SPM Chemistry

Homologous Series

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Homologous Series

    1. A series of compounds with similar chemical properties, in which members differ from one another by the possession of an additional CH2 group, is called a homologous series.
    2. Each homologous series must have the following four characteristics:
      1. Every member in the homologous series can be represented by a common formula. For example:
        1. Alkane: CnH2n+2
        2. Alkene: CnH2n
        3. Alcohol: CnH2n+1OH
        4. Carboxylic acid : CnH2n+1COOH
        5. Ester: CnH2n+1COOCmH2m+1
      2. Every member differ from others by the group -CH2– which has the relative mass 14. For example:
      1. Members of the same homologous series can be prepared through one common method. For examples, All alkenes can be prepared by dehydration of relevant alcohols.

Preparing Ethene (Dehydration of ethanol):

C2H5OH → C2H4 + H2O

Preparing Propene (Dehydration of propanol)

C3H7OH → C3H8 + H2O

    1. Members of the homologous series have the same chemical properties. For example:All alkenes show addition reaction as added into bromine.

Functional Group

Functional group is a group of atoms tied together which will take part in a chemical reaction therefore determine the chemical properties of the series.
  1. Members of the same homologous series have the same chemical properties because of the presence of a common functional group in all the molecules of the series.
  2. Functional group is a group of atoms tied together which will take part in a chemical reaction therefore determine the chemical properties of the series.
  3. Therefore, the homologous series is a family of organic compound that is made out of members with common functional group, common chemical properties, and consecutive members are differ by one carbon atom and two hydrogen atom, namely, –CH2-.
  4. Table below shows some homologous series and their respective general formula and functional group.
 
Carbon Compounds General Formula Functional group
Alkane CnH2n+2 Carbon-carbon single bond – C – C –
Alkene CnH2n Carbon-carbon double bond – C = C –
Alcohol CnH2n+1OH Hydroxyl group – OH
Carboxylic Acids CnH2n+1COOH Carboxyl group – COOH
Esters CnH2n+1COOCmH2m+1 Carboxylate group – COO –