CHAPTER 11: Unit 8. Definition: Alkenes and Alkynes

Opening Essay

Our modern society is based to a large degree on the chemicals we discuss in this chapter. Most are made from petroleum. Earlier in this chapter we noted that alkanes—saturated hydrocarbons—have relatively few important chemical properties other than that they undergo combustion and react with halogens. Unsaturated hydrocarbons—hydrocarbons with double or triple bonds—on the other hand, are quite reactive. In fact, they serve as building blocks for many familiar plastics—polyethylene, vinyl plastics, acrylics—and other important synthetic materials (e.g., alcohols, antifreeze, and detergents). Aromatic hydrocarbons have formulas that can be drawn as cyclic alkenes, making them appear unsaturated, but their structure and properties are generally quite different, so they are not considered to be alkenes. Aromatic compounds serve as the basis for many drugs, antiseptics, explosives, solvents, and plastics (e.g., polyesters and polystyrene).

The two simplest unsaturated compounds—ethylene (ethene) and acetylene (ethyne)—were once used as anesthetics and were introduced to the medical field in 1924. However, it was discovered that acetylene forms explosive mixtures with air, so its medical use was abandoned in 1925. Ethylene was thought to be safer, but it too was implicated in numerous lethal fires and explosions during anesthesia. Even so, it remained an important anesthetic into the 1960s, when it was replaced by nonflammable anesthetics such as halothane (CHBrClCF₃).

Definition

An unsaturated hydrocarbon is a hydrocarbon that contains one or more carbon-carbon multiple bonds. Three main classes of unsaturated hydrocarbons exist: alkenes, alkynes  and aromatic hydrocarbons.

Unsaturated hydrocarbons have similar physical properties similar to those of saturated hydrocarbons but their chemical properties are different. Whenever a specific portion of a molecule governs its chemical properties, that portion of the moelcule is called FUNCTIONAL GROUP. A functional group is a structural feature in an organic molecule that is directly involved in most chemical reactions when the molecule participate in reaction.

An alkene is a an acyclic unsaturated hydrocarbon that contains one or more carbon carbon double bonds. Thus alkene functional group is C=C group.  In terms of family names, they differ only in end name: -ene vs. -ane.

A cycloalkene is cyclic unsaturated hydrocarbon that contains one or more carbon carbon double bonds within the ring system. The simplest cycloalkene is cycolpropene.C3H4.

An alkyne is an acyclic unsaturated hydrocarbon that contains one or more carbon-carbon triple bonds. Therefore the alkyne functional group is C-C triple bond.