X-Ray Diffraction in Liquids: A Comparison of Certain Primary Normal Alcohols and their Isomers

Abstract

This x-ray diffraction study of certain isomers of the liquid primary n-alcohols confirms, in general, present conceptions concerning the alteration in shape of the chain molecule with the attachment of branches, and it emphasizes the importance of the molecular space array in liquids, or the cybotactic state, and an investigation of it by x-ray diffraction. One isomer of n-butyl, five of n-amyl, one of n-hexyl, and two of n-heptyl alcohol were tested. It is found that the attachment of C${\mathrm{H}}_3$ as a side branch alters the "diameter" by 0.6A and of OH by 0.4A (Ångstrom units, ${10}^{-8\mathrm{}}$ cm.) The attachment of C${\mathrm{H}}_3$ and of OH to the same atom in the chain increases the diameter by only 0.65A. The diameter is in a sense a mean value, but in the case of the di-n-propyl carbinol there is a better resolution and the side attachment of OH increases the diameter in one direction by 0.45A and in the other not at all. The existence is shown of associated polar groups giving two molecules in a continuous chain for each longitudinal spacing. When the OH group is not attached to the last or next to last atom in the chain this association disappears and there is but one molecule for each longitudinal spacing. This is in agreement with other experiments. In the one case carefully tested, triethyl carbinol, the first, third and fifth orders of the side spacing peak are apparently present. The assumption of a molecular space array seems the only one consistent with the facts.