COMMUKICATIONS TO THE EDITOR
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reverse procedure (OH- into Hg”’), CY was the of X were determined by an isotope dilution same. n‘hen Xg+ was added to excess OH - > CY technique. was 1.016. For the reverse addition, CY was signiThe activities theoretically to be expected of X ficantly smaller; all values were less than 1.013. arising exclusively b y an “internal” path from N Thus, for a given sample to Xg20,it appears possible (path A) and by an “external” path (path B) to deduce the order in which the reactants were were calculated by a graphical integration method. originally mixed b y measurement of the 019’016 x _t-_ -_ --r ratio. These results are suggestive of a kinetic isotope effect. For the H g + + aquo-ion, the preferential rupture of O16-H relative to OI8-H would concentate OI6 in the HgO. For -1gf, the situation is kn more complex. =In alternative mechanism for the The data for activity of N V.Y. time and for moles fractionation involves the equilibrium isotopic of X formed z’s. time were plotted. A reaction exchange between OH- and H 2 0 , favoring the period of f minutes was subdivided into arbitrarily concentration of 0l6in the OH-.3 small time increments. If XI is the number of Further work is in progress to extend the data and examine other cations. TVe appreciate the moles of X formed during time increment 1, Xr discussions with R. IV. Parry, the assistance of during time increment 2 , etc., and Xt during time \Ir. E. Barrett with the mass spectrometer, and the f, CYO the original activity of N, cy1 the average financial support of the C. and the 1Iichigan activity of N during time increment 1, etc., then the activity of X to be expected by the “internal” Memorial-Phoenix Project. path (-1) is a l X l ‘X, a2X2,’Xt . cViXi/Xt. (3) T h e ionic hydration fractionation effect of Feder and Tauhe Since the only source of activity in the system is (J. Ckem. Phys., 20, 1335 (1952)) wherein 0 1 8 concentrates in t h e N,3the activity of M A a t any time is CY(, - cuj aquo-ion relative t o t h e solvent water would prohahly he of negligible where a, is the average activity of F-7 during any importance since most of t h e experiments were carried o u t a t low concentrations. given time increment. T h e activity of X to be expected by the “Fxternal” path (B) is therefore DEPARTMENT OF CHEMISTRY
r-=-
+
+
+
+
+
RICHARD B BERNSTEIY (CY” - a&Y1 ‘Xt UNIVERSITYOF MICHIGAX (CYo - az)XsiXt . . . (a,, ANNARROR,MICHIGA~U HARK\G PARS ai rXi ‘Xt. .Application of these calculations to the RECEIVFD TU\F 11,19.7.5 data show t h a t after three minutes and ten minutes
31.4% and 43.4% respectively of the X present has been formed by path A . 4 ON THE STEREOCHEMISTRY AND MECHANISM ,Uhough epimerization of iLI to X via enolic O F THE DIELS-ALDER REACTION’ species formally satisfies the requirement t h a t part Sir: of t h e isomerization occur b y path -1, we consider We wish to report the first conclusive evidence this an unlikely mechanism, since the isomerization t h a t isomerization of a bicyclic Diels-A\lder adduct does not appear to be catalyzed b y bases and does of endo-stereochemistry to the ?so-isomer can occur not occur with the dihydro derivative of N.’C without dissociation into kinetically free addends.’ The results also do not formally exclude a mechWhen the endo cyclopentadiene-maleic anhydride anism involving intermediate formation of a diin the radical followed by hydrogen transfer, as proposed adduct ( N ) , radioactively labeled with carbonyl carbons, was heated with an equimolar by Craig,2c or a diffusion-controlled secondary quantity of non-radioactive maleic anhydride recombination of original partners.5 However, we (XI)in boiling decalin (190”),rapid exchange of h’ consider the most probable detailed description with M A occurred simultaneously with slower of path to be conversion of X to a complex in conversion of -?$ to the eso-isomer (X). (Exchange which ATA\ and cyclopentadiene are relatively free of X with h1.X was small a t short reaction times to rotate with respect to each other, followed by recombination of the fragments in either stereochemically possible sense. On the basis of the present evidence, this is indistinguishable from primary recombination in a Franck-Kabinowitch solvent cage. I l y e are ilidebted to the Office of Ordnance KeCO search for financial support and to Professors A. IV. s x :\damson and S . IT.Renson for helpful discussion. The variation of endo-activity with time was O F CHEMISTRY determined b y re-isolating N from quenched re- II)EPARTMEST A . RERSOS T N I V E R S I T Y O F SOUTHERN CALIFORSIA JEROME action mixtures. Since isolation of X directly I,OS ANGELES 7 , CALIFORNIA from reaction mixtures a t small concentrations of RECEIVED J U L Y 11, 195.5 . X was not feasible, the activity and concentratioii -!:3) This is true i f the exchange of X with M A is neglected. T h e ,,‘d
(1) Sponsored by the Office of Ordnance Research, U. d. Army. ( 2 ) For previous discussion of the mechanism of such isomerizations see (a) R. B XVoodward and H. Baer, THISJOGRNAL, 6 6 , fj45 (1941); ( b ) E(.Alder and W. Trimborn, A n n . , 566, 58 (1920); (c) D. Craig, THISJOURNAL, 73, 4889 (1951); (d) D. Craig, J. J. Shipman, J . Kiehl, F. Widmer, R. Fowler and A . Hawthorne, ibid., 7 6 , 4573 (1954); (e) J . A . Berson and R. Swidler, ibid , 7 6 , 1721 (1953)
uxrection for this is small. I t s omission leads t o a slightly more wnservative estimate of t h e importance of t h e “internal” path ( A ) . (4) The ratio of t h e amounts of X formed by the two paths is (kePi /:5X),’(kdXI.C5Hl, - k s X ) . This need not be time invariant i f ki # k j or if Cafh is being irreversibly removed from t h e system, e . R . , by polymerization. ( 5 ) Cf.inlev a l i a , R hI. Xoyes, 1.Chem. Phys., 22, 1349 (1954).
