Preparation of Alkyl halides From Alcohols

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Preparation of Alkyl Halides From Alcohols by the Action of Hydrogen Halides on Alcohols

The Action of HX on Alcohols:

  • The reaction is a nucleophilic substitution.
  • Methyl and primary alcohol undergo SN¹ mechanism while secondary and tertiary alcohols undergo SN² mechanism.
  • Among halogen halides, HCl is least reactive in nature. Because the chloride ion is a weaker nucleophile than bromide or iodide ions. Hence hydrogen chloride is mixed with anhydrous ZnCl2.
  • In these reactions, anhydrous ZnCl2 not only acts as dehydrating agent but also helps in the cleavage of C-O bond of the alcohol. ZnCl2 is Lewis acid and coordinates with the oxygen atom and thus weakens C-O bond. This results in the formation of carbocation which combines with Cl- ion to form chloroalkane. Tertiary alcohols are highly reactive hence for tertiary alcohol ZnCl2 is not required.
  • This reaction is a nucleophilic substitution. The stability of carbocations is of order tertiary > secondary > primary. Hence the order of reactivity also follows the same order tertiary > secondary > primary.
  • Bond dissociation energy of H-X bond is of order H-Cl > H-Br > H-I. hence the reactivity of halogen acids follows the order HI > HBr > HCl.
  • Unlike alkyl chlorides, the secondary and tertiary bromides and iodides cannot be obtained from their respective alcohols. It is because of the secondary and tertiary alcohols on heating with concentrated H2SO4 undergo dehydration to form alkenes. Hence for these preparations dilute H2SO4 is used.

General Reaction:

                R–OH  +            HX          →     R–X       +    H2O

Alcohol     halogen acid       alkyl halide

Order of Reactivity:

Tertiary alcohol > Secondary alcohol > Primary alcohol

and HI > HBr > HCl.



Preparation of Alkyl Chlorides:

General Reaction:

  • When alcohol is treated with Lucas reagent alkyl chloride is obtained. Lucas reagent is a solution of a concentrated hydrochloric acid with zinc chloride. This reaction is known as Groove’s process.

                  R–OH  +            HCl               R–Cl       +    H2O

Alcohol     Conc.Hydrochloric acid     Alkyl chloride

Example – 1: Preparation of ethyl chloride (Chloroethane) from ethyl alcohol (Ethanol):

              C2H5OH  +            HCl                  C2H5Cl       +    H2O

Ethyl alcohol   Conc.Hydrochloric acid       Ethyl chloride



Example – 2: (Preparation of isopropyl chloride (2-Chloropropane) from isopropyl alcohol (Propan-2-ol):

Example – 3: (Preparation of tert- Butyl chloride (2-Chloro-2-methylpropane) from tert- Butyl alcohol (2-Methylpropan-2-ol):

Preparation of Alkyl Bromides:

General Reaction:

  • When alcohol is treated with concentrated hydrobromic acid (NaBr + H2SO4) alkyl bromide is obtained.

                         R–OH  +            HBr                R–Br       +    H2O

           Alcohol    Hydrobromic acid               Alkyl chloride

Example – 1: Preparation of ethyl bromide (Bromoethane) from ethyl alcohol (Ethanol):

              C2H5OH         +       HBr                 C2H5Br       +    H2O



Ethyl alcohol    Hydrobromic acid                          Ethyl bromide

Example – 2: (Preparation of isopropyl bromide (2-Bromopropane) from isopropyl alcohol (Propan-2-ol):

Example – 3: (Preparation of tert- Butyl bromide (2-Bromo-2-methylpropane) from tert- Butyl alcohol (2-Methylpropan-2-ol):

Preparation of Alkyl Iodides:

General Reaction:

  • When alcohol is refluxed with Potassium or sodium iodide with 95% phosphoric acid and heated, alkyl iodide is obtained.

                R–OH  +            HI                R–I       +    H2O

Alcohol    Hydroiodic acid                 Alkyl iodide



Example – 1: Preparation of ethyl iodide (Iodoethane) from ethyl alcohol (Ethanol):

              C2H5OH         +       HI                 C2H5I       +    H2O

Ethyl alcohol    Hydroiodic acid                         Ethyl iodide

Example – 2: (Preparation of isopropyl iodide (2-Iodopropane) from isopropyl alcohol (Propan-2-ol):

Preparation of Alkyl Halides From Alcohols by Action of Phosphorous Halides:

  • This method is suitable for preparation of primary and secondary alkyl halides.
  • A good yield of tertiary alkyl halides cannot be obtained by this method.
  • The reaction of an alcohol with PX3 does not involve the formation of carbocation and usually occurs without rearrangement of the carbon skeleton.

Preparation of Alkyl Chlorides By Action of Phosphorus Trichloride PCl3 on Alcohol:

General Reaction:

  • When alcohol is treated with phosphorous trichloride, alkyl chloride and phosphorous acid are obtained.

3 R-OH  +            PCl3            3 R-Cl       +    H3PO3

       Alcohol       phosphorous trihalide    alkyl halide    phosphorous acid



Example – 1: Preparation of ethyl chloride (Chloroethane) from ethyl alcohol (Ethanol):

    3C2H5OH  +            PCl3                3C2H5Cl       +    H3PO3

Ethyl alcohol   Phosphorous trichloride            ethyl chloride    Phosphorus acid

Both the products are in the liquid state and are separated by fractional distillation.

Example – 2: (Preparation of isopropyl chloride (2-Chloropropane) from isopropyl alcohol (Propan-2-ol):



Preparation of Alkyl Chlorides By Action of Phosphorus Pentachloride PCl5 on Alcohol:

General Reaction:

  • When alcohol is treated with phosphorous pentachloride, alkyl chloride and phosphoryl chloride (phosphorous oxychloride) are obtained.

ROH         +     PCl5                  RCl         +     POCl3           +            HCl

Alcohol   Phosphorous pentachloride        Alkyl chloride    Phosphoryl chloride    hydrogen chloride

Example – 1: Preparation of ethyl chloride (Chloroethane) from ethyl alcohol (Ethanol):

    C2H5OH  +            PCl5               C2H5Cl       +    POCl3           +            HCl

Ethyl alcohol   Phosphorous pentachloride        Ethyl chloride    Phosphoryl chloride    hydrogen chloride

Example – 2: (Preparation of isopropyl chloride (2-Chloropropane) from isopropyl alcohol (Propan-2-ol):

Preparation of Alkyl Bromides by the Action of Mixture of Phosphorous and Bromine/ Iodine on Alcohols:

  • Unlike PCl3, the compounds PBr3 and PI3 are not stable and hence they are to be prepared when they are to be used by treating bromine and iodine with red phosphorous.
  • Compounds PBr5 and PI5 do not exist.

Preparation of Alkyl Bromides by the Action of Mixture of Phosphorous and Bromine on Alcohols:

  • Alkyl bromides are prepared by the action of bromine, in presence of red phosphorus, on alcohols.  Phosphorus bromide PBr3, which is unstable is formed as intermediates in the reaction.

3 R-OH  +            PBr3            3 R-Br       +    H3PO3



       Alcohol       phosphorous tribromide    alkyl bromide    phosphorous acid

Example – 1: Preparation of ethyl chloride (Chloroethane) from ethyl alcohol (Ethanol):

    3C2H5OH  +            PBr3               3C2H5Br       +    H3PO3

Ethyl alcohol   Phosphorous tribromide            ethyl bromide    Phosphorus acid

Preparation of Alkyl Iodides by the Action of Mixture of Phosphorous and Iodine on Alcohols:

  • Alkyl iodides are prepared by the action of iodine, in presence of red phosphorus, on alcohols.  Phosphorus iodide PBr3, which is unstable is formed as intermediates in the reaction.

3 R-OH  +            PI3            3 R-I       +    H3PO3



Example – 1: Preparation of ethyl iodide (Iodoethane) from ethyl alcohol (Ethanol):

       Alcohol       phosphorous triiodide    alkyl iodide    phosphorous acid

3C2H5OH  +            PI3              3C2H5I       +    H3PO3

Ethyl alcohol   Phosphorous triiodide          ethyl iodide    Phosphorus acid

Preparation of Alkyl chlorides From Alcohols by the Action of Thionyl Chloride (Darzen’s Procedure):

General Reaction:

  • When alcohol is refluxed with thionyl chloride, alkyl chloride, sulphur dioxide and hydrogen chloride are obtained. Better yield is obtained if pyridine is added in a small amount.

           ROH  +        SOCl2        RCl  + SO2 ↑­  + HCl  ­↑

Alcohol    Thionyl chloride              Alkyl chloride

Example – 1: Preparation of ethyl chloride (Chloroethane) from ethyl alcohol (Ethanol):

                           C2H5OH  +        SOCl2        C2H5Cl  + SO2 ↑­  + HCl  ­↑



Ethyl alcohol        Thionyl chloride                 Ethyl chloride

Example – 2: (Preparation of isopropyl chloride (2-Chloropropane) from isopropyl alcohol (Propan-2-ol):

  • The use of thionyl chloride for preparation of alkyl chlorides is most convenient because the other products of reaction (SO2 and HCI) being gases go off  (or can be expelled during distillation very easily). and hence the method requires no special purification/separation.

C5H11OH on bromination gives compound C5H11Br. Suggest all possible structures of alcohol and bromide.

Preparation of Alkyl Halides
 

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

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