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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 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
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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 NO 2 +
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Nitration
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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
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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
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Friedel-Crafts reactions: electrophilic aromatic substitution Alkylation – Lewis acid catalyzed addition of alkyl halide RCl + AlCl 3 AlCl 4 - + R +
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Friedel-Crafts reactions: electrophilic aromatic substitution
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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
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