What Is the Mechanism for The Reaction of Cyclohexane with Bromine in The Presence Of Sunlight?

In this article, we talk about the Br₂ light reaction with a common cycloalkane, cyclohexane, and its applications in the practical world.

Introduction:
Before we get in to Br₂ light reaction, let’s quickly review the basics of the elements involved in this reaction. Present in the halogen group of the periodic table, Bromine is quite a rare element. It is found in the earth’s crust and is rather rare as compared to its other similar metals. Mostly found as natural salt deposits and Brines, it can be filtered in pure form where it exists as a reddish-Brown liquid with Brown fumes. If exists in Br₂, the bond is a nonpolar molecule and does not have any significant polarity in its molecular structure.

Cyclohexane (C₆H₁₂) on the other hand is a cycloalkane, has a nonpolar molecule, is a cyclic version of hexane, and has a sharp smell. It has a ring containing six carbon atoms and exists as a liquid at room temperature/pressure. This molecule doesn’t have a nucleophile nature due to the unavailability of unsaturation present in its structure. A similar molecule is C₆H₁₂ or Cyclohexene which resembles the cyclohexane in its chemical structure but differs due to being an unsaturated hydrocarbon. This nature comes from its double bond between two of the carbon atoms in its molecular structure as compared to single bonds between each carbon atom of Cyclohexane.  

Reaction Of Cyclohexane with Bromine

Their reaction is a typical “halogenation” reaction where a series of reactions result in a product called cyclohexyl Bromide (3-Bromocyclohexene) represented by chemical formulae of C₆H₁₁Br.  The reaction can be stated as follows:

C₆H₁₂ + Br₂ → C₆H₁₁Br + HBr

As mentioned before, cyclohexane has no available electrons for bonding with other chemicals and that is why it cannot react with other elements like Bromine in normal reaction conditions. But the reaction above can be executed in the presence of a catalyst as it can accelerate the Breaking of C-H bonds to introduce Bromine in them. But even this process is pretty slow due to the strong bonding between the C-H bonds and somehow low polarity of Bromine as compared to other similar elements like Chlorine.

The most common catalyst for their combination is UV light. In this Br₂ light reaction, Cyclohexane replaces one of its hydrogen atoms with a Bromine atom after Breaking of C-H bonding in its molecular structure. This results in C₆H₁₁Br with HBr giving away as a byproduct. The catalyst here, which is the UV light triggers a free-radical substitution reaction between them.

In this reaction type, the influence of light triggers the generation of Br•radicals from the cleavage within the Br₂ molecules. These radicals influence cyclohexane and generate a C₆H₁₁•radical which reacts with another Bromine molecule to form cyclohexyl Bromide or C₆H₁₁Br. Note that this reaction of cyclohexane with Bromine can also be accelerated with the help of heat instead of UV light.

Applications of Cyclohexane & Bromine Reaction:

Since their reaction falls in the halogenation category, both act as an important industrial agent involved in the synthesis of different halogenated organic compounds. The resultant product from this reaction is also used as a solvent in various other reactions where the extraction of solvents is crucial. It is also used as a functional group involved in the production of many other complex organic molecules. Moreover, this reaction is frequently used for educational and industrial research purposes where it plays roles to act as a “substituted cyclohexanes” used in material sciences and other front-end chemistry studies.

Conclusion:

To conclude the Br₂ light reaction, it can be stated that their reaction can be regarded as a typical example of halogenation. The two participants in this reaction result in 3-Bromocyclohexene and HBr where light acts as a catalyst which also can be replaced by heat. The reaction is quite slow though and once triggered, the resultant solution will have a plethora of free radicals ready to react. The reaction holds strong significance for the organic synthesis of various chemical compounds and is also a great way for further research of organic compounds.