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CBSE Important Questions

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Homologous Series

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A group of organic compounds having similar structures and similar chemical properties in which the successive compounds differ by CH2 group.

Two adjacent homologous groups differ by 1 carbon atom and 2 hydrogen atoms .

General formula of Alkanes --- CnH2n+2

Characteristics of a Homologus Series

  1.  All members represented by same general formula.
  2. Two adjacent homologues differ by a CH2
  3. Difference between molecular masses of any 2 homologues is 14 μ
  4. All compounds show similar chemical properties.
  5. There is gradual change in their physical properties with increase in molecular masses. (melting , boiling points and densities increase)

Alkenes --- CnH2n

Alkynes --- CnH2n-2

Functional Groups

An ‘atom’ or a ‘group of atoms’ which makes a carbon compound reactive and decides its properties.

  1. Halo Group :   X  (X can be  Cl, Br or I)
    Chloro , Bromo and Iodo group ----
    When 1 H atom of an alkane is replaced by a halogen atom, we get Haloalkane.
    Eg. CH4 → Replace one H by Cl → CH3Cl

    General  formula  is  CnC2n+1 X   (  X  may be Cl ; Br; I  )

    Chloromethane - CH3Cl   ---- methyl chloride
    Bromomethane - CH3Br   ---- methyl  bromide
    Chloroethane    -  C2H5Cl  ----  ethyl  chloride
    Chloropropane  -  C3H7Cl  ----  propyl chloride

  2. Alcohol Group :  OH  ( alcoholic  /  hydroxyl group)
    Alcohol group is made up of  1 oxygen and 1 hydrogen atom joined together.
    1 H atom is replaced by an alkane  by a hydroxyl group ( OH  group)
    CH4  → Replace  1 H by  OH → CH3OH

    General formula  is  CnC2n+1OH
    Methanol  - CH3OH  ---- methyl  alcohol
    Ethanol     -  C2H5OH ---- ethyl alcohol
    Propanol  -   C3H5OH ----  propyl  alcohol

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Submitted By Mrs. Kritika Bhola
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