Question

Methyl bromide is widely used as a fumigant to prevent the spread of diseases and pests in agricultural products, and it can be prepared from methane via radical bromination. Recently, a new method for preparing methyl bromide was developed, involving genetically engineered microorganisms that are capable of producing large quantities of the compound.

1. In practice, radical bromination of methane produces many by-products. For example, ethane is obtained in small quantities. Suggest a mechanism for the formation of ethane.

Draw the first step of the mechanism (initiation stage). Include all nonbonded electrons (lone pairs and radicals) in your answer.

Answer 1

The formation of ethane as a by-product in the radical bromination of methane occurs when two methyl radicals (•CH₃) produced in the reaction combine. The first step of this mechanism, the initiation stage, involves the homolytic cleavage of molecular bromine into bromine radicals.

Step-by-step explanation:

The question involves the mechanism by which radical bromination of methane can lead to the formation of by-products such as ethane. The first step in this mechanism is the initiation stage, where a homolytic cleavage occurs due to the energy provided by heat or light, producing two bromine radicals (Br•).

The chemical equation for this initiation step can be depicted as:

Br₂ → 2Br•

Each bromine atom now has an unpaired electron. This bromine radical can then abstract a hydrogen atom from methane (CH₄), leading to the formation of methyl radical (•CH₃) and hydrobromic acid (HBr), as shown below:

•Br + CH₄ → •CH₃ + HBr

The methyl radical can then react with another bromine molecule to form methyl bromide (CH₃Br). However, two methyl radicals can also combine to form ethane (C₂H₆) as a side product in the following reaction:

•CH₃ + •CH₃ → C₂H₆