12.8 Aromatic Compounds

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.

Learning Goal Describe the bonding in benzene; name aromatic compounds, and draw their line-angle structural formulas.

The space-filling model for

benzene consists

of a six-carbon ring with

six hydrogen

atoms.

Benzene, an Aromatic Compound

Benzene isan 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.

Aromatic Compounds

Some common examples of aromatic compounds that we use for flavor are anisole from anise, estragole from tarragon, and thymol from thyme.

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.

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.

Naming Aromatic Compounds

When a benzene ring is a substituent, — C6H5, it is named as a phenyl group.

Naming Aromatic Compounds

When there are two or more substituents, the benzene ring is numbered to give the lowest numbers to the substituents.

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.

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.

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).

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.

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.

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.

Study Check

Select the correct name for each compound.1. A. chlorocyclohexane B. chlorobenzene C. 1-chlorobenzene2. A. 1,2-dimethylbenzene B. 1,4-dimethylbenzene C. 1,3-dimethylbenzene

Solution

Select the correct name for each compound.1. B. chlorobenzene 2. C. 1,3-dimethylbenzene

Concept Map