In the last three articles, we have studied the methods of preparations of alkyl halides from alkanes, alkenes, alcohols. In this article, we shall study the method of the preparation of alkyl halids by halide exchange method. 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 bromide Mercurous fluoride  Methyl fluoride  Mercurous bromide

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        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 > Organic Chemistry > Halogen Derivatives of Alkanes > Preparation of Alkyl Halides by Halide Exchange Method

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