Aromatic Compounds

Aromatic Systems are Characterized by Their Chemical Stability

• Note the chemical stability of the aromatic systems to the reaction conditions in the following slides

Note aromatic system’s stability toward hydrogenation

Note aromatic systems’ stability toward strong reducing agent LiAlH4

Note the (two) aromatic systems’ stability toward Br2

Note the aryl iodide’s stability toward SN2 substitution(SN2 substitution occurs at the sp3 hybridized carbon)

Note the aryl iodide’s stability toward the strong base (potassium tert-butoxide) used to effect elimination

Reactions of Aromatic Systems: Electrophilic Aromatic Substitution

The Friedel-Crafts Reaction

Treatment of Aryl Halides with Extremely strong bases (amide anions, NaNH2, pKa of ammonia = 38) can cause substitution reactions…

(note that the above table shows the conjugate acids only)

But the mechanism involves a two-step process of elimination-addition.

Treatment of aryl halides having strongly electron-withdrawing substituents (at the 2- and the 4-position) can also cause substitution reactions

But…

The mechanism involves addition-elimination.

Reactions of Side Chains and Attached Functionality on Aromatic Compounds

Treatment of amines with nitrous acid (HONO) generates diazonium salts

The Sandmeyer Reaction

Swiss chemist (1854-1922) after whom reaction is named

Benzylic Positions Can be Selectively Oxidized (all the way to the carboxylic acid) by Potassium Permanganate

The Clemmensen Reduction