A reaction where two or more molecules combine to form a larger one is an addition reaction.

These reactions occur to change the unsaturated compound to saturated compound.
In dehydrohalogenation reaction, alkyl halides give alkenes. Rearrangement gives the structural isomers of the reactant, while in substitution reaction one of the group or an atom is replaced by other group. Therefore, the process of converting alkyl halides into alcohols involves substitution reaction.

Reaction of alkyl halides:
Primary alkyl halides
Secondary alkyl halides
Tertiary alkyl halides