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