Dehydrohalogenation means an elimination reaction that eliminates (remove) a hydrogen halide from a substrate. Leaving group tendency : I⊖>Br⊖>Cl⊖>F⊖ The ease of dehydrohalogenation of alkyl halide with alcoholic KOH is 3∘>2∘>1∘ but iodide which is least basic of four common halide ( F,Cl,Br and I) is the best leaving group among them. Fluorine is least effective leaving group because fluoride ion is most basic. Rate of dehydrohalogenation depends on the leaving group. Good leaving group means high rate of dehydrohalogenation. The rate of dehydrohalogenation is