Unsaturated hydrocarbons. 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.

Alkenes and cycloalkenes. An alkene is an acyclic unsaturated hydrocarbon in which one or more carbon - carbon double bonds are present. A cycloalkene is a cyclic unsaturated hydrocarbon that contains one or more carbon - carbon double bonds within the ring system.

Alkene nomenclature. Alkenes and cycloalkenes are given IUPAC names using rules similar to those for alkanes and cycloalkanes, except that the ending -ene is used. Also, the double bond takes precedence both in selecting and in numbering the main chain or ring.

Cis - trans isomers. Because rotation about a carbon - carbon double bond is restricted, some alkenes exist in two isomeric (cis - trans) forms. Cis - trans isomerism is possible when each carbon of the double bond is attached to two different groups.

Physical properties of alkenes. Alkenes and alkanes have similar physical properties. They are nonpolar, insoluble in water, less dense than water, and soluble in nonpolar solvents.

Addition reactions of alkenes. Numerous substances, including H2, Cl2, Br2, HCl, HBr, and H2O, add to an alkene carbon - carbon double bond. When both the alkene and the reactant are unsymmetrical, the addition proceeds according to Markovnikov's rule: The carbon atom of the double bond that already has the greater number of H atoms gets one more.

Addition polymers. Addition polymers are formed from alkene monomers that undergo repeated addition reactions with each other. Many familiar and widely used materials, such as fibers and plastics, are addition polymers.

Alkynes and cycloalkynes. Alkynes and cycloalkynes are unsaturated hydrocarbons that contain one or more carbon - carbon triple bonds. They are named in the same way as alkenes and cyclo-alkenes, except that their parent names end in -yne. Like alkenes, alkynes undergo addition reactions. These occur in two steps, an alkene forming first and then an alkane.

Aromatic hydrocarbons. Benzene, the simplest aromatic hydrocarbon, and other members of this family of compounds contain a six-membered ring with a cyclic, delocalized bond. This aromatic ring is often drawn as a hexagon containing a circle.

Nomenclature of aromatic hydrocarbons. Monosubstituted benzene compounds are named by adding the substituent name to the word benzene. Positions of substituents in disubstituted benzenes are indicated by using a numbering system or the ortho- (1,2), meta- (1,3), and para- (1,4) prefix system.

Reactions of aromatic hydrocarbons. Aromatic hydrocarbons undergo substitution reactions rather than addition reactions. Important substitution reactions are alkylation and halogenation.