General processes of preparation of Alkyl Halides

         (1) From alkanes

         (i) By halogenation :

         This reaction tends to free radical mechanism.

• Order of reactivity of X₂ for a provided alkane is, F₂ > Cl₂ > Br₂ > I₂.
• The reactivity of the alkanes follows the sequence :

          3°alkane >  2°alkane >  1°alkane.

         (ii) With sulphuryl chloride :

•          This reaction is a fast due to in presence of light and trace of an organic peroxide.

         (2) From alkenes (Hydrohalogenation by Electrophillic addition)

•         Addition of HBr to alkene in the existence of organic peroxide take place due to peroxide effect or Kharasch's effect.
•         This addition take place by two methods, Peroxide initiates free radical mechanism.
•         Markownikoff's addition by electrophillic mechanism.        
•         The sequence of reactivity of halogen acids is, HI > HBr > HCl.

(3) From alcohols

         (i) By the action of halogen acids

            Groove's process

•          The reactivity sequence of HX in the above reaction is : HI > HBr > HCl > HF.
•          Reactivity order of alcohols 3⁰ > 2⁰ > 1⁰ > MeOH.
•          2° and 3° alcohols undergo S_N¹; whereas 1° and MeOH undergo S_N² mechanism.
•          Concentrated HCl + anhy. ZnCl₂ is known as lucas reagent.

         (ii) Using PCl₅ and PCl₃ :

•          Bromine and iodine derivatives cannot be obtain from the above reaction, because PBr₅ or PI₅ are unstable.
•          This method gives good yield of primary alkyl halides but poor yields of secondary and tertiary alkyl halides.

         (iii) By the action of thionyl chloride (Darzan's process) : Reaction takes place through SN² mechanism.

         (4) From silver salt of carboxylic acids (Hunsdiecker reaction, Decarboxylation by Free radical mechanism) 

•          The reactivity of alkyl group is 1⁰ > 2⁰ > 3⁰.
•          Only bromide are found in good yield.
•          Not suitable for chlorination due to yield is poor.
•          In this reaction iodine forms ester instead of alkyl halide and the reaction is called Birnbourn-Simonini reaction,

           .

         (5) By Finkelstein reaction (Halide exchange method) : 

         r Alkyl fluorides cannot be prepared by that process. They may be found from corresponding chlorides by the action of Hg₂F₂ or antimony trifluoride.

         (6) Other method

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