The Concept of Homology

Groups of compounds, such as the hydrocarbons called the alkanes (and each separately, the alkenes, the alkynes, and the arenes ) that have similar structures, frequently have similar chemical and physical properties. The members of such structurally related groups are said to constitute a homologous series and, given the properties of some of its members, it is frequently possible to estimate the properties of others. 

Branching in alkanes, which provides structural variation and alters the effects of intermolecular interactions, does not lend itself to the same kind of analysis. However, in general, branching (in compounds with the same molecular weight) lowers the boiling point. 

For alkenes (Table 3.3), alkynes, and arenes, the concept of homology as pertains to addition of alkyl substituents also continues to apply, although positional isomerism and stereochemistry play important roles. Addition of further carbon-carbon double bonds, triple bonds, and/or aryl groups, as well as branching, confounds widespread prediction however, in a closely related series, the principle is maintained. 

The boiling point and melting point values presented are largely from Rappoport, Z. CRC Handbook of Tables for Organic Compound Identification, 3rd edition, CRC Press, Cleveland, OH, 1977. Values for -AHf ( 0.5) are largely from Cox, J. D. Pilcher, G. Thermochemistry of Organic and Orgarwmetallic Compounds, Academic Press, New York, 15170. 

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Jean Baptiste-Andr Dumas, 1800-1884. Professor of chemistry at the Athenaeum and at the Sorbonne. He devised a method of determining vapor density, and developed the theory of types m organic chemistry, which he defended against Berzelius duahstic electrochemical theory. From a study of the aliphatic alcohols, Dumas and Pehgot developed the conception of homologous series, See also ref. (62). 

Panchen, A.L., Richard Owen and the concept of homology, in Homology The Hierarchical Basis of Comparative Biology, Hall, B.K., Ed., Academic Press, San Diego, CA, 1994, pp. 21-62. 

Remane, J., The concept of homology in phylogenetic research its meaning and possible applications, Palaeontol. Z., 57, 267-269, 1983. 

I identified three milestones in the development of the concept of homology (1) Owen s distinction and clarification of homology and analogy (and homologue and analogue), (2) Patterson s distinction between taxic and transformational homology, and (3) Nelson s introduction of the concept of relationship into studies of homology. 

Kaplan, D.R., The concept of homology and its central role in the elucidation of plant systematic relationships, in Cladistics Perspectives on the Reconstruction of Evolutionary History, Duncan, T. and Stuessy, T.F., Eds., Columbia University Press, New York, 1984, pp. 51-70. 

Kaplan, D. R. (1984). The concept of homology and its central role in the elucidation of plant systematic... 

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