A series of organic compounds that have similar structural features but differ from adjacent members by (-CH2) group is referred to as homologous series. Each member of homologous series is called homolog.
1. CH3-OH, CH3-CH2-OH, C3H7-OH are the members of a homologous series called “Alcohols”. 2. CH3-COOH, CH3-CH2-COOH, C3H7-COOH are the members of homologous series called “Carboxylic acids”. 3. CH3-O-CH3, C2H5-O-CH3, C3H7-O-CH3 are the members of homologous series called “Ethers”.
CHARACTERISTICS OF A HOMOLOGOUS SERIES
1. STRUCTURAL FORMULA All the members of a homologous series have same structural formula. 2. MOLECULAR FORMULA Molecular formula of different members of a homologous series differs from previous and next member by CH2. 3. NATURE OF ELEMENT All the members of series have same type of elements. 4. FUNCTIONAL GROUP All the members of homologous series contain same functional group. 5. GENERAL FORMULA Members of a homologous series can be represented by a general formula.
FOR EXAMPLE: ALKANE : CnH2n+1 ALKENE : CnH2n ALKYNE : CnH2n-2 ALCOHOL : CnH2n+1 OH ETHERS : CnH2nO Where n = number of carbon atoms.
6. MOLECULAR MASS Molecular mass of any two consecutive members differ by 14 units
7. CHEMICAL PROPERTIES All the members of a homologous series have same chemical properties due to same functional group.
8. METHODS OF PREPARATION Generally all the members of a series can be prepared by using same method.
9. PHYSICAL PROPERTIES Physical properties of the members of a homologous series vary with the increase in atomic weight.