Aromatic Compounds Benzene and derivatives

Aromatic compounds Originally named for smell Aliphatic/aromatic compounds are called arenes; called aryl groups as a substituent

Structure of benzene Historical problems Benzene is unsaturated (formula C 6 H 6 ) but does not react with Br 2, H 2 /Pd or H 2 O/H 2 SO 4 Undergoes substitution rather than addition C 6 H 6 + Cl 2  C 6 H 5 Cl + HCl (with FeCl 3 catalyst) CH 3 (CH 2 ) 3 CH=CH 2 + Cl 2  Cl Cl | CH 3 (CH 2 ) 3 CH-CH 2

Kekule’s structure

Resonance structure Due to Linus Pauling Each structure has equal energy and contributes equally

Resonance Actual structure is a combination of the two Hückel rule: systems with 4n+2 pi electrons are aromatic

Other aromatic systems Cyclopentadienyl anion (6 pi e-) pKa of cyclopentadiene is around 16.

Other aromatic systems cycloheptatrienyl cation

Nomenclature of benzene derivatives Monosubstituted benzene rings – substituent named followed by “benzene” chlorobenzene isopropylbenzene

Nomenclature of benzene derivatives Common names retained by IUPAC  Toluene  Styrene  Phenol  Anisole  Aniline  Benzaldehyde  Benzoic acid  Xylene Systematic name  Methylbenzene  Ethenylbenzene  Hydroxybenzene  Methoxybenzene  Aminobenzene  Phenylmethanal  Phenylformic acid  Dimethylbenzene

Phenyl group 2-phenyl-1-propanol

Disubstituted rings Numbering system – substituents numbered in alphabetical order 1-chloro-3-ethylbenzene 1-bromo-2-(methylethyl)phenol

Locator system ortho (o-) 1, 2 meta (m-) 1, 3 para (p-) 1, 4 m-methylaniline o-xylene

Three or more substituents Substituents are named in alphabetical order, with the first getting the lowest number 2,4,6-trinitrotoluene (TNT)

1-bromo-4-ethyl-2-nitrobenzene Trisubstituted rings

Reactions of Benzene and derivatives Halogenation Proceeds via cationic mechanism only with iron catalyst – called electrophilic aromatic substitution Presence of a halogen tends to destabilize the ring to further substitution Nitration Proceeds via cationic mechanism using NO 2 +

Nitration

Presence of nitro group is deactivating Nitro group can be reduced to amine (-NH 2 ) with H 2 /Ni Provides a route from benzoic acid to PABA – used in sunscreens and in synthesis of nucleic acids

Sulfonation Ph-H + H 2 SO 4  PhSO 3 H + H 2 O Products are strong acids Used in alkylbenzenesulfonate detergent synthesis CH 3 (CH 2 ) 10 CH 2 Ph + H 2 SO 4, NaOH  CH 3 (CH 2 ) 10 CH 2 PhSO 3 - Na + (sodium 4-dodecylbenzenesulfonate, an anionic detergent) Nonpolar end dissolves in grease, polar end in water

Friedel-Crafts reactions: electrophilic aromatic substitution Alkylation – Lewis acid catalyzed addition of alkyl halide RCl + AlCl 3  AlCl R +

Friedel-Crafts reactions: electrophilic aromatic substitution

Acylation – same process using acyl chloride – product is a ketone

Phenols Phenol and some derivatives have antiseptic properties Phenols tend to be acidic

Phenols Phenols react with hydroxide bases to give phenoxide salts