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Aromatic Compounds

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

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Aromatic Compounds

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  1. Aromatic Compounds Benzene and derivatives

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

  3. 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

  4. Kekule’s structure

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

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

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

  8. Other aromatic systems • cycloheptatrienyl cation

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

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

  11. Phenyl group 2-phenyl-1-propanol

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

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

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

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

  16. 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+

  17. Nitration

  18. 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

  19. 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

  20. Friedel-Crafts reactions: electrophilic aromatic substitution • Alkylation – Lewis acid catalyzed addition of alkyl halide • RCl + AlCl3 AlCl4- + R+

  21. Friedel-Crafts reactions: electrophilic aromatic substitution

  22. Friedel-Crafts reactions: electrophilic aromatic substitution • Acylation – same process using acyl chloride – product is a ketone

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

  24. Phenols • Phenols react with hydroxide bases to give phenoxide salts

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