Understanding the Product of Cyclohexene with Bromine Under UV Light and Heat

When cyclohexene reacts with bromine in the presence of either heat or UV light, a specific product is formed. This reaction is a classic example of a free radical substitution reaction. In this article, we will explore the process and deduce the product formed - 3-bromocyclohexene.

Rationale Behind the Reaction Mechanism

The reaction between cyclohexene and bromine (Br2) in the presence of UV light or heat follows a complex mechanism, but the end product is always 3-bromocyclohexene. This product is formed due to the stability and the ease of formation of the radical intermediates involved in the reaction.

Formation of the Product: 3-Bromocyclohexene

When cyclohexene reacts with bromine (Br2) in the presence of UV light, the reaction can be described as follows:

Initial Formation of Bromine Free Radical: UV light (or heat) causes the bromine molecule to break down into bromine free radicals:

Br2

Formation of Cyclohexene Allylic Radical: The cyclohexene molecules undergo a homolytic cleavage, forming a cyclohexene allylic radical:

Cyclohexene Br2 → Cyclohexene allylic radical Bromine free radical

Substitution Reaction: The bromine free radical then attacks the allylic position of the cyclohexene allylic radical, forming 3-bromocyclohexene:

Cyclohexene allylic radical Bromine free radical → 3-Bromocyclohexene

Formation of New Bromine Free Radical: The 3-bromocyclohexene product then breaks down, forming another bromine free radical, which can start the process again:

3-Bromocyclohexene → Cyclohexene allylic radical Bromine free radical

Reaction Mechanism in Detail

The detailed mechanism can be broken down as follows:

Bromine Homolysis: The first step involves the homolysis of bromine in the presence of UV light or heat. This results in the formation of bromine free radicals:

Br2 → 2 Br?

Formation of Cyclohexene Allylic Radical: Cyclohexene undergoes a homolytic cleavage at the allylic position, forming the cyclohexene allylic radical:

Cyclohexene → Cyclohexene allylic radical 3-H?

Attacking Bromine Free Radical: The bromine free radical then attacks the cyclohexene allylic radical at the allylic position, forming a new radical intermediate:

Cyclohexene allylic radical Br? → 3-bromocyclohexene H?

Termination or Further Reaction: The 3-bromocyclohexene radical can either reform bromine, continuing the reaction, or react further, depending on the conditions.

Conclusion

In conclusion, the reaction of cyclohexene with bromine in the presence of UV light or heat results in the formation of 3-bromocyclohexene. This reaction is a classic example of a free radical substitution reaction and occurs due to the stability of the intermediate radicals formed. Understanding this reaction helps in the design of synthetic pathways in organic chemistry.