The given reaction is a substitution reaction.
Step-by-step explanation:
CH₃ - CH₂ -CH₂ - OH + HBr → CH₃ - CH₂ -CH₂ - Br + H₂O
When propanol is treated with hydrogen halide say HBr that is hydrogen bromide then the bromide ion from HBr replaces hydroxyl group in propanol, so it is called as substitution reaction, since -OH group is substituted by Br group. Mostly alcohols undergo nucleophilic substitution reaction.
Alcohols when treated with hydrogen halide -OH is substituted by -Br group, so it is called as substitution reaction.