Alc. KOH is used for dehydrohalogenation.

When a haloalkane with -Hydrogen atom is heated with alcoholic solution of KOH, there is elimination of hydrogen atom from -carbon a halogen atom from - carbon atom. As a result, an alkene is formed as a product. Since -hydrogen atom is involved in elimination it is often called - elimination. It is also known as dehydrohalogenation. If there is possibility of formation of more than one alkene due to availability of more than one -hydrogen atoms, usually one alkene is formed as a major product. This is observed by Russian chemist Alexander Zaitsev (or Saytzeff), who formulated a rule, which is summerised as,-" In dehydrohalogenation reactions, the preferred product is that alkene which has the greater number of alkyl groups attached to doubly bonded carbon atoms. It is shown by the following example-

In both the reaction products the major product will be pent-2-ene because according to Satzeff rule, the preferred product should be most substituted

Hence, pent-2-ene will be and pent-1-ene will be