When the mixture of hydrocarbon and halogen is heated at 520-670 K in dark or is subjected to ultraviolet light at room temperature, the free radical substitution reaction takes place.
The reactivity of hydrogen towards free radical substitution is 3° > 2° > 1°. For example, when butane is used in free radical substitution reaction, the products we get are 1-Chlorobutane and 2-Chlorobutane. 2-Chlorobutane being 2° is the major product.
Free radical substitution reaction is a chain reaction. Let's take the example of CH4 and Cl2 to understand the mechanism. The free radical substitution reaction consists of the following steps :
When a mixture of CH4 and Cl2 is heated at 520-670 K or is subjected to ultraviolet light at room temperature, Cl2 absorbs energy and undergoes homolytic fission.
Propagation step consists of two sub steps
The chain reaction may terminate if two of the same or different free radicals combine among themselves without producing new free radicals.
Alkenes react with halogen acids to form haloalkanes. Markovnikov's rule is applied in case of unsymmetrical alkenes.
Markovnikov's rule states that in the addition reactions of unsymmetrical alkenes, the negative part (e.g. halogen) of reagent attaches to the carbon having less number of hydrogen atoms. For example, prop-1-ene reacts with HBr to give 2-bromopropane as a major product.
If HBr is added to alkene in the presence of peroxide, the negative part (i.e. Br-) attaches to the carbon having more number of
hydrogen atoms. Since the peroxide effect is contrary to Markovnikov's rule, it is also known as antimarkovnikov's rule. For example,
Prop-1-ene reacts with HBr in the presence of peroxide to give 1-bromopropane as a major product.