$C{H_2}OH - CHOH - C{H_2}OH\xrightarrow[{or\,{H_2}S{O_4}\,,\,\Delta }]{{KHS{O_4}\,\,or\,\,{P_2}{O_5}}}\,C{H_2} = CH - CHO\, + \,2{H_2}O$