Substitution Reactions - Organic Chemistry

Substitution Reactions - Organic Chemistry

A reaction in which one group or atom is replaced by another...


Substitution Reactions
A reaction in which one group or atom is replaced by another is called a substitution reaction.

  1. Free Radical Substitution: Alkenes undergo free radical substitution in the presence of light

    CH4 + Cl2CH3Cl + HCl

    Light energy or heat causes homolytic fission of chlorine producing chlorine radicals which attack methane to methyl chloride.

    When the ratio of CHto Cl2 is high, CH3Cl is formed predominantly and when Clis in excess, all the hydrogens are replaced to give CCl4.

    Ratio of rate of substitution.
     
      1H 2H 3H
    In Cl: 1 : 3.8 : 5
    In Br2 : 1 : 82 : 1600

    The number of types of H present in an alkane is equal to the number of monohalogen substituted products obtained by the halogenation of that alkane.

    For example, Isopentane has 4 types of H, hence, isopenthane on chlorination will give four alkyl chlorides.



  2. Nucleophilic Substitution: The reaction which involves displacement of a nucleophile by another nucleophile is called nucleophilic substitution (SN) reaction. For example, hydrolysis of alkyl halide.



  3. Electrophilic (aromatic) Substitution: Due to having negative -electron cloud, benzene ring discourages the nucleophilic attack and encourages electrophilic attack.

    The typical reactions of benzene and its derivatives are electrophilic substitution.


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