But-1-ene reacts with HBr to form 2-bromobutane(acc. To markovnikoff’s rule) and further 2-bromobutane is treated with alcoholic KOH in the presence of heat to form But-2-ene with the removal of HBr.(alcoholic KOH removes H from the carbon which has less number of hydrogens just next to the carbon to which halide is attached and gives the major product i.e. Saytzeff rule)