What is Homologous Series? : definition, example
Homologous Series
Such a series of organic compounds in which organic compounds are placed in increasing order of molecular weight, in which each member has the same functional group and in which the molecular formulas of two successive members differ only by CH2, is called a homogeneous series.
Each member of a homologous series is called a homologue of the other members. The following are the main features of homogeneous categories:
- All the members of a homogeneous series can be represented by a general formula.
- All the members of a homogeneous series can be constructed by the same methods.
- The physical constants of the members of a homologous series increase or decrease with the increase of their molecular masses.
- Due to the presence of similar functional groups, most of the chemical properties of all the members of a homologous series are similar. There is a greater difference in the chemical properties of the first and other members of some homogeneous series.
Homologous series are of great importance in the study of organic chemistry. The concept of homologous series has greatly helped in simplifying the study of organic chemistry.
As it is clear from the characteristics of homogeneous categories, after studying the initial members of a homogeneous category, sufficient knowledge is gained about the methods and properties of the rest of the members.
Thus the study of more than two million organic compounds has been limited to the study of a few homogeneous categories. The following is a brief description of some of the major homologous categories.
1. Alkane series - The members of this series have the general formula CnH2n+2. The members of this category are called alkanes, paraffins, or aliphatic saturated hydrocarbons. The first member of this series is methane (CH4) and the second member is ethane (C2H6).
The group of atoms obtained by removing one hydrogen atom from a molecule of an alkane is called an alkyl group. The alkyl group is denoted by R. On this basis, the general formula of alkanes is also RH. Following are the structures and names of some alkyl groups-
一CH3 一C2H5 一C3H7
methyl group ethyl group propyl group
2. Alkene series - The members of this series have the general formula CnH2n. The members of this category are called alkenes or olefins. A carbon-carbon double bond is present in the structure of each member of this series.
3. Alkyne series - The members of this series have the general formula CnH2n-2. The members of this category are called alkynes. A carbon-carbon triple bond is present in the structure of each member of this series.
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