The Double Bond and Cis- Trans-Isomerism

The double C=C bond isi to some extent susceptible to quantum mechanical treatment Pauling,Slater, E. Hiickel, and more recently R. S. Mulliken in particular have tried, by employing different methods, to derive criteria for the form of the charge distribution corresponding to the aliphatic and the aromatic double bond. It has been proved repeatedly that this chemical primary valence is produced by 4 electrons, two of 

Researches of Eucken and his co-workers on ethylene have in fact afforded very striking confirmation of this concept. They deduce from the effect of temperature on specific heat a torsional or turning oscillation of fairly high frequency (750 cm - 10 per sec.) so that the molecule represents a highly impeded one dimensional oscillator whose properties have been studied by Teller and Weigert. Comparison of measurement and theory shows that the barrier of potential which has to be scaled in the transition from cis- to trans-form, lies far above kT] on this depends the stability of the two isomers and the possibility of isolating them by the methods of synthetic organic chemistry. 

To obtain an insight into the probable behavior we shall refer to Pig. 14 

The energy difference U between the minima of the two troughs is the heat of conversion of the two modifications into one another it determines for the mixture ratio of the isomers and can be calculated from 

Stuart also has investigated this problem by calculating the reciprocal effect of the two Cl atoms, and has arrived at very plausible results. Actually the cis-position was found to be preferred but its calculated stability, 1150 cal/mol, was greater than that given by experiment. We must, however, be content with this agreement in view of the inadequacy of the theory underlying the calculation. 

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