Question

In the presence of a small amount of bromine, the following light -promoted reaction has been observed.

(a) Write a mechanism for this reaction. Your mechanism should explain how both products are formed. (Hint: Notice which H atom has been lost in both products)

(b) Explain why only this one type of hydrogen atom has been replaced, in preference to any of the other hydrogen atoms in the starting material.

Short answer

Mechanism showing how both products are formed

(b) The bond dissociation enthalpy for allylic hydrogen is lower due to the formation of more stable allylic free radical.

Step-by-step solution

Free radicals

An atom or group of atoms containing odd or unpaired electron is known as the free radical. The unpaired electron is represented by a single unpaired dot in the formula. Free radicals are electrically neutral. They are highly reactive species formed by homolytic fission of a covalent bond.

Steps involved in free radical chain reaction

In a free-radical chain reaction, free radicals are generally created in the initiation steps. A free radical and a reactant is combined to yield a product and another free radical in the propagation steps. Lastly, the number of free radicals generally decrease in the termination steps.

Mechanism and explanation

(a) The mechanism consists of three parts which are initiation step, propagation step I and propagation step II.

Mechanism showing how both products are formed

(b) The bond dissociation enthalpy for allylic hydrogen is lower due to the formation of more stable allylic free radical.