Aromaticity

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Aromatic Compounds: Benzene and compounds resembling benzene in chemical behavior.

Structure of Benzene

_• C₆H₆

• 1858 - Friedrich August Kekule’ proposed that carbon atoms joint to form chains

• 1865 - Carbon chains sometimes close to form rings

• Benzene yields only one monosubstitution product - C₆H₅Y

• Benzene yields three disubstitution products

Stability of Benzene

• Undergoes substitution rather than addition

• Heats of hydrogenation and combustion are lower than expected

• All C-C bonds are equal and are intermediate in length between single and double bonds

Aromatic Character

• Hückel 4n+2 rule: Aromatic compounds must contain cyclic clouds of delocalized p electrons above & below the plane of the molecules and the p clouds must contain a total of (4n+2) p electrons

Polynuclear Aromatic Hydrocarbons: Naphthalene