Preparation of Alkyl Halides from Alkanes

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Science > Chemistry > Halogen Derivatives of AlkanesYou are Here
  • Alkyl halides can be prepared from alkanes by their halogenation.

General Reaction:

R – H  +  X– X  →  R – X     +     H – X

Alkane    Halogen    Alkyl halide  Halogen acid

Conversion of -H (Alkanes) into – X (Alkyl halides)

Order of Reactivity for Halogens: 

F2 > > Cl2 > Br2 > I2



Benzylic, allylic > tertiary > secondary > primary > vinylic, aryl

Preparation of alkyl chlorides from alkanes (photohalogenation):

General Reaction:

  • When alkane is treated with chlorine in presence of ultraviolet light or diffused sunlight alkyl chlorides or chloroalkanes are obtained. The reaction gives a mixture of all possible chloroalkanes.

R – H  +  Cl– Cl   R – Cl     +     H – Cl

                    Alkane         Chlorine                Alkyl chloride          Hydrogen chloride

  • As the reaction is taking place in the presence of ultraviolet light or diffused sunlight the reaction is known as photohalogenation of alkanes.

Example –1: Preparation of methyl chloride (Chloromethane) from methane

CH4  +   Cl2      CH3Cl       +      HCl



             Methane      Chlorine         Methyl Chloride    Hydrogen Chloride

Example –2: Preparation of ethyl chloride (Chloroethane) from ethane

C2H6     +    Cl2   C2H2Cl         +      HCl

               Ethane       Chlorine                     Ethyl  Chloride       Hydrogen Chloride

Preparation of alkyl bromides from alkanes:

General Reaction :

  • When alkane is heated with bromine in presence of anhydrous AlBr3 as catalyst alkyl bromide or bromoalkane is obtained.

R – H  +  Br– Br   R – Br     +     H – Br

                    Alkane         Bromine                Alkyl bromide          Hydrogen bromide



Example- 1: Preparation of ethyl bromide (Bromoethane) from ethane:

C2H6     +    Br2     C2H2Br         +      HBr

                 Ethane       Bromine               Ethyl  bromide       Hydrogen  bromide

Example- 2: Preparation of methyl bromide (Bromomethane) from methane:

CH4  +   Br2     CH3Br       +      HBr

             Methane      Chlorine         Methyl Chloride    Hydrogen Chloride

Preparation of alkyl iodides from alkanes:

  • With iodine reaction is reversible. Thus we can not get a good yield of alkyl iodide. Hence direct iodination is difficult.

R – H      +  I – I     ⇌   R – I     +       H – I



                    Alkane         Iodine      Alkyl iodine     Hydrogen iodine

  • If the reaction is carried out in the presence of iodic acid HIO3, or mercuric oxide HgO, or dilute HNO3 which can oxidise HI formed and the lodoalkane can be obtained. The iodination reaction stops at mono-iodo state.

General Reaction Using Iodic Acid HIO3:

       5 R-H   +  2 I2 +   HIO3  →    5 R-I  +  3 H2O

Alkane   Iodine  Iodic acid    Alkyl iodide

General Reaction Using Mercuric Oxide HgO:

                  2R-H  +  2 I2  +    HgO      →       2 R-I  +  Hg I2 + H2O

Alkane   Iodine  mercuric oxide        Alkyl iodide



General Reaction Using Dilute Nitric Acid HNO3:

   8R-H  + 4 I2    +  HNO3  →       8R-I   + 3H2O    +   NH3

Alkane   Iodine    nitric acid     Alkyl iodide                ammonia

Example-1: Preparation of ethyl iodide (Iodoethane) using HIO3:

     5 C2H6   +  2 I2 +   HIO3  →    5 C2H5I  +  3 H2O

Ethane   Iodine  Iodic acid         Ethyl iodide



Example-2: Preparation of ethyl iodide (Iodoethane) using HgO:

                  2C2H6 +  2 I2  +    HgO      →       2 C2H5I  +  HgI2 + H2O

Ethane    Iodine  mercuric oxide     Ethyl iodide

Example-3: Preparation of ethyl iodide (Iodoethane) using HNO3:

   8C2H6 + 4 I2    +  HNO3  →       8C2H5I   + 3H2O    +   NH3

Ethane   Iodine  Nitric acid             Ethyl iodide        Ammonia

Drawbacks/Disadvantages of direct halogenation (Photohalogenation):

  • After forming alkyl halide the reaction proceeds further and poly-substitution takes place and gives a mixture of di, tri, tetra etc. haloalkanes.

CH4  +   Cl2  photohalogenation CH3Cl  (Chloromethane) +  HCl

CH3Cl   +   Cl2 → CH2Cl2 (Dichloromethane) +  HCl



CH2Cl2  +   Cl2 →  CHCl3  (Trichloromethane) +  HCl

CHCl3  +   Cl2  →  CCl4  (Tetrachlromethane) +  HCl

  • This reaction continues till all halogens in the alkane are replaced one by one by chlorine or bromine. Thus we get mixture of di, tri, tetra etc. haloalkanes.
  • The mixture contains less amount of alkyl chlorides or bromides. Chlorination and bromination reactions are not selective. Hence they may give isomers of the monohalogen derivatives of alkanes. Besides the separation of constituents is difficult.
  • Iodination of alkanes is reversible reaction it requires additional reagents like mercuric oxide or iodic acid or nitric acid for obtaining alkyl iodes. Hence direct halogenation is not the suitable method for the preparation of alkyl halides.

Notes:

  • Thermal chlorination of alkanes is called Hare and Mc Bee reaction. It is carried about at 673K.
  • Diffused sunlight causes homolytic fission of halogen, hence the reaction is a free radical substitution.
  • If alkanes are used in excess, the major product is monohalogen derivatives of alkanes. i.e. alkyl halides.
  • The ease of substitution of different types of a hydrogen atom is Benzylic, allylic > tertiary > secondary > primary > vinylic, aryl
  • Thus, an alkane containing a tertiary hydrogen atom would be more reactive towards substitution reaction than with only primary hydrogen atom or primary and secondary hydrogen atoms. Isobutane would undergo substitution more rapidly than propane or ethane.
  • Alkyl fluorides are not prepared by fluorination of alkanes, because the reaction is highly explosive in nature.
  • Propane on chlorination gives a mixture of 2- Chloropropane (55 %) and 1- Chloropropane (45 %)
  • Butane on chlorination gives a mixture of n-butyl chloride (28 %) and sec-butyl chloride (72 %)
  • iso-Butane on chlorination gives a mixture of tert-butyl chloride (64 %) and isobutyl chloride (36 %)
  • Bromination of alkanes is electrophilic substitution reaction, as it forms AlBr4 ion and Br+ ion. Br+ ion takes part in substitution.

 

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

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