Nomenclature Alkenes contain double bonds and alkynes contain triple bonds. The lUPAC suffixes are -ene for alkenes and -yne for alkynes.

Geometry of Alkenes. In alkenes, the double-bonded carbons and the four groups attached to these carbons lie in the same plane. Because rotation about the double bond is restricted, alkenes may exist as geometric or cis-trans isomers. This type of stereoisomerism is possible when each double-bonded carbon is attached to two different groups.

Physical properties of Alkenes. The physical properties of alkenes are very similar to those of the alkanes. They are nonpolar, insoluble in water, less dense than water, and soluble in nonpolar solvents.

Alkene Reactions. The characteristic reaction of alkenes is addition. Three important addition reactions are: bromination (an example of halogenation) to give a dibrominated alkane, hydration to produce an alcohol, and the reaction with H-X to give an alkyl halide. The addition of H2O and H-X are governed by Markovnikov's rule ("Them that has--gets.")

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

Alkynes. The alkynes contain triple bonds and possess a linear geometry of the two carbons and the two attached groups. Alkyne names end in -yne. The physical and chemical properties of alkynes are very similar to those of the alkenes.

Aromatic Compounds and the Benzene Structure. Benzene, the simplest aromatic compound, C₆H₆, and other members of the aromatic class contain a six membered ring. This aromatic ring is often drawn as a hexagon containing a circle which represents the six electrons of three double bonds that move freely around the ring. Friedrich August Kekule was the first to correctly describe the structure of benzene. Benzene was discovered by Michael Faraday in 1825. The characteristic reaction of aromatic compounds is substitution. All organic compounds that do not contain an aromatic ring are called aliphatic compounds.

Nomenclature of Benzene derivatives. There are several acceptable lUPAC names for many benzene compounds. Some lUPAC names are based upon widely used common names, such as toluene and aniline. Other compounds are named as derivatives of benzene or by designating the benzene ring as a phenyl group and using the prefix phenyl. There is no cis-trans isomerism in aromatics as there is in other ring compounds as the ring is perfectly flat with bond angles of 120^o.

Properties and Uses of Aromatic Compounds. Aromatic hydrocarbons are nonpolar and have physical properties similar to the alkanes and alkenes. Benzene does not readily undergo addition reactions that are typical of alkenes. Benzene and toluene are key industrial chemicals. Other important aromatics include phenol, aniline, and styrene. Benzene is carcinogenic

Heterocycles. A ring compound that contains at least one atom that is not carbon is known as a heterocycle.