Aromatic Hydrocarbons

A class of fragrant compounds with unique properties Benzene – the basis of all aromatic compounds Friedrich August Kekule first proposed the structure of benzene to be a ring with alternating single and double bons C6H6

1. Alkenes react to decolorize bromine solutions, but benzene does not Benzene behaves very differently from most compounds that have double bonds 1. Alkenes react to decolorize bromine solutions, but benzene does not Br2 Color fades Br2 Color fades Br2 No reaction – stays reddish brown

2. Alkenes react with cold potassium permanganate to give diols and brown MnO2, benzene does not KMnO4 Brown color 0°C KMnO4 Brown color 0°C OH OH KMnO4 0°C No reaction, stays purple

3. Alkenes are hydrogenated with a platinum catalyst at room temperature while benzene resists hydrogenation at room temp. (benzene requires heat and pressure to be hydrogenated) 3H2 Pt, Δ, pressure

The structure of benzene is not adequately represented by the two Kekule formulas, but is a resonance hybrid of the two The structure of benzene is better represented by

Nomenclature of Aromatic Hydrocarbons 1. Monosubstituted derivatives are named as derivatives of benzene Ex.

Certain monosubstituted structures retain their “common” names toluene styrene phenol

2. Disubstituted derivatives have their position on the ring designated by the following: ortho – adjacent positions on the ring meta – positions separated by one carbon para – opposite positions OR…they can be numbered

Common name for dimethyl benzenes are xylene

3. Polysubstituted benzenes have their position numbered Lowest possible numbers are used (don’t forget to alphabetize) When the common name of a parent compound is used, the number 1 position is always assigned to the carbon bearing the functional group of the parent compound

4. When an aromatic group is a constituent of a much larger chain, it’s sometimes easier to name it as a substituent group phenyl group benzyl group

Name the following structures.

5. Fused ring derivatives of benzene have their positions numbered naphthalene anthracene phenanthrene

Name the following structures.