Organic Chemistry – Chapter 18: Aromatic Compounds

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Toluene
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Phenol
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Aniline
Aniline
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Acetophenone
Acetophenone
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Benzaldehyde
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Benzoic acid
Benzoic acid
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ortho-Xylene
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Styrene
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Naming aromatic compounds
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For monosubstituted benzenes, benzene is used as the parent name
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Ortho
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Groups are in a 1,2 relationship
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Meta
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Groups are in a 1,3 relationship
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Para
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Groups are in a 1,4 relationship
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Naming compounds with more than two substituents
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Lowest possible numbers are used Listed alphabetically Any of the monosubstituted aromatic compounds can serve as the parent name
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Stability of benzene
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Aromatic rings don’t undergo addition reactions
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Aromaticity
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Cyclic, conjugated Unusually stable Planar Undergo substitution that retain conjugation
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4n+2 rule
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A molecule is aromatic only if it is planar, has a monocyclic system of conjugation, and contains 4n+2 pi electrons Molecules with 2,6,10,14,18….are aromatic These numbers fill bonding orbitals
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Anti-aromatic
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Molecules with 4n pi electrons (4,8,12,16) Delocalization of the pi electrons would destabilize the ring
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Cyclobutadiene example
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Four pi electrons Anti-aromatic Highly reactive
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Benzene example
Benzene example
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Contains six pi electrons (4n+2) where n = 1 Aromatic
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Cyclooctatetraene example
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Eight pi electrons Not aromatic Electrons localized on four double bonds No cyclic conjugation Not planar (tub-shaped)
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Frost circles
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1. Draw circle 2. Draw a polygon so one point is on the bottom of circle 3. Draw horizontal line at each connection point 4. Draw dotted line through center 5. Identify bonding molecular orbitals (below the line), nonbonding MOs (on the line), and antibonding MOs (above the line)
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Heterocyclic compounds
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Contains two or more different elements in the ring (usually N, O, or S with carbon)
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Pyridine
Pyridine
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Each carbon contains one pi electron The nitrogen contributes one pi electron The lone pair is not involved in the pi system, as it’s in a plane
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Pyrimidine
Pyrimidine
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Two nitrogen atoms that each contribute a pi electron
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Pyrrole
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Aromatic Six pi electrons Each sp2 orbital contributes one pi electron The lone pair is in a p orbital and so two pi electrons are contributed
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Imidazole
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Both nitrogens are sp2 One N contributes one pi electron The other N contributes two pi electrons from the lone pair, as it is not in a double bond
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Contributing pi electrons
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If the nitrogens are in a double bond, they contribute one pi electron If the nitrogens are not in a double bond, they contribute two pi electrons

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