Preparations of Alkene:
Following methods are used to prepare alkenes-
1) By dehydration of Alcohol:
When an alcohol is heated to about 170ْC in presence of conc. H2SO4 a molecule of water is eliminated and an alkene is formed.
CH3 — CH2—OH + Conc. H2SO4 —-170ْC—> CH2 = CH2 + H2O
2) By dehydrohalogenation of Alkyl halide:
When an alkyl halide is heated with an alcoholic solution of KOH, a molecule of hydrogen halide is eliminated and an alkene is formed. e.g.
CH3 — CH2—Br + Alco. KOH ——-> CH2 = CH2 + KBr + H2O
3) By hydrogenation of Alkyne:
Alkene can be prepared by hydrogenation of alkyne in presence of finely divided palladium and BaSO4.
H—C ≡ C—H + H2 —-[Pd/BaSO4 ]—-> CH2 = CH2