Chemistry MECHANISM OF HALOGENATION OF ALKANES
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MECHANISM OF HALOGENATION OF ALKANES

Mechanism of Halogenation of Alkanes :

`text(Halogenation :)` See fig.1.

`undersettext(excess)(CH_4) + X_2 overset(U.V.)-> CH_3-X +HX`

`CH_4 +X_2 overset(U.V.)-> CH_3-X -> CH_2X_2-> CHX_3 -> CX_4`

`undersettext(excess)(CH_3-CH_3) +Cl_2 oversettext(sunlight)oversettext(diffused)-> CH_3CH -Cl + HCl`

`CH_3-CH_3 + undersettext(excess)(Cl_2) undersettext(sunlight)oversettext(diffused)-> CH_3CH_2Cl +` mixture of product obtained by polychlorination

See fig.2.

Mono halogenation of propane and Butane : See fig.3.

Hence these reactions show that the reaction depends upon the abstracting power of the halogen atoms as well as the number of `H`-atoms present in the organic molecule.

The abstracting power of the halogen atoms refers to the selectivity factor which includes the `HX` bond energy as well as bond dissociation energy of the `C-H` bond.

Mechanism of Halogenation of `CH_4` (monohalogenation) :

`CH_4 +X_2 oversettext(sunlight)oversettext(diffused)-> CH_3X +HX`

`text(Initiation)` `text(Step-I)` `X_2 -> 2X^*..................(i)`

`text(Propogation)` `text(Step-II)` `X^* + CH_3-H -> HX + CH_3^* .....................(ii)`

`CH_3^* + X-X -> CH_3X + X^*..................(iii)`

`X^* +CH_3-H -> HX......................(iv)`

In this whole reaction the reactive species is `X^*`. The formation depends upon the abstracting power of halogen which in turn depends upon the bond strength of `HX` formed. Which is `HF > HCl > HBr > HI` and also bond dissociation energy of `X_2` which is `Cl_2 > Br_2 > F_2 > I_2`. See fig.

`text(Termination Step- III)` The free Radicals present in the reactions are `X^*` and `CH_3^*`. The different ways in which they can combine for terminating the reaction is

`overset(.)(CH_3) +overset(.)(X) -> CH_3 -X`

`overset(.)(CH_3) +overset(.)(CH_3) -> CH_3-CH_3`

`overset(.)(X) +overset(.)(X) -> X_2`

Reactivity of different `H`-atom :

The general order of reactivity of different `H`-atoms is tertiary-H > secondary-H > primary-H because the stabilities of the corresponding free radicals are tertiary free radical > secondary free radical > primary free radical. The basic reactivity of `H`-atom is tertiary > secondary > primary but their reactivity ratio with chlorine or bromine differs.

p : s : t

The reactivity order with chlorine `1 : 3.8 : 5`

The reactivity order with Bromine `1 : 82 : 1600`

The reaction of alkane with `I_2` can be performed by addition of oxidising agents like `HNO_3` or `HIO_3`.

`CH_4 + I_2 ⇄ CH_3 - I + HI`

This reaction is highly reversible with reactions proceeding more in backward direction. But on adding `HNO_3` or `HIO_3` the `HI` gets oxidised to `I_2` thereby preventing the reduction of `CH_3-I` to `CH_4`.

Laboratory Chlorination :

The laboratory chlorinations are usually done using `SO_2Cl_2` (sulphuryl chloride) in the presence of alkyl peroxides as initiator.

`R' -H + SO_2Cl_2 overset(R-O-O-R)-> R'- Cl + SO_2 + HCl`

`text(Initiation Step-I)` `R -O - O -R -> 2R - O^*`

`text(Step-II)` `R -O + R' -H -> R - OH + [R']^*`

`text(Propagation Step-III)` `[R']^* + SO_2Cl_2 - R'Cl + S^* O_2Cl`

`text(Step-IV)` `S^* O_2Cl - > SO_2 + Cl^*`

`text(Step-V)` `Cl^* + R' -H -> [R']^* + HCl`

Then there is repetition of step `III`, `IV` and `V`.

`text(Termination :)`

The different free radicals are `R'^*, S^* O_2Cl` and `Cl^*, R- O^*` which can combine in many ways.

Evidence in support of free radical mechanism :

(i) `text(Effect of U. V. light :)`

The reaction does not take place at room temperature. This is because energy is needed to cleave chlorine molecule homolytically into chlorine free radical.

(i) `text(Effect of number of photon :)`

For each photon or quantumn of light used, several thousand molecules of alkylhalide are formed.

(ii1) `text(Effect of Adding Inhibitors :)`

Inhibitors are substances which slow down the rate of reaction. It has been observed that when oxygen is added in the reaction mixture, halogenation of alkanes slows down due to the formation of less reactive peroxy alkyl free radical.

`R^(.)·+O= O^(-)`
 
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