7381
than the first by ca. 5 units,26the pK, of V (T-) may be estimated a t 16.0. Finally, taking the ApK, between T* and V as equal to that existing between H3+O and H 2 0 gives a pK, of ca. - 1 for T*. Since this value is considerably less than the pK2 of succinic acid, the k2 step in eq 10 should be diffusion controlled. While data sufficient to generate a Bronsted plot for the general base catalyzed hydrolysis of M F d o not exist, extrapolations from the literature make it appear unlikely that the coefficient would approach 0. The p value for general base catalyzed hydrolysis of ethyl dichloroacetate, a n ester considerably more reactive than M F , in general base catalyzed hydrolysis, is 0.47.'* The /3 decreases t o = 0.2 for ethyl trifluorothiolacetate which has a much better leaving group than M F , and is equal t o 0 only for the hydrolysis reactions of relatively basic catalysts with methyl S-trifluoroacetylmercaptoacetate which has an even better leaving The mechanism of eq 10 can therefore be ruled out. The mechanism of eq 9 is kinetically indistinguishable from that of eq 11 involving specific base, general acid c a t a l y s i ~ . ~ 'Such a mechanism would predict a methoxyl-180kinetic isotope effect that is less than that obtained for alkaline hydrolysis, since the attack by hy-
H
QI
HO
H
nHB e HO-C-0-HI C-0 I
OMe
n
n
-B
-+
I
OMe
+
+
+
B- (11) droxide ion on the ester stabilized by proton donation from BH should result in an earlier transition state than in unassisted attack. The observed isotope effect while small is significantly larger for general base catalyzed than for alkaline hydrolysis and thus argues against the mechanism of eq 11. Leaving group kinetic isotope effects thus provide a new means for resolving kinetic ambiguity in general acid-base catalysis reactions. 27 The remaining mechanism for consideration is that shown in eq 9. The present results provide no information on whether or not the anionic tetrahedral intermediate depicted is stable enough t o be at equilibrium with respect t o transport processes.28 They do require that k , be rate determining if the intermediate exists, and, in either case, an early transition state involving little C-OMe bond loosening for the rate-determining step. HC0,-
Hf
MeOH
(28) M . I. Page and W. P. Jencks, J . Amer. Chem. Soc., 94, 8828 (1972); W. P . Jencks and I 9 9 % reaction. I n a control experiment a sample containing all reagents used in kinetic runs except N : j : j ~ W hwhich , was replaced by 0.65 M sodium tetraphenylboride t o mimic the ionic strength of kinetic runs. was held 40 hr a t 60". N o cyclohexene was formed by glpc analysis. and the solution contained no bromide ion by titration. Similarly. two control solutions, the first containing cyclohexene. thiophenol, Ns,ij3SPh,and 2.6-lutidine, and the second containing cyclohexyl phenyl sulfide. thiophenol, N.ia:isSPh, and 2,6-lutidine, were completely stable according to glpc analyses for times corresponding to those required for > 9 9 % reaction of cyclohexyl bromide at 60". These controls demonstrate that the substitution and elimination products are stable under kinetic conditions. Kinetics in DMF. A solution containing all reagents except cyclohexyl bromide was prepared at room temperature under nitrogen. Aliquots were transferred to serum-capped test tubes. To each tube at - 78 a fixed amount of cyclohexyl bromide was added with a 10-pl Hamilton syringe fitted with a Chaney adapter. Bromide determination for each data point. treatment of data, and product distributions were done by the methods described for kinetics in methanol. Control experiments analogous to those run in methanol demonstrated that cyclohexyl bromide did not solvolyze and that cyclohexene and cyclohexyl phenyl sulfide were stable under the reaction conditions. In independent experiments secondorder rate constants for two runs a t 20.1" and 0.095-0.099 M N,,,,SPh differed by 1.6%. Kinetics in N.1??6B??z6.For each kinetic run a solution containing all reactants except cyclohexyl bromide was prepared in an inert atmosphere in a flask fitted Nith a silicone rubber septum. The sclution was equilibrated at the desired temperature, and cyclohexyl bromide was added by syringe with magnetic stirring. Aliquots were withdrawn with a gas-tight syringe. and bromide ion content for each data point was determined by a method modified from that described for kinetics in methanol. N???eBi??einterfered with the diphenylcarbazone-mercuric nitrate titration. Consequently. each molten salt aliquot was quenched in 4 ml of 0.25 .M sulfuric acid and the mixture was extracted four times with benzene and three times with chloroform. The resulting aqueous solution was free of N22?6B?226and contained all the bromide ion according t o control experiments which simulated kinetic conditions. Rate constants were determined by the method described for kinetics in methanol, but data points were slightly more scattered from the least-squares lines than in methanol or D M F experiments. However, rate constants obtained at 45.25" and 0.12-0.13 .M Na3.?SPhin two batches of N2??6Ba??6prepared over 7 months apart differed by only 5.8% (see the last two lines of Table I). (22) F. W. Cheng, Microchem. J., 3 , 537 (1959). (23) A. A . Frost and R. G. Pearson, "Kinetics and Mcchanism," Znded, Wiley,NewYork, N . Y . , 1961,pp 14, 17.
Kinetics of Molten Triethyl-n-hexylammonium Triethyl-n-hexylboride
7386 Product distributions were determined after >99% disappearance of cyclohexyl bromide by extraction of the N1226B2126 solution with 5-6 portions of 1-2 ml of pentane and glpc analysis of the pentane extracts. Cyclohexene and cyclohexyl phenyl sulfide were determined using octane and tetradecane as internal standards on matched 6.5 ft X 0.25 in. 11 Apiezon L columns with temperature programming to obtain both analyses from a single injection. In control experiments the sum of products determined by this method accounted for 97% of the initial cyclohexyl bromide when the reaction was carried out in sealed glass ampoules, but some cyclohexene
was lost when the reaction was carried out in septum-capped flasks. In the latter cases cyclohexyl phenyl sulfide was determined by the glpc method and the cyclohexene was assumed to account for the remainder of the cyclohexyl bromide consumed. Control experiments showed that this assumption gave results within 2 % of those obtained by glpc analysis of both products.
Acknowledgment. We thank the U. S. Public Health Service (GM 16909) and the Science FoundatiOn (GP 38493) for support of this research.
Mercury ('PJ-Sensitized Decomposition of cis- and trans-3,4-Dimethyl- 1-pyrazoline. The Kinetics of Highly Vibrationally Excited Triplet lY2-DimethyltrimethyleneBiradicals Edgar B. Klunder and Robert W. Carr, Jr.* Contribution from the Department of Chemical Engineering and Materials Science, Unicersity of Minnesota, Minneapolis, Minnesota 55455. Received May 16, I973 Abstract : The Hg( 3P1)-photosensitized decomposition of both cis- and trans-3,4-dimethyl-l -pyrazoline was studied from 0.2 to 4 and 5 Torr, respectively, and in the presence of Ar at pressures up to 1 atm. The major products are nitrogen, cis- and trans-dimethylcyclopropane, cis- and trans-2-pentene, 3-methyl-1-butene, and
ethane. The product distributions from both reactant isomers are nearly identical. For the cis reactant @Nz = 0.93 i 0.05 at 4 Torr, and for the trans reactant aX2= 0.95 f 0.05 at 5 Torr. Total hydrocarbon quantum yields are independent of pressure from -1 to 760 Torr (pressurized with Ar). Highly vibrationally excited triplet 1,2-dimethyltrimethylenebiradicals are formed by N z elimination from both pyrazolines. The hot biradicals either dissociate to methyl and methallyl radicals or cyclize to dimethylcyclopropanes, which are also sufficiently excited to experience geometrical and structural isomerization, the latter only being observed at pressures below about 1 Torr. Pentenes and ethane are formed by radical-radical combination reactions.
D
irect and triplet-sensitized photodecomposition of 1-pyrazolines has frequently been postulated to proceed via nitrogen elimination t o yield 1,3 biradicals. l-g The spin multiplicity of the intermediates and their subsequent reactions depend on the primary photophysical processes occurring in the precursor, though in general direct photolysis is thought t o yield singlet biradicals while triplet photosensitizers are thought to produce triplet biradicals. Direct photolysis of 1-pyrazolines yields cyclopropanes, presumably via biradical cyclization, and olefins as the major products both in vapor and condensed phases. 3 , 7 , 9 Olefins and cyclopropanes are also formed in two vapor phase systems which are thought t o involve triplet 1,3 biradicals as the olefin precursor. These are the addition of CH2(3B1)t o double bonds'O9 l 1 and the triplet photosensitization of dimethylcyclo(1) P. Dowd, J . Amer. Chem. Soc., 88, 2587 (1966). (2) S . D. Andrews and A. C. Day, Chem. Commun., 667 (1966). (3) R. Moore, A. Mishra, and R. J. Crawford, Can. J . Chem., 46,
3305 (1968). (4) P. Scheiner, J . Amer. Chem. Soc., 90, 988 (1968). ( 5 ) E. L. Allred and R. L. Smith, ibid., 91, 6766 (1969). (6) E. l3. Klunder and R. W. Carr, Jr., Chem. Commun., 742 (1971). ( 7 ) S.D. Nowacki, P. B. Do, and F. G. Dorer, ibid., 273 (1972). ( 8 ) P. B. Condit and R. G. Bergman, ibid., 4 (1971). (9) D . H. White, P. B. Condit, and R. G. Bergman, J . Amer. Chem. Soc., 94, 1348 (1972). (10) F. J. Duncan and R. J. Cvetanovic, ibid., 84, 3594 (1962); H. M. Frey, ibid., 82, 5947 (1960). (11) D. C. Montague and F. S. Rowland, J . Chem. SOC., Chem. Commwi.,193 (1972).
butanone vapors. l 2 3 l 3 In contrast, triplet photosensitization of 1-pyrazolines in solution yields cyclopropanes but n o ~ l e f i n s . ~ JIf, ~triplet biradicals are important intermediates in these reactions, there is a n unexplained effect of phase on the reaction products. O n the other hand, triplet biradicals may not be involved in all cases, or the biradicals may have different properties, dependent upon their mode of formation. Tn this work we have investigated the reasons for the above noted differences in behavior. No one mechanism can explain the results of the photolytic or pyrolytic decomposition of all the compounds in which diradical intermediates are thought to play a role. The diverse modes of reaction can be attributed to different structures and/or electronic states of the intermediates. We initiated a n investigation of the vapor phase triplet-sensitized decomposition of 1-pyrazolines in order t o compare and contrast behavior with that obtained by triplet sensitization in solution and with direct photolysis. cis- and trans-3,4-dimethyl-lpyrazoline were chosen for study since the expected triplet biradicals would be similar t o those obtained by adding CH2(3B1)t o cis- or t r a n s - 2 - b ~ t e n e ' ~ ~or" by the triplet-sensitized decomposition of cis- and trans-2,3dimethylcyclobutanone. 12,l 3 The investigation was expected to reveal whether the triplet biradicals produced (12) J. Metcalfe and E. I
