An Electron-Diffraction Investigation of Several Unsaturated Conjugated Molecules

Citation

Lu Valle, James Ellis (1940) An Electron-Diffraction Investigation of Several Unsaturated Conjugated Molecules. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/XRRJ-7H19. https://resolver.caltech.edu/CaltechETD:etd-04272004-162550

Abstract

[Summary]

The electron-diffraction investigation of formaldehyde, glyoxal, dimethylgloxal, oxalyl chloride, and vinyl ether has shown that the conjugating power of two carbon-oxygen double bonds is of the same magnitude as the conjugating power of two carbon-carbon double bonds. The criterion for the magnitude of the conjugating power is the decrease in the carbon-carbon single bond distance. The carbon-carbon single bond distance in glyoxal and dimethylgloxal is 1.47Å (lg), and the carbon-carbon single bond distance in butadiene and cyclopentadiene is 1.46Å (lh). These distances are the same within the experimental error of the determination.

The investigation of formaldehyde (li) gave a reliable value of the carbon-oxygen double bond distance of 1.21Å.

In oxalyl chloride the carbon-carbon bond distance is only 1.50Å instead of 1.47Å as in glyoxal and dimethylgloxal (lg). The carbon-chlorine bond distance in the. same molecule is 1.72Å, a shortening of of 0.04Å. The value of 1.50Å may be explained by the competition of a free electron pair on the chlorine atom with the carbon-carbon single bond for the carbon-oxygen double bond.

In vinyl ether a bond shortening of only 0.02Å in each bond or 0.04Å total shortening is reported. The shortening may be summed in this molecule as the heat of hydrogenation data(10) show that the amount of resonance in vinyl ether is the same as in ethyl vinyl ether. The shortening is due to the resonance between a tree electron pair on the oxygen atom and the two carbon-carbon double bonds. This shortening is less than that shown by the chloroethylenes(3) but the difference in electronegativity of chlorine and oxygen may well explain this fact.

The molecules glyoxal, dimethylgloxal, oxalyl chloride and vinyl ether are all found to be coplanar and trans with the exception of oxalyl chloride in which the (COCl) groups are rotated 17° from the trans position. The potential barrier restricting rotation in the first three molecules was found to be greater than five kcals. and it is indicated that the potential curve for oxalyl chloride will show a double minimum. No estimate was made at the potential barrier in vinyl ether.

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