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Note on Homologous Series

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A homologous series is a group of organic compounds, which has the same functional group, but two successive members differ by CH2 group.

http://i.imgur.com/23Nf6jP.gif

Characteristics of homologous series

  1. All the members of the same series can't be represented by the same general formula. For example, the series of alkanes can be represented by general formula CnH2n+2.
  2. Each successive member of a homologous series differs by CH2.
  3. All members of the series show similar chemical properties.
  4. All members of the series can be prepared by general methods of preparation.
  5. All members can show a gradual change in their physical properties such as specific gravity, melting point, etc.
  6. All members of the same series have the same functional group. For example, Alcohol (-OH) , ether (-O)

Alkyl group:

The hydrocarbon unit derived by the removal of one hydrogen atom from an alkane is called alkyl group or the alkyl radical. The name of an alkyl radical is written by replacing the ending "ane" of the alkane with "ayl".

- H

i.e. Alkane -> Alkyl radical

S.No Alkanes Alkyl Radicals
1. CH4 (Methane) CH3 (Methyl)
2. C2H6 (Ethane) C2H5 (Ethyl)
3. C3H8 (propane) C3H7 (propyl)
4. C4H10 (Butane) C4H9 (butyl)

Functional group:

It is defined as an atom or group of atoms (radical) which determines the chemical behavior of the organic compounds.

S.No Name of functional group Symbol of functional group Organic compounds
1. Hydroxyl OH Alcohol
2. Ether O Ether
3. Amino NH2 Amme
4. Carboxylic acid COOH Acid
5. Aldehyde (formyl) CHO Aldehyde
6. Keto CO Ketone

When an alkyl group combines with a functional group, an organic compound is formed. Example:

-CH3+ -OH \(\longrightarrow\) CH3OH

  • All the members of the same series can’t be represented by the same general formula.
  • Each successive member of a homologous series differs by CH2.
  • All members of the series can be prepared by general methods of preparation.
  • All members can show a gradual change in their physical properties such as specific gravity, melting point etc.
  • All members of the same series have the same functional group. For example: Alcohol (-OH) , ether (-O)
.

Very Short Questions

A series of related hydrocarbons that have the same functional group and can be denoted by the common general formula is called homologous series.

Formula Common Name IUPAC Name
CH3 – CH2 – CH2 – CH3 n – Butane Butane
CH3 – CH = CH2 Propylene Propane
HC ≡ CH Acetylene Ethyne

The characteristics of homologous series are:
1. Each successive member differs by CH2.
2. All members of the series can be prepared by similar method of preparation.
3. All the members show similar physical and chemical properties.
4. All members must have same functional group.

A homologation reaction is a chemical process which converts one member of a homologous series to the next member.

Following are the characteristics of homologous series:

  1. All the members of the series can be represented by a general formula.
  2. The adjacent members of a homologous series differ in their formulae by CH2.
  3. All members of the series can be prepared by general methods of preparation.
  4. All members of the series show similar chemical properties.

A series of related hydrocarbons that have the same functional group and can be denoted by the common general formula is called homologous series.

The individual member of homologous series is called homologue.


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  • ______ is used  for purifying organic compounds by extraction.

    ethanoic acid
    alcohol
    ethanol
    diethyl ether
  • A group of organic compounds, which has the same functional group, but two successive members differ by CH is ________.

    homologuos series 


    alkene


    alkane


    alkyl group


  • Each successive member of a homologous series differs by _______.

    CH2


    CH


    CH3


    CH4


  • The functional group of alcohol is ______.

    O


    OH


    CH


    none of above


  • The hydrocarbon unit derived by the removal of one hydrogen atom from alkane is called ______.

    functional group


    homologous series


    alkyl group


    ethyl group


  • The alkyl radical of CH4 is_______.

    propyl
    ethyl
    methyl
    butyl
  • _______ is defined as an atom or group of atoms (radical) which determines the chemical behavior of the organic compounds.

    alkyl group
    Functional group
    none of above
    Ethyl group
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