Benzene and Aromatic Ring System

• An aromatic hydrocarbon is an unsaturated cyclic hydrocarbon.
• Aromatic hydrocarbons do not readily undergo addition reactions.

Benzene

• Benzene is the simplest aromatic hydrocarbon.
• The molecular formula of Benzene is C6H6.
• Benzene is a flat, symmetrical molecule.
• The structural formula for benzene involves a six-membered carbon ring in which there are three double bonds.

Aromatic Ring System

• The "circle-in-the-ring" denotes the electrons associated with the double bonds that move "around" the ring.
• Each carbon atom in the ring participates in three conventional (localized) bonds, two C-C bonds and one C-H bond.
• Each carbon atom participates in one delocalized bond (the circle) that involves all six carbon atoms.
• A delocalized bond is a covalent bond in which electrons are shared among more than two atoms.
• Delocalized bonds in benzene and its derivatives cause resistance to addition reactions.
• The aromatic ring system is a functional group present in aromatic compounds.
• It is a highly unsaturated carbon ring system in which both localized and delocalized bonds are present.

Nomenclature of Aromatic Hydrocarbons

• Replacing one or more hydrogen atoms on benzene with other groups produces benzene derivatives.
• Compounds with alkyl groups or halogen atoms attached to the benzene ring are commonly encountered.
• The IUPAC system to name monosubstituted benzene uses the substituent name as a prefix to the name benzene.
• Some monosubstituted benzenes have names where the substituent and the benzene ring together form a new parent name.
• Monosubstituted benzene structures are often drawn with the substituent at the "12 o'clock" position.
• All the hydrogen atoms in benzene are equivalent; it does not matter at which carbon the substituted group is located.
• If the the benzene ring has a group impossible to name as a substituent, the benzene ring is treated as a group attached to the substituent.
• In this reversed designnation, the benzene ring attachment is called a phenyl group.
• Name the compound according to the rules for naming alkanes, alkenes, and alkynes.

Benzene Derivatives with Two Substituents

• Attaching two substituents, either the same or different, to a benzene ring allows for three isomeric structures.
• To distinguish these three isomers, specify the positions of the substituents relative to one another using numbers or nonnumerical prefixes.

Numbered Isomers

• When numbers are used, dicholorbenzenes are named with the following set of names:
  • 1,2-Dichlorobenzene
  • 1,3-Dichlorobenzene
  • 1,4-Dichlorobenzene

Prefix System

• The prefix system uses the prefixes ortho- (o-), meta- (m-), and para- (p-).
• Ortho- means 1,2 disubstitution; the substituents are on adjacent carbon atoms.
• Meta- means 1,3 disubstitution; the substituents have one carbon between them.
• Para- means 1,4 disubstitution; the substituents have two carbons between them, on opposite sides of the ring.
• When one of the two substituents in a disubstituted benzene imparts a special name to the compound, the compound is named as a derivative of that parent molecule.
• The special substituent is assumed to be at ring position 1.
• Name substituents cited in alphabetical order before the ending -benzene when neither substituent group imparts a special name.
• The carbon of the benzene ring bearing the substituent with alphabetical priority becomes carbon 1.
• Xylene is a benzene ring bearing two methyl groups.
• Xylene has three isomers.
• Xylene is not named as dimethylbenzenes or as methyl toluenes.
• Xylenes are good solvents for grease and oil.
• Xylenes are used for cleaning microscope slides and optical lenses and for removing wax from skis.

Benzene Derivatives with Three or More Substituents

• If more than two groups are present on the benzene ring, their positions are indicated with numbers.
• Number the ring in such a way as to obtain the lowest possible numbers for the carbon atoms that have substituents.
• If there is a numbering choice where two systems give the same lowest set, the group that comes first alphabetically is given the lower number.

Physical Properties of Aromatic Hydrocarbons

• Aromatic hydrocarbons are insoluble in water.
• They are good solvents for other nonpolar materials.
• Aromatic hydrocarbons are less dense than water.
• Benzene, monosubstituted benzenes, and many disubstituted benzenes are liquids at room temperature.
• Benzene is a colorless, flammable liquid that burns with a sooty flame because of incomplete combustion.
• Benzene was once widely used as an organic solvent.
• Benzene use as a solvent has been discontinued because of its short- and long-term toxic effects.
• Benzene inhalation can cause nausea and respiratory problems.

Sources of Aromatic Hydrocarbons

• Coal tar was once the main source of aromatic hydrocarbons, but petroleum is now the primary source.
• Saturated hydrocarbons obtained from petroleum can be converted to aromatic hydrocarbons at high temperatures with special catalysts.

Chemical Reactions of Aromatic Hydrocarbons

• Substitution reactions occur when groups of atoms replace hydrogen atoms in a hydrocarbon molecule.
• Alkylation
• Halogenation

Alkylation

• An alkyl group (R-) from an alkyl chloride (R-CI) substitutes for a hydrogen atom on the benzene ring.
• Alkylation requires a catalyst, AlCl3.
• Alkylation is the most important industrial reaction of benzene.

Halogenation

• Bromine or chlorine can replace a hydrogen atom on a benzene ring when the benzene is treated with Br2 or Cl2 in the presence of a catalyst.
• The catalyst is usually FeBr3 (Iron Bromide) for bromination, or FeCl3 (Iron Chloride) for chlorination.

Fused-Ring Aromatic Hydrocarbons

• A fused-ring aromatic hydrocarbon is an aromatic hydrocarbon whose structure contains two or more rings fused together.
• Two carbon rings that share a pair of carbon atoms are said to be fused.
• The three simplest fused-ring aromatic compounds are naphthalene, anthracene, and phenanthrene.
• All three are solids at room temperature.

Fused-Ring Aromatic Hydrocarbons and Cancer

• A number of fused-ring aromatic hydrocarbons are known to be carcinogens.
• The most potent carcinogens are 1,2-Benzanthracene, 1,2,5,6-Dibenzanthracene, and 3,4-Benzpyrene.
• Fused-ring aromatic hydrocarbons are often formed when hydrocarbon materials are heated to high temperatures.
• These resultant compounds are present in low concentrations in tobacco smoke and automobile exhaust.
• They are found sometimes in burned (charred) food, such as the charred portions of a well-done steak cooked over charcoal.