By Rodebush W. H.
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Extra info for Chemical Constants and Absolute Entropy
Hence, according to Scheme I, when the rate-determining step is no longer the formation of the intermediary ozone-olefin adduct but its conversion into primary ozonide, the overall process is nucleophilic in nature provided that the final step is the ring-closure involving O". [This interpretation offers a justification for Cvetanovic's hypothesis ( I I ) that k << k. 0 la* Figure 7. Verification of Equation 12 for cis-1,2-disubstituted ethylenes Conversely, the results obtained for the monosubstituted ethylenes can be interpreted in terms of a reduced complex formation under the influence of electron-withdrawing groups, resulting in a relatively smaller contribution of pathway 2 and in a lowering of the rate constant.
233). re re The dependence of the relative reaction rates on olefin geometry can be discussed with reference to Equation 14. As pointed out b y Murray and co-workers (25), the complex formation occurs with cis-olefins rather than with their trans isomers for steric reasons; hence, K (cis) > K (trans). However, during the formation of the primary ozonide b y either path the olefinic carbon atoms change i n hybridization, from sp to sp . The bond angles thus decrease from 1 2 0 ° to 1 0 9 ° ; i n the cis isomers, this results i n a compression of the substituents van der Waals radii.
Both, therefore, have the trans configuration. It is remarkable that the trans ozonide i n this case is the only product, even when pure cis-l,4-dibromo-2,3-dimethyl-but-2-ene is ozonized. The configurations of the other ozonides are based on the assumption that the trans isomers have the lower dipole moments i n comparison with the cis compounds. As a measure of the dipole moment the retention time i n liquid phase or gas chromatography was used. The isomer with the lower retention time should be the trans compound.