Preparation of Alkyl Halides by Halide Exchange Method

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  • This method is typical for preparations of alkyl iodides/fluorides/bromides.

Preparation of Alkyl iodides by Halide Exchange Method(Finkelstein Reaction)

General Reaction:

  • When alkyl chlorides or bromides when treated with NaI in presence of dry acetone give alkyl iodides.

R–Cl or R–Br  +  NaI → RI + NaCl  or NaBr

Alkyl chloride / bromide   sodium iodide   →    Alkyl iodide sodium chloride / bromide

Example – 1: Preparation of Ethyl iodide (Iodoethane) from ethyl chloride (Chloroethane):

C2H5-Cl    +      NaI  C2H5I      +   NaCl

Ethyl chloride    sodium iodide            Ethyl iodide       sodium chloride



Example – 2: Preparation of Ethyl iodide (Iodoethane) from ethyl bromide (Bromoethane):

C2H5-Br    +      NaI  C2H5I     +   NaBr

Ethyl bromide    sodium iodide                  Ethyl iodide       sodium bromide

Preparation of Alkyl fluorides by Halide Exchange Method(Swart’s Reaction)

  • The direct reaction of alkanes with fluorine is highly explosive in nature, hence it can’t be produced by direct fluorination of alkanes.

General Reaction:

  • When alkyl halides are treated with salts like AgF, Hg2F2, CoF3, SbF3 fluoroalkanes can be obtained.

Example – 1: Preparation of Ethyl fluoride (Fluoroethane) from ethyl chloride (Chloroethane):

C2H5Cl          +     AgF    →  C2H5F           +   AgCl



Ethyl chloride   silver fluoride   Ethyl fluoride       silver chloride

Example – 2: Preparation of Methyl fluoride (Fluoromethane) from methyl bromide (Bromoethane):

2  CH3Br          +         Hg2F2     →       2 CH3F           +   Hg2Br2

Methyl chloride   mercurous fluoride     Methyl fluoride   mercurous chloride

  • For replacement of two or three halogens CoF3, SbF3 are used

Example – 3



Preparation of Alkyl bromides from silver salt of fatty acid (Borodine Hunsdiecker Reaction)

General Reaction:

When silver salt of fatty acid is refluxed with bromine in CCl4, alkyl bromide is obtained.

RCOOAg       +          Br2    Bromoethane    RBr         +      CO2 +  AgBr

Silver salt of fatty acid               bromine         alkyl bromide                 silver bromide

Example -1: Preparation of Methyl Bromide (Bromomethane) from silver acetate:

CH3COOAg       +          Br2        CH3Br         +      CO2 +  AgBr



Silver acetate                bromine             methyl bromide            silver bromide

Example -2: Preparation of Ethyl Bromide (Brommethane) from silver propionate:

C2H5COOAg    +     Br2        C2H5Br         +      CO2 +  AgBr

Silver propionate            bromine          ethyl bromide                      silver bromide

Note:

  • Chloroalkanes can be obtained by this method but yield is very low. Iodoalkanes can not be obtained by this method because iodine forms easter with  silver salt of fatty acid.

 

Science > Chemistry > Halogen Derivatives of AlkanesYou are Here
Physics Chemistry  Biology  Mathematics

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