Definition of Substitution: A substitution reaction is a chemical process where one atom or functional group in a molecule is replaced by another atom or group. In the context of alkanes, a hydrogen atom is swapped for a halogen atom.
Reaction Requirement: For this reaction to proceed, a halogen molecule (such as or ) must collide with an alkane molecule. However, this collision lacks sufficient energy to break the bonds unless external energy is provided by ultraviolet (UV) radiation.
The Role of UV Light: UV light provides the energy required to break the covalent bond within the diatomic halogen molecule, creating highly reactive particles that can then attack the stable alkane structure.
General Equation: The reaction typically follows the pattern: .
Specific Examples:
The Haloalkane Family: The products, known as halogenoalkanes or haloalkanes, are used industrially as solvents, refrigerants, and precursors for pharmaceuticals.
Identify the Conditions: Whenever a question mentions an alkane reacting with bromine or chlorine, immediately check for the presence of UV light or sunlight. If the reaction is kept in the dark, no reaction occurs.
Don't Forget the Byproduct: A common mistake is to only write the haloalkane. Always include the hydrogen halide (e.g., or ) as the second product to ensure the equation is balanced.
State the Reaction Type: Be prepared to identify the reaction specifically as 'substitution'. Examiners look for this term to distinguish it from the addition reactions found in alkenes.
