International General Certificate of Secondary Education (IGCSE) Chemistry Practice Exam

Which conditions are necessary for the reaction of methane with bromine to form bromomethane?

1. Heat

2. UV light

3. Pressure

4. An alkaline environment

The correct answer is: UV light

The reaction of methane with bromine to form bromomethane is a type of substitution reaction known as free radical halogenation. This reaction typically requires the presence of ultraviolet (UV) light to initiate the process. When UV light is applied, it provides the energy necessary to break the bromine molecular bond (Br-Br), generating bromine radicals. These radicals are highly reactive and can then abstract hydrogen atoms from methane, which leads to the formation of bromomethane and hydrogen bromide. Without UV light, the reaction does not proceed effectively under normal conditions since sufficient energy to break the bromine bond is not supplied. Therefore, the necessity of UV light in this context is crucial for enabling the formation of the reactive species that drive the substitution reaction forward. While heat can also drive some reactions, it is not the preferred method for this specific reaction and does not provide the same mechanism for generating radicals as UV light. Pressure or an alkaline environment is not required for this reaction to occur and does not play a role in facilitating the substitution process. Therefore, UV light is the correct condition necessary for this reaction.