A specific reagent for dehydrohalogenation of alkyl halides to get alkenes is;

Alkyl halides react with the alcoholic $\mathrm{KOH}$ to give alkenes and hydrogen halides. Following is the reaction that takes place. It is known as a dehydrohalogenation reaction. As the name suggests, the reaction involves the removal of hydrogen and halogen.

${\mathrm{CH}}_3-{\mathrm{CH}}_2\mathrm{X}\to {\mathrm{CH}}_2={\mathrm{CH}}_2+\mathrm{HX}$