Ethyne and Bromine Reaction in CCl4: Mechanism and Synthesis of 1,2-Dibromoethane

Ethyne, also known as acetylene, is a simple alkene with a unique triple bond. It has a wide range of industrial and chemical applications. In the presence of bromine in carbon tetrachloride (CCl4), ethyne undergoes an electrophilic addition reaction to form 1,2-dibromoethane (2-bromopropane). This article will explore the reaction mechanism, provide a step-by-step overview, and discuss the role of each compound involved.

Overview of the Reaction

Ethyne (C2H2) and bromine (Br2), when combined in carbon tetrachloride (CCl4), react to form 1,2-dibromoethane (HBr2CHBr2). This reaction is a classic example of an electrophilic addition reaction. The reaction does not only involve the formation of a new bond but also generates bromonium ions, which serve as key intermediates.

Reaction Steps

Formation of a Bromonium Ion

The first step in this reaction involves the attack of the π bond of ethyne on one of the bromine atoms. This leads to the formation of a bromonium ion intermediate. Simultaneously, the other bromine atom is released as a bromide ion (Br-).

Nucleophilic Attack

In the second step, the bromide ion (Br-) attacks the more substituted carbon of the bromonium ion. This nucleophilic attack results in the formation of a vicinal dibromide, which is 1,2-dibromoethane (HBr2CHBr2) in this case.

Overall Reaction Summary

The overall reaction can be summarized by the following equation:

text{C}_2text{H}_2 text{Br}_2 xrightarrow{text{CCl}_4} text{C}_2text{H}_2text{Br}_2

This results in the formation of 1,2-dibromoethane (HBr2CHBr2). The reaction is characterized by the addition of bromine across the triple bond of ethyne, converting it into a saturated compound, 1,2-dibromoethane.

Mechanism Summary

The reaction mechanism is described as follows:

Electrophilic Addition

The π bond in ethyne acts as a nucleophile and attacks the bromine atom.

Bromonium Ion Formation

A cyclic bromonium ion is formed, which is a crucial intermediate in the reaction.

Nucleophilic Attack by Br

The bromide ion attacks the bromonium ion, leading to the formation of the dibrominated product (1,2-dibromoethane).

Additional Insights

In a typical reaction setup, bromine solution in CCl4 is used. Initially, one mole of bromine reacts with one mole of ethyne to form 1,2-dibromoethene. Further addition of bromine will convert 1,2-dibromoethene into 1,2,2,2-tetrabromoethane.

The reaction mechanism can be represented by the following step-by-step reaction:

H-C≡C-H —

Br2

→ CHBrCHBr —

Br2

→ Br2HC-CHBr2

As the bromine solution is added, the color changes from red to colorless due to the consumption of bromine.