(1) alcohol dehydration

Alcohol reacts with acids (such as sulfuric acid) by heating. At a certain temperature, alcohol removes one molecule of water to produce olefins. For example, ethanol reacts with concentrated sulfuric acid at 160℃ to generate ethylene.

CH3CH2OH

—→

CH2=CH2

+

H2O

(2) Dehalogenation of halogenated hydrocarbons

In the presence of alkaline reagents, monoalkyl halides also lose a hydrogen halide molecule, and olefins are generated by 1, 2- elimination reaction;

ch 3c H2 ch 2cl~→

CH3CH=CH2

If there are more than 100 different β-hydrogen atoms in alkyl halides, several different olefin isomers can be generated, and which olefin is the main product is related to the structure of raw materials and reaction conditions.

(3) Dehalogenation of o-dihalide

Under the action of zinc = magnesium, o-dihalide loses two halogen atoms and produces olefins. In forensic medicine, metals provide a pair of electrons for the breaking of carbon-halogen bonds and the formation of double bonds. The metal first takes away a halogen atom to generate carbanion, and then the carbanion loses the β-halogen atom to generate olefin.

In addition to the above three methods, alkyne hydrogenation, carboxylic ester and quaternary ammonium salt cracking, Wittig reaction, sulfonate sulfonation and so on. It is also a method for preparing olefins.