Reaction of Ethene with Bromine under UV Light: A Comprehensive Guide

When ethene (CH2CH2) reacts with bromine (Br2) in the presence of ultraviolet (UV) light, a fascinating photochemical reaction takes place. This process, known as the halogenation of ethene, is not only important in organic chemistry but also finds applications in various industries. In this article, we will explore the mechanistic details of this reaction, its significance, and its practical uses.

Mechanism of the Reaction

The reaction of ethene with bromine under UV light occurs through a free radical mechanism, which is a type of photochemical reaction. This reaction is distinct from the typical electrophilic addition reaction of bromine with alkenes, as it requires the presence of UV light to initiate the process.

Initiation Step

The key to initiating this reaction is the absorption of UV light by the bromine molecules. UV light causes the dissociation of bromine into bromine radicals (Br·). This step is crucial for the subsequent reaction to occur.

Initiation: UV light causes the dissociation of bromine molecules into bromine radicals.
Br2 → 2 Br·

Propagation Step

The bromine radicals (Br·) then react with ethene (CH2CH2) to form a bromoethyl radical (C2H3Br·).

Propagation: The bromine radical reacts with ethene to form a bromoethyl radical.
Br· CH2CH2 → C2H3Br·

Termination Step

The bromoethyl radical can then react with another bromine radical, leading to the formation of 1,2-dibromoethane (C2H4Br2). This process continues until the radicals are consumed.

Termination: The bromoethyl radical can then react with another bromine radical to form 1,2-dibromoethane.
C2H3Br· Br· → C2H4Br2

This reaction results in the formation of 1,2-dibromoethane, a compound characterized by its absence of color, in contrast to the orange bromine initially present. The presence of UV light is crucial for initiating the radical formation, making this reaction distinct from typical electrophilic addition reactions of bromine with alkenes.

Significance and Applications

The reaction of ethene with bromine under UV light is not only a fundamental example in organic chemistry but also has practical applications in testing for unsaturation in organic compounds. The characteristic red-brown color of bromine disappears as it adds across the double bond, making this reaction a visual indicator of unsaturation.

In addition to its educational value, this reaction is also used in the synthesis of 1,2-dibromoethane, which is a useful intermediate in the production of other organic compounds. The ability to control the reaction through the use of UV light allows for precise and controlled synthesis, which is crucial in the pharmaceutical and polymer industries.

Conclusion

The reaction of ethene with bromine under UV light is a clear example of a photochemical halogenation reaction. Understanding this reaction is essential for both educational and practical purposes. Whether in the classroom or in industrial settings, this reaction showcases the power of UV light in driving chemical reactions and its applications in organic synthesis.