UBC Theses and Dissertations
Stability relationships for cis-trans olefin pairs Page, Brian Denis
An investigation was undertaken to examine some of the factors which may affect the thermodynamic position of equilibrium for a number of cis-trans olefin pairs. The positions of equilibrium were measured for selected aldehydes, ketones, nitriles and propenyl ethers. For these unsaturated compounds it has been found that their position of equilibrium may be attributed to four different factors. These factors are: i) a steric factor involving van der Waals repulsion between cis groups; ii) a polar repulsive term involving electrostatic repulsion between polar groups; iii) an effect termed differential resonance stabilization, which results from a conjugative stabilization by resonance which is greater in the trans isomer than in the cis isomer, where steric distortion from coplanarity of the carbonyl group and the double bond is present; and iv) an attractive interaction arising between cis substituents. The previously unreported Z isomers of crotonaldehyde and tiglaldehyde have been prepared as a mixture with the E isomers by photoisomerization of the E isomers. The presence of the Z isomers was readily detected by n.m.r. spectroscopy. Several β-halo α, β-unsaturated aldehydes have been prepared. The differential resonance stabilization, which resulted in >98% of the E isomer in the crotonaldehyde and tiglaldehyde equilibrium, was found to be partially compensated by an additional halogen-formyl polar repulsive term. The replacement of the β-methyl group in β-chlorocrotonaldehyde by a tert-butyl group increased the differential resonance stabilization effect such that the presence of the E isomer in the equilibrium was not detected. The effect of this tert-butyl group is partially compensated by the replacement of the β-halogen by methoxyl. The equilibrium positions for the series of aldehydes were compared to those of the corresponding ketones and esters. The preferential stabilization of a more polar isomer of a given isomer pair by a polar solvent has been investigated for several nitriles and haloölefins. The magnitude of the solvent stabilization effect in the above compounds was found to depend on the difference in polarity of the two solvents and the difference in polarity of the isomeric pair. With the above information, it was possible to extend the work of Gardner and McGreer on the additivity of free energy terms (multiplication of equilibrium ratios) to predict olefinic equilibria. In most cases correlation of the equilibrium ratio to a non-polar solvent, gave reasonable agreement between the predicted and experimental positions of equilibrium.
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