Question

Expalain the free radical substitution of alkane?

Answer 1

See below:

Explanation:

Alkanes can be converted to Halogenalkanes through a free radical substitution as free radicals are highly reactive.

This is best broken down into 3 steps: Initiation, Propagation and Termination

Lets use the reaction between Chlorine and Methane (`CH_4`), which can occur in the atmosphere.

Initation

1. `Cl_2 -> 2Cl^.` A chlorine molecules is broken down by UV light and undergoes homolytic fission (electrons in the split covalent bond goes to each of the two atoms, which turn into free radicals- a species with an unpaired electron= reactive.)

2. Propagation
   These free radicals will go on and react with other molecules around them: like Methane.

`Cl^.+CH_4 -> HCl + CH_3^.`

The free radical will donate it's lone electron to form a new covalent bond with Hydrogen, causing the C-H bond in Methane to split and creating a new radical (a methyl radical) which will go on reacting.

`CH3^.+Cl_2 -> CH3Cl + Cl^.`

This step can form the halogenalkane, chloromethane.

3 Termination
When two radicals react together to from a new molecule which will not go on reacting. This can also form the desired product.

`CH3^. + Cl^. ->CH3Cl`

This also produced Chloromethane.

Or form a new alkane in this case:

`CH3^. + CH3^. -> C_2H_6`

Which produces Ethane.