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 C6H6) but does not react with

Br2, H2/Pd or H2O/H2SO4

Undergoes substitution rather than addition

C6H6 + Cl2 C6H5Cl + HCl

(with FeCl3 catalyst)

CH3(CH2)3CH=CH2 + Cl2

Cl Cl

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CH3(CH2)3CH-CH2

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 NO2

+

Nitration

Nitration

Presence of nitro group is deactivating Nitro group can be reduced to amine (-NH2)

with H2/Ni Provides a route from benzoic acid to PABA –

used in sunscreens and in synthesis of nucleic acids

Sulfonation

Ph-H + H2SO4 PhSO3H + H2O

Products are strong acids Used in alkylbenzenesulfonate detergent

synthesis CH3(CH2)10CH2Ph + H2SO4, NaOH

CH3(CH2)10CH2PhSO3-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 + AlCl3 AlCl4- + R+

Friedel-Crafts reactions: electrophilic aromatic substitution

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