Homologous Series

HOMOLOGOUS SERIES, groups of organic compounds with the same chemical function, but differing from each other by one or more methylene (CH₂) groups. If in the simplest compound of the series of saturated hydrocarbons—methane, CH₄—one of the H atoms is replaced by a CH₃ group, we obtain CH₃-CH₃ (ethane); from ethane by the same replacement propane can be formed, CH₃-CH₂-CH₃, from it butane, CH₃-CH₂-CH₂-CH₃, and so on. Methane, ethane, propane, butane, etc., differ from each other by one CH₂ group and constitute a homologous series of saturated hydrocarbons, in which each subsequent member is a homolog of the preceding ones. In a similar manner other homologous series are formed: alcohols, aldehydes, ketones, amines, acids, ethers, etc. All compounds belonging to the same series possess qualitatively identical properties, are obtained by analogous methods, and behave more or less identically toward specific reagents. Often the first members of a homologous series exhibit some deviations from the properties of the remaining members of the same series; for example, formic acid (the series of fatty monobasic acids) and oxalic acid (the series of fatty dibasic acids) are very energetic reducing agents, whereas the remaining members of these series do not possess reducing properties. Analogous deviations are observed in other homologous series. Changes in certain physical properties in homologous series proceed quite regularly and can often be expressed by definite quantitative ratios. Molecular volumes always increase from one member to another, and this increase is expressed by a more or less constant value of 22. E.g.: Saturated fatty monobasic acids Molecular weight

22 Propionic C₂H₅COOH ....

22 The heat of combustion of each subsequent homolog increases by a constant value (154-158 calories), which is therefore taken as the heat of combustion of the CH₂ group. This constancy served as the basis for calculating atomic heats of combustion and heats of formation (energy of formation) of bonds between two C atoms (C-C) and between C and H atoms (C-H). Molecular refraction and molecular dispersion in homologous series increase quite uniformly. Thus, each CH₂ group causes an increase in molecular refraction (for the D line) by 4.6 units. The melting point in homologous series with increasing molecular weight changes most often in the direction of an increase. However, the increase in melting point occurs far from uniformly, and in the higher members the influence of homology almost completely disappears. Alternation phenomena are often observed, i.e., upon transition from one member of a homologous series to the next, there is an increase in melting point, then a decrease, then an increase again, and so on. This can be observed particularly clearly in the homologous series of fatty acids, where each member with an odd number of C atoms melts at a lower temperature than the even homolog following it. The boiling point of homologs mostly increases by 19-30° depending on the nature of the series. In higher members, the difference in boiling point is significantly smaller than in lower ones.