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CHEM2213 Topic 6.docx

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Chemistry 1027A/B
Mark Workentin

Topic 6 Benzene & Derivatives  Aromatic Compound: term used to classify benzene and its derivatives  Arene: aromatic hydrocarbon  Aryl Group: group derived from an aromatic compound by the removal of an H  Ar–: symbol used for an aryl group What Is The Structure Of Benzene?  Molecular formula of benzene is C H6,6molecule is highly unsaturated  Yet benzene does not show the chemical properties of alkenes  All 6C and 6H are equivalent  C skeleton of benzene forms a regular hexagon with C-C-C and H-C-C bonds angles of 120° 2 o For this type of bonding, C uses sp hybrid orbitals o C-C bonds are neither single nor double bonds, but something intermediate  Resonance Energy: difference in energy between a resonance hybrid and most stable of its hypothetical contributing structures  Although the resonance energy of benzene is less than the strength of a C-C single bond, it is greater than the strength of H bonding in water and alcohols What Is Aromaticity?  Contain high degrees of unsaturation yet fail to undergo characteristic alkene addition and oxidation-reduction reactions  To be aromatic, a ring must: o One 2p orbital on each of its atoms o Be planar, or nearly planar for continuous overlap of all 2p orbitals of the ring o 2, 6, 10, 14, 18 and so forth pi electrons in the cyclic arrangement of 2p orbitals  Heterocyclic Compound: organic compound that contains 1 or more atoms other than C in its ring How Are Benzene Compounds Named And What Are Their Physical Properties?  Phenyl Group: C H6–5aryl group derived by removing a H from benzene  Benzyl Group: C H6C5 – 2lkyl group derived by removing a H from the methyl group of toluene  When two substituents occur on a benzene ring, 3 constitutional isomers are possible: o Ortho (o): refers to groups occupying positions 1 and 2 o Meta (m): refers to groups occupying positions 1 and 3 o Para (p): refers to groups occupying positions 1 and 4  Benzene, has no substituents and is flat, can pack its molecules very closely giving it a considerably higher melting point than many substituted benzenes  Polynuclear Aromatic Hydrocarbon: hydrocarbon containing two or more fused aromatic rings What Is The Benzylic Position And How Does It Contribute To Benzene Reactivity? 3  Benzylic Carbon: sp hybridized C bonded to a benzene ring  Benzene is unaffected by strong oxidizing agents (H CrO2) 4  Side chain methyl group is oxidized to carboxyl group but aromatic ring is unchanged  If a benzylic H exists, then the benzylic C is oxidized to a carboxyl group (COOH) and all other C of the side chain are removed  If no benzylic H exists, then the side chain is not oxidized What Is Electrophilic Aromatic Substitution?  Most characteristic reaction of aromatic compounds is electrophilic aromatic substitution  Involves substitution of one of the ring H of benzene for an electrophilic reagent What Is The Mechanism OF Electrophilic Aromatic Substitution?  Electrophilic Aromatic Substitution: reaction in which an electrophile E + substitutes for a H on a+ aromatic ring o Reagent(s)  E o Attack of the electrophile on the aromatic ring
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