Summary
Highlights
Alkenes, or unsaturated hydrocarbons of the ethylene series, are rarely found in nature due to their high chemical activity. Industrially, they are obtained by processing petroleum products at high temperatures through dehydrogenation of saturated hydrocarbons. Key methods include: cracking of alkanes (decomposition at high temperatures into an alkane and an alkene), dehydrogenation (removal of hydrogen at high temperatures with a nickel catalyst), dehydrohalogenation of haloalkanes (reaction with concentrated alcoholic alkali solution), dehalogenation of dihaloalkanes (reaction with metal to form an alkene and salt), and dehydration of alcohols (removal of water upon heating in the presence of concentrated sulfuric acid).
Important rule for reactions like dehydrogenation, dehydration, dehydrohalogenation, and dehalogenation: hydrogen atoms are predominantly removed from less hydrogenated carbon atoms, following Aleksandr Mikhailovich Zaitsev's rule.
Alkenes are widely used as chemical raw materials due to their high reactivity. Ethylene is used to produce: 1,2-dichloroethane (a solvent and agricultural pesticide), chloroethane (used for local anesthesia), ethyl alcohol (for synthetic rubber, dyes, and medicines), and polyethylene (for packaging, bags, toys, and household items). Ethylene also accelerates the ripening of fruits. Propylene is used for synthesizing glycerin, alcohols, and polypropylene (for ropes, cables, and packaging materials).