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12.8 Aromatic Compounds In 1825, Michael Faraday isolated a hydrocarbon called benzene, which consists of a six-carbon ring with alternating double bonds and the molecular formula C6H6. The space-filling model for benzene consists of a six-carbon ring with six hydrogen atoms. Learning Goal Describe the bonding in benzene; name aromatic compounds, and draw their line-angle structural formulas. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Benzene, an Aromatic Compound Benzene is • an aromatic compound. • a ring of six C atoms, each bonded to one H atom. • a flat ring structure drawn with three alternating double bonds. • represented by two structures, because the electrons are shared equally among all the C atoms. • represented by a line-angle structural formula using a circle in the center instead of the alternating double bonds. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Aromatic Compounds Some common examples of aromatic compounds that we use for flavor are anisole from anise, estragole from tarragon, and thymol from thyme. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Benzene • Each carbon atom uses three valence electrons to bond to the hydrogen atom and two adjacent carbons. The fourth valence electron was thought to be shared in a double bond with an adjacent carbon. • In 1865, August Kekulé proposed that the carbon atoms in benzene were arranged in a flat ring with alternating single and double bonds between the adjacent carbon atoms. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Naming Aromatic Compounds • Aromatic compounds containing a benzene ring and a single substituent are named as benzene derivatives. • Since the ring contains only one substituent, the ring is not numbered. • Some common names such as toluene, aniline, and phenol are allowed by IUPAC rules. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Naming Aromatic Compounds • When a benzene ring is a substituent, — C6H5, it is named as a phenyl group. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Naming Aromatic Compounds • When there are two or more substituents, the benzene ring is numbered to give the lowest numbers to the substituents. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Naming Aromatic Compounds • When a common name such as toluene, phenol, or aniline can be used, • the carbon atom attached to the methyl, hydroxyl, or amine group is numbered as carbon 1. • prefixes are used to show the position of the two substituents. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Naming Aromatic Compounds • The common name xylene is used for the isomers of dimethylbenzene. • If three or more substituents are attached to the benzene ring, they are numbered in the direction to give the lowest set of numbers and then named alphabetically. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Chemistry Link to Health: Common Aromatic Compounds Toluene is used as a reactant to make drugs, dyes, and explosives such as TNT (trinitrotoluene). The benzene ring is also found in some amino acids (the building blocks of proteins). General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Chemistry Link to Health: Common Aromatic Compounds The benzene ring is found in pain relievers such as aspirin, acetaminophen, and ibuprofen; and in flavorings such as vanillin. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Properties of Aromatic Compounds The flat, symmetrical structure of benzene allows for interactions between the cyclic structures, which contributes to the higher melting points and boiling points of benzene and its derivatives. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Properties of Aromatic Compounds • Aromatic compounds are less dense than water, although they are usually more dense than other hydrocarbons. • Halogenated benzene compounds are denser than water. • Aromatic hydrocarbons are insoluble in water and are used as solvents for other organic compounds. • Only aromatic compounds containing strongly polar functional groups such as — OH or — COOH will be somewhat soluble in water. • Benzene and other aromatic compounds are resistant to reactions that break up the aromatic system, although they are flammable. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Study Check Select the correct name for each compound. 1. A. chlorocyclohexane B. chlorobenzene C. 1-chlorobenzene 2. A. 1,2-dimethylbenzene B. 1,4-dimethylbenzene C. 1,3-dimethylbenzene General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Solution Select the correct name for each compound. 1. B. chlorobenzene 2. C. 1,3-dimethylbenzene General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Concept Map General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc.