Substitution ("Reaction type")
is a chemical "reaction in which an atom, usually a halogen, replaces a hydrogen atom in a hydrocarbon." (Zumdahl, Chemistry)


radical substitution ("Reaction mechanism")
describes the changes during the reaction. The name of this mechanism is radical m. because radicals are participating.

Three steps of radical substitution

1. Start reaction (Photo above): Formation of two bromine radicals


2a Chain reaction 1: Attack of a bromine radical
 
 
 
 

2b Chain reaction 2: Attack of the resulting hexyl radical,
                               Reforming a bromine radical.
Start and chain reactions stop as soon as bromine (Br2) is consumed.
3. Terminating reaction: 2 hexyl radicals join to produce the 12-C alkane C12H26.