Question

Show mechanism for 2 methylpropane free radical monobromination.

Explain why pryon undergoes substitute reaction like benzene but not addition reaction at the C=C bond

Answer 1

a) The monobromination of 2-methylpropane is shown below in the figure. It is a free radical chain reaction mechanism. Generally, free radical reactions are described in three steps: initiation, propagation and termination. The first step in the reaction mechanism is the homolytic splitting of a Br2 bond to give a pair of Br free radicals by the addition of uv light. This is called the initiation step because it initiates the reaction. The next step is the abstraction of the hydrogen atom from the tertiary carbon by a Br radical to generate a new radical at the tertiary carbon atom. The tertiary radical is more stable then the primary radical. Finally the tertiary radical then reacts with another molecule of bromine to form the product and regenerating a Br radical in the process.

The detailed reaction scheme and the mechanism is shown below in the figure.

b) Pyron undergoes substitution reaction like benzene and not addition reaction at C=C double bond. This is due to the enhanced stability of pyron due to the presence of conjugated double bonds in the system of pyron, just like benzene. Therefore, any addition to C=C double bond would only make the system less stable and hence least likely reaction to occur similar to benzene.

Given below is the mechanism for the monobromination of 2-methylpropane