aromatic compounds
DESCRIPTION
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 - PowerPoint PPT PresentationTRANSCRIPT
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Aromatic Compounds
Benzene and derivatives
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Aromatic compounds
Originally named for smell Aliphatic/aromatic compounds are called
arenes; called aryl groups as a substituent
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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
| |
CH3(CH2)3CH-CH2
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Kekule’s structure
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Resonance structure
Due to Linus Pauling
Each structure has equal energy and contributes equally
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Resonance
Actual structure is a combination of the two
Hückel rule: systems with 4n+2 pi electrons are aromatic
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Other aromatic systems
Cyclopentadienyl anion (6 pi e-)
pKa of cyclopentadiene is around 16.
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Other aromatic systems
cycloheptatrienyl cation
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Nomenclature of benzene derivatives
Monosubstituted benzene rings – substituent named followed by “benzene”
chlorobenzene isopropylbenzene
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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
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Phenyl group
2-phenyl-1-propanol
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Disubstituted rings
Numbering system – substituents numbered in alphabetical order
1-chloro-3-ethylbenzene 1-bromo-2-(methylethyl)phenol
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Locator system
ortho (o-) 1, 2 meta (m-) 1, 3 para (p-) 1, 4
m-methylaniline o-xylene
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Three or more substituents
Substituents are named in alphabetical order, with the first getting the lowest number
2,4,6-trinitrotoluene (TNT)
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1-bromo-4-ethyl-2-nitrobenzene
Trisubstituted rings
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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
+
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Nitration
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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
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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
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Friedel-Crafts reactions: electrophilic aromatic substitution Alkylation – Lewis acid catalyzed addition of
alkyl halide RCl + AlCl3 AlCl4- + R+
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Friedel-Crafts reactions: electrophilic aromatic substitution
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Friedel-Crafts reactions: electrophilic aromatic substitution Acylation – same process using acyl chloride
– product is a ketone
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Phenols
Phenol and some derivatives have antiseptic properties
Phenols tend to be acidic
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Phenols
Phenols react with hydroxide bases to give phenoxide salts