Chapter 22 – Summary

22.1 Alkenes and Alkynes – Structure and Naming

Any hydrocarbon containing either a double or triple bond is an unsaturated hydrocarbon. Alkenes have a carbon-to-carbon double bond. The general formula for alkenes with one double bond is C_nH_2n. Alkenes can be straight chain, branched chain, or cyclic. Alkynes have a carbon-to-carbon triple bond. The general formula for alkynes with one triple bond is C_nH_2n-2.  or cyclic. Simple alkenes and alkynes often have common names, but all of them can be named by the system of the International Union of Pure and Applied Chemistry. The physical properties of alkenes are quite similar to those of alkanes. Like other hydrocarbons, alkenes are insoluble in water but soluble in organic solvents.

• Addition of hydrogen (hydrogenation): CH₂=CH₂ + H₂ → CH₃CH₃
• Addition of halogen (halogenation): CH₂=CH₂ + X₂ → XCH₂CH₂X (where X = F, Cl, Br, or I.)
• Addition of water (hydration): CH₂=CH₂ + HOH → HCH₂CH₂OH

Markovnikov’s Rule applies to unsymmetrical additions.  In the addition of HX or H₂O to an alkene, the more highly substituted carbocation is formed as the intermediate rather than the less highly substituted one.  There are several tests used to determine the presence of a double or triple bond: bromine test and permanganate test.

22.4 Aromatic Compounds – Structure and Naming

The cyclic hydrocarbon benzene (C₆H₆) has a ring of carbon atoms. The molecule seems to be unsaturated, but it does not undergo the typical reactions expected of alkenes. The electrons that might be fixed in three double bonds are instead delocalized over all six carbon atoms. A hydrocarbon containing one or more benzene rings (or other similarly stable electron arrangements) is an aromatic hydrocarbon, and any related substance is an aromatic compound. One or more of the hydrogen atoms on a benzene ring can be replaced by other atoms. When two hydrogen atoms are replaced, the product name is based on the relative position of the replacement atoms (or atom groups). A 1,2-disubstituted benzene is designated as an ortho (o-) isomer; 1,3-, a meta (m-) isomer; and 1,4-, a para (p-) isomer. An aromatic group as a substituent is called a phenyl group. A polycyclic aromatic hydrocarbon (PAH) has fused benzene rings sharing a common side.

22.5 Aromatic Reactions

Aromatic compounds undergo substitution and oxidation reactions.  A benzene ring can be substituted with a halogen atom (using X₂ and FeX₃), a substituted benzene can be halogenated (add X), a benzene can be nitrated (adding -NO₂) or sulfonated (adding -HSO₃), and a substituted benzene can be hydroxylated (add -OH).  Side chain oxidation occurs with an alkylated benzene leaving the benzene ring intact and changing any carbon chain to benzoic acid. Friedel-Crafts reactions allow for substitution of an alkyl chain onto a benzene ring.

Except where otherwise noted, this page is written and adapted by David Wegman, Adrienne Richards and Samantha Sullivan Sauer from “13.S: Unsaturated and Aromatic Hydrocarbons (Summary)“,  in Basics of General, Organic, and Biological Chemistry (Ball et al.) by David W. Ball, John W. Hill, and Rhonda J. Scott via LibreTexts, licensed under CC BY-NC-SA 4.0. / A LibreTexts version of Introduction to Chemistry: GOB (v. 1.0), CC BY-NC 3.0.