The direction of ozone cleavage of selected unsymmetrical olefins RlR2C=CR3R4 to give (a) RlR2C0 + R3R4CCOO-and (b) RIR,CCOO-+ R3R4C0 has been measured, with particular emphasis on the mode of participation of alkyl substituents in distributing the reaction paths a and b. The interpretation, in terms of substituent effects in stabilizing potential zwitterionic carbo-cations in the transition states corresponding to paths a and b, considers (i) the "hyperconjugative stabilization" of positive sites, where the stabilizing abilities of the substituents are in the order CH, > C2H5 > i-C3H7 > tert-C4H9, and (ii) the participation of inductive effects. The latter appear to be particularly important when the substituents on the two C atoms of the primary ozonide act in the same direction in favoring its electrocyclic cleavage. In the ozonolysis of monosubstituted ethylenes the inductive effects are predominant, i.e. the R-C+HOO-formation is favored by the R groups in the order tert-C4H9 > i-C3H7 > C2H: > .CH3, whereas in trans-and cis-l,2-disubstituted ethylenes "hyperconjugation" governs the dlrectlon of cleavage. The cleavage of selected 1,l-disubstituted, tri-, and tetrasubstituted ethylenes is also studied.

Extended Huckel LCAO-MO calculations indicate that the half-chair conformation of the five membered 1,2,3-trioxolane ring is more stable than the envelope form. The equatorial conformation is preferred by all substituents in the primary ozonides of monosubstituted ethylenes. In the primary ozonides of trans-1,Zdisubstituted ethylenes, the equatoria1,equatorial conformation is preferred by small substituents (methyl, ethyl npropyl), whereas the bulkier isopropyl and t-butyl groups prefer the axia1,axial conformation. In the primary ozonides of cis-1 ,Zdisubstituted ethylenes, the largest group prefers the equatorial conformation when both substituents are small, and the axial conformation when one of the substituents is at least as large as the n-propyl group. When one of the substituents is as large as the t-butyl group, the primary ozonide does not appear to possess a "stable" conformation. he very unstable 1,2,3-trioxolanes (primary ozon-T ides) I, which results from the electrophilic 1,3dipolar cycloaddition of ozone to a carbon-carbon double bond, have been the object of recent studies. Because of the great instability of the primary ozonides, any experimental study with these intermediates in the ozonolysis reaction must be performed at low temperatures (-78 t o -130'). ICriegee and Schroder,2 and Greenwooda have isolated a-diols by the treatment of ozonized solutions of olefins (-1 lSO) in ether by an isopropyl Grignard reagent. The configurations of the a-diols obtained(2) R.

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R e~u le 14 octobre 1966For the ozonization of ring-substituted styrenes, the proportions of the two ways of cleavageyielding (a) ArCHO + H C H O~ and (6) ArCHOO + HCHO are related by the equation log x / ( l -x ) = -AAGo*/2.3RT + (pa -Pb)u to Hamrnett's u and p constants (x = proportion of cleavage following route a ; AAGo* (0.22 kcal/mole a t 25 "C) = Gibbs activation energy difference between assumed transition states of the primary styrene ozonide). The interpretation in terms of transition states concludes that the zwitterion is preferentially formed a t the carbon atom with the highest electron density.Canadian Journal of Chemistry. Volume 45, 533 (1967) E n dkpit d u nombre considkrable de travaux ayant trait au clivage par l'ozone d e s d o u b l e s liaisons olkfiniques, n o u s sommes encore remarquablement ma1 renseignks quant au sens de rupture des molozonides d'olkfines asym6triques. Des rkcentes ktudes effectukes sur l'indhne ( I ) e t le styrhne (2) tendent, en effet, A montrer que lee effets klectroniques invoquCs lors d e la prkvision du sens de rupture sont souvent surestimks. Ainsi les deux voies de clivage prhvues par la thkorie de Criegee

Id, is not enough to cause the planar geometry to be as stable as the nonplanar geometry. Upon strengthening the resonance interaction with both the ring and the fluorines, a planar geometry is increasingly favored. This effect is illustrated in the difluorocyclopropylcarbonium ion, le, where the planar conformation is the most stable geometry. It is much more stable than any geometry with a significant outof-plane deviation. This is in accord with Pauling's24 expectation that the planar geometry is increasingly more stable relative to nonplanar geometry as the elec-6953 troné gativity difference between C and F decreases.