Dehydrohalogenation reaction order is alkyl halide relative reactivity order is $3^{\circ}>2^{\circ}>1^{\circ}$ Alkyl halide reactivity order is $\mathrm{I}>\mathrm{Br}>\mathrm{Cl}>\mathrm{F}$ (in $\beta$ elimination of $\mathrm{F}$ is rarely used). When heated with strong base, alkyl halides typically undergo a 1, 2-elimination reaction to generate alkenes.
