculated the extra electrostatic work the nucleophilic participation of the neighboring W , assuming a mechanism of the carbonium type, 2-substituents. To evaluate these driving forces, on the same basis as The results, suinwe compared the observed acetolysis rates k with inarized in Table 11, predict values of KC '/I.+ of the the rates kc predicted for a mechanism of the observed rate for the cis-2-chloro ester and ' / ' 6 carbonium type. Whenever k significantly ex- of the observed rate for the cis-2-bromo ester. ceeded the estimate of kc, a nucleophilic driving These factors axe large enough to make it appear force due to the neighboring group was indicated. unlikely that mechanisms of the carbonium type In the case of the monatomic or structurally (4) The author is grateful to Dr. R. B. Loftfield of the Massachusimple neighboring groups such as halogen or OH, setts General Hospital for providing him with pure samples of these values of kc were calculated2 from the extra elec- compounds. (5) The rate constants listed in Table I measure the acetolysis of the trostatic worka W needed to generate the C-0 group, not of the less reactive halogen atoms dipole MI of the partly ionized p-bromobenzene- p-bromobenzenesulfonte which may also solvolyze. The solvolysis rates of the halogen atoms Experimental Twenty-five ml. of a standardized solution of selenious acid was pipetted into a 3 5 - c ~ .platinum crucible. With the fluorine cell operating, the platinum delivery tube was introduced into the solution through a hole in the cover. The solution was maintained at steam-bath temperature. Following fluorination, aliquots were analyzed for unoxidized selenious acid.
?
r
1
(1) Visiting Associate Chemist, Brookhaven National Laboratory, JuneSeptember, 1950; Department of Chemistry, Florida State University. (2) S. Winstein, E. Grunwald and I, 1,. Ingraham, T H I S JOWRNAL. TO, 821 (1948). (3) J. G . Kirkwood and F H Westheimer, J Chcm. Phrs , 6, 506, 513 (1938)
may be estimated from the relative reactivities, cyclohexyl p-bromobenzenesulfonate: cyclohexyl bromide: cyclohexyl chloride 1 :3 X lo-:: 1 X 10 - 4 ; and k/kE for a cis- or trans-2-p-bromobenzenesulfoxr substituent - 7 X 10-6 (ref. 2). Thus one would expect reaction rates 1,'10,000 of the observed rates if the cis-chlorine atom was sovolyrine, and 1/500 of the observed rates if the cis-bromine atom was solvolyzinp. (6) S. \\'instein and E. Grunrrald. THISJOIJRXAI., 70, 828 (1948).
Nov., 1951
5439
NOTES
presence of 0.017 m potassium acid phthalate proceeded with complicated kinetics. Calculated first-order rate constants increased during the reaction, as is illustrated in Table 111, again presumably due to a consecutive reaction TABLE I involving acetate. The specific rate listed in Table I was FIRST-ORDERACETOLYSISRATE CONSTANTS OF Z-SUB- obtained by extrapolation to zero time. Calculation of W.-Study of Fisher-T+ylor-HL$hfelder STITUTED CYCLOHEXYL ~BROMOBENZENESULPONATES, 74.9 ' models revealed that electron attracting trans-l,2cyclohexR O S O ~ C E H I B ~KOCsHsOua me substituents choose only the polar-trans positions of the !&Substituent m m 108 k, sec. - 1 chair form of cyclohexane' where they are nearly coplanar trans-C1 0.0393 , . . . 5 .97b with their carbon atoms and oriented trans. In this way the ,0736 ,... 5.94 dipole-interaction energies W are minimized. For two .0434 0.0168 6.3c electron-attracting cis-substituents, there seems little to choose between the 1,2-positions in either the chair-form or Cis-C1 .0392 .... 1.58 the boat-form of cyclohexane. The angles and distances for ,0778 ,... 1.58 all possible positions appear to be very comparable. Also, ,0470 ,0168 2.32 a t the angles and distances typical of cis-l,2-substituents, cis-Br ,0446 .... 1.51 the interaction energy W is not very sensitive to small changes in geometry. Therefore, it seemed logical to com,0723 .... 1.58 pute W for cis-bromine and cis-chlorine neighboring groups .0481 .0167 2.58 using the same model as before,2 %.e., a model where the trans-Brd .05 .... 1250 trans-bonds are coplanar. a KOCgHj03, potassium acid phthalate. * Previous (7) R C. Fuson and H. R. Snyder, "Organic Chemistry," John value2 5.64. Extrapolated value a t zero time. Data Wiley and Sons, New York, N Y . , 1942, p. 14. taken from reference 2 . CHEMISTRY DEPARTMENTS Experimental BROOKHAVEN NATIONAL LABORATORY Materials .-cis-2-Chlorocyclohexyl p-bromobenzenesul- UPTON, L. I., NEW YORK, AND fonate, m.p. 93-94', trans-2-chlorocyclohexyl p-bromobenFLORIDA STATE UNIVERSITY zenesulfonate, m.p. 79-80', and cis-2bromocyclohexyl p - TALLAHASSEE, RECEIVED J U N E 21, 1951 FLORIDA bromobenzenesulfonate, m.p. 95.4-95.6', were prepared and furnished by Dr. R. B. Loftfield. The syntheses will be described by him elsewhere. Glacial acetic acid, acetic anhydride, and potassium acid The Partial Molal Entropy of Cuprous Ion phthalate were reagent grade chemicals. BY 2. 2. HUGUS,JR. Rate Measurements .-The sealed ampoule technique of rate measurement described previously2 was used. TitraThe Bureau of Standards tables' give as the tions were done with perchloric acid or potassium acid phthalate in gladal acetic acid to brom phenol blue* or partial molal entropy of cuprous ion, -6.3 e.u. tropadin 00 indicator. Rates were followed to about Such a value seems too negative in view of the 10% reaction, and over this range the kinetics was first order known entropies of other plus one ions.2 within the experimental error of 5% for the cis-2-fiahsters. Apparently the literature contains only two For the acetolysis of trans-2-chlorocyclohexylp-bromobenzenesulfonate in the absence of potassium acid phthalate, determinations of equilibrium in the reaction first-order kinetics was observed within t h e , experimental Cu(c) c u + + = 2cu+ error of 574, but the end-points of the titration were not
are the major reaction paths in the acetolysis of the cis-2-halocyclohexyl p-bromobenzenesulfonates.
+
over a range of temperatures. Kawassiadis3 studied this equilibrium in sulfate solutions in the temperature range 50-200". His results when plotted as - R In K against 1/T seem erratic, k X lo8, kc X rps, asd have been discarded as unreliable. Heinerth4 2-Substituent W , kcal./molea sec. sec. -1 studied the same equilibrium in sulfate and pertrans-Cl (5.30)" (5.9) 5.9 chlorate solutions in the temperature range 20-60". 0.11 1.58 cis-c1 8.03 He found that variation in cupric concentration trans-Br 4.90 10.4 1250 caused no change within experimental error in the cis-Br 7.36 0.30 1.55 quotient (Cu++)'/*/(Cu+). Thus it appeared that a The value of the parameter i t l l / D ~was obtained from the data for trans-2-chlorocyclohexyl p-bromobenzenesul- the activity coefficients of cuprous and cupric fonate. W = R T l n ( k B / k c ) , as in reference 2. ions tended to cancel. Heinerth regarded his results in perchlorate solutions 8s uncertain since TABLE I11 there was evidence that perchlorate was reduced, ACETOLYSIS OF tt'anS-2-CHLOROCYCLOHEXYL p-BROMOBENand with t& in mind only his results in sulfate ZENESULFONATE IN THE PRESENCE OF 0.0168 m POTASSIUM solutim have been taken into account. 74.9" ACTDPHTHALATE, From Heinerth's data one may calculate for the ROSOOCbHaBr, KOCaHsOa, 108 K,4 above reaction Time, hr. rn m sec. - 1 TABLE I1 PREDICTEDVALUES O F kc FOR THE 2-HALOCYCLOHEXYL ~-BROMOBENZENESULFONATES, 74.9'
0.0 0.04345 89.5 ,04255 267.5 .04068 .03838 476.0 767.7 .03464 976.3 ,03234 K = [In a / ( a - x ) ] / t .
0.01676 .01586 ,01399 ,01169 ,00795 .00565
(6.3) 6.49 6.84 7.24 8.20 8.40
very sharp and reaction mixtures to which a slight excess pf potassium acid phthalate was added turned newtral in a few minutes due to wme secondary reaction. AGetofysis of trans-2-chlorocyclohexyl p-bromobenzuwulhnate h tbc
AF& = S8.29 kcal.
and AH29s = f18.78 kcal.
the uncertainty in these values being k O . 1 kcal. Using the known entropies of Cu(c), 7.97 e.u. (1) Selmted Vdues of Chemical Thermodynamic Properties, Series I , National Bureau of Standards, Washington, D. C. (1&47, d s a ) . (2) See for example, K. K. Relley, U. S Bur Mines Bull., No 477 (1950). (3) C. T. ,Kawassiadis, P r ~ k l i k aAksd. Atknon, 10, 391 (1035) (4) E. Heinerth, Z. EWrtrocbcm., 87, 81 (1931).
