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Solvent effect on the position of Olefin equilibrations and additivity of free energy contributions of vicinal groups Kaushal, Dharam Paul

Abstract

The effect of solvent: on the equilibrium position for a number of cis-trans pairs of di- and trisubstituted haloethylenes and unsaturated nitriles have been investigated in carbon tetrachloride and pentane by the photo-induced bromine-catalyzed reaction. These equilibrium values have been compared to the values for neat samples. In all cases it has been observed that the proportion of the more polar isomer (higher dipole moment) of a pair of isomers decreased and the proportion of the less polar isomer increased in the non-polar solvents. The bromine-catalyzed equilibrium data for nine 1,2-disubstituted olefins (having Br, Cl, CN, CH₃ groups) have been obtained as neat and as 10% solutions in carbon tetrachloride and pentane at room temperature. The data thus obtained have been used to calculate the expected equilibrium position for nineteen trisubstituted olefins, assuming, as did Gardner and McGreer (45), that interactions which are present in nine 1,2-disubstituted olefins can be combined in a linear free energy sense. In eleven of the twelve olefins equilibrated, the agreement between the calculated and the experimental values has been found to be good. The equilibrium data for some unsaturated nitriles and unsaturated esters have been achieved catalytically (Br₂ / CCl₄) or thermally. For these olefins it has been found that their equilibrium positions may be attributed to many factors. These factors are: I) a steric effect involving only traditional steric interactions between bulkier groups; II) an attractive nonbonded interaction which is present between many groups; III) a polar repulsive term involving electrostatic repulsive interactions between polar groups; and IV) a conjugative stabilization effect where conjugative stabilization by resonance is greater in trans isomer than in cis isomer, when the carbonyl group is markedly sensitive to steric effects of cis substituents. It has been observed in this study that steric factors are dominant for equilibria of the unsaturated esters and polar effects mainly are dominant for equilibria of the nitriles.

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