Why do halogenation reactions take place only at high temperatures or in the presence of light?
1 Answer
The halogenation of alkanes is a free radical reaction.
The initiation step involves the homolytic cleavage of a Cl-Cl or Br-Br bond.
It takes energy to break a covalent bond. This energy can come from one of two sources.
1. Heat
A high temperature makes the molecules move more rapidly and have more energetic collisions.
It also makes the bonds vibrate with greater amplitude, so that the bonded atoms are almost falling away from each other.
So a high temperature promotes the initiation step.
2. Ultraviolet Light
The electrons in the bonding MO of the Cl-Cl bond can absorb a quantum of ultraviolet energy. This can excite them to an antibonding MO.
The weakened bond plus the energy of molecular collisions again leads to homolytic cleavage of the Cl-Cl bond.
So heat and light are the conditions that favour halogenation of alkanes.
