Alkenes, a significant class of unsaturated hydrocarbon, possess a carbon–carbon double bond. They can fall into two broad categories, namely, acyclic alkenes and cyclic alkynes. In accordance with the containing number of carbon–carbon double bond, alkenes can also be divided into several classes such as mono-enes, diene with one or two carbon–carbon double bond, respectively. Alkenes are manufactured industrially by hydrocarbon cracking. Raw materials are from intensively natural gas condensate components, which are broken apart to produce a mixture of primarily aliphatic alkenes and other products. The mixture is feedstock and temperature dependent, and subsequently separated by fractional distillation. This is mainly used for the production of small alkenes (up to six carbons).
Fig. 1 common compounds of alkenes
Alkenes are colorless, nonpolar, and flammable. The physical state depends on their molecular mass. Like the corresponding saturated hydrocarbons, the most basic alkenes, containing four carbons or below, exhibit gas state at room temperature. Linear alkenes of about five to sixteen carbons are liquids, and higher alkenes are waxy solids. The melting point of the solids also increases as growing in molecular mass. Alkenes are relatively stable in chemical activity, but have much stronger reactiveness than the corresponding alkanes, either accounting for the reactivity of the carbon–carbon pi-bond or the presence of allylic CH centers. Most reactions of alkenes participate in additions to this pi bond, forming new single bonds.
Alkenes serve as a feedstock for the petrochemical industry resulting from their extensive participation in various reactions, prominently polymerization and alkylation. The most fundamental alkenes, ethylene and propylene, are the two most vital organic chemicals produced industrially. About 127 million metric tons of ethylene and 54 million metric tons of propylene are manufactured worldwide every year in order to synthesize polyethylene, acetic acid, acetaldehyde, ethylene glycol and a great amount of other substances.
Fig.2 Industrially important compounds manufactured from ethylene and propylene (Organic Chemistry-9th Edition, David Klein, 2015)
Alkenes can be used as plant hormones that play a crucial role in growth and development of plants. For instance, ethylene and gibberellin are widely applied in agriculture to ripen fruits, vegetables, corns and many other crops.
Fig. 3 two mainly used alkenes for plant hormones
A large varieties of alkenes can be used as drugs or medical intermediates. Polyenes are a kind of antibiotics widely used to cure fungal infections. Nystatin and amphotericin B are mainly clinical-applied drugs. Terpenes, whose general molecular formulas is (C5H8)n, are a kind of alkenes and have various biological activities, which entail them to apply in natural medicines and clinical candidates.
Fig. 4 mainly used antifungal of polyenes