Allylic Substitution and the Allyl Radical

When propene reacts with bromine or chlorine at low temperatures, the reaction that takes place is the usual addition of halogen to the double bond  

However, when propene reacts with chlorine or bromine at very high temperatures or under conditions in which the concentration of the halogen is very small, the reaction that occurs is a substitution. These two examples illustrate how we can often change the course of an organic reaction simply by changing the conditions. (They also illustrate the need for specifying the conditions of a reaction carefully when we report experimental results.) 

At high temperature (or in the presence of a radical initiator) and low concentration of Xg a substitution reaction occurs. 

In this substitution a halogen atom replaces one of the hydrogen atoms of the methyl group of propene. 

Allylic hydrogen atom and allylic substitution are general terms as well. The hydrogen atoms of any saturated carbon atom adjacent to a double bond are called allyUc hydrogen atoms. Any reaction in which an allylic hydrogen atom is replaced is called an allyUc substitution. 

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