Retention data on alkanes are re-examined from a structural point of view with an emphasis on the molecular connectivity. Rather than adopting empirical parameters for selected structural groupings an assumption that bond contributions depend on the formal carbon valencies in a hydrogen suppressed molecular graph is made. Except for several highly branched systems (which give problems in almost all empirical correlations) this single assumption produces a good linear correlation with the connectivity index which compares favorably with alternative schemes using ten (and more) structural empirical parameters. Furthermore, a close examination of the skeletal forms for the worst cases indicates that these are characterized by a larger number of methyl groups three bonds apart, hence by an additional assumption, that such methyl-methyl fragments cause departures from the correlation (given by the quadratic function of the number of fragments) retention indices for all alkanes considered can be well represented by a linear correlation with the connectivity index. The significance of the work is in demonstration of a dramatic reduction of the structural assumptions required. The approach should be considered as complementary to purely empirical correlations of the retention data with a large number of structural indices, pointing rather to the significance of the structural fragments involved then competing in precision with more flexible alternative schemes. © 1978.
