5. Aromatic Compounds Pi3 Aromatic was used to described some fragrant compounds in early 1gth century Not correct: later they are grouped by chemical behavior (unsaturated compounds that undergo substitution rather than addition) Current: distinguished from aliphatic compounds by electronic configuration
5. Aromatic Compounds • Aromatic was used to described some fragrant compounds in early 19th century –Not correct: later they are grouped by chemical behavior (unsaturated compounds that undergo substitution rather than addition) • Current: distinguished from aliphatic compounds by electronic configuration p.153
CH CH CH Benzene Toluene Xylene Indene (bp 801c) (bp llc) (bp: ortho, 144C; (bp 182 C) meta, 139\C; para, 138C Naphthalene Biphenyl Anthracene Phenanthrene (mp 801C) (mp 71C) (mp216C) (mp lonc) Thomson· Brooks cole
5.1 Stracture of benzene p.154 The Kekule proposal Benzene's unusual structure All its c-c bonds are the same length: 139 pm between single(154 pm) and double (134 pm) bonds Electron density in all six C-C bonds is identical Structure is planar, hexagonal CC- C bond angles 120° Each C is sp and has a p orbital perpendicular to the plane of the six-membered ring 1.5 bonds on average H C H emhm9o%sC幽
Benzene’s Unusual Structure: • All its C-C bonds are the same length: 139 pm — between single (154 pm) and double (134 pm) bonds • Electron density in all six C-C bonds is identical • Structure is planar, hexagonal • C–C–C bond angles 120° • Each C is sp2 and has a p orbital perpendicular to the plane of the six-membered ring 5.1 Stracture of Benzene: The Kekule Proposal p.154
Drawing benzene and Its derivatives The two benzene resonance forms can be represented by a single structure with a circle in the center to indicate the equivalence of the carbon-carbon bonas This does indicate the number of t electrons in the ring but reminds us of the delocalized structure We shall use one of the resonance structures to represent benzene for ease in keeping track of bonding changes in reactions
Drawing Benzene and Its Derivatives: • The two benzene resonance forms can be represented by a single structure with a circle in the center to indicate the equivalence of the carbon–carbon bonds • This does indicate the number of p electrons in the ring but reminds us of the delocalized structure • We shall use one of the resonance structures to represent benzene for ease in keeping track of bonding changes in reactions
p155 Molecular Orbital Description of benzene The 6 p-orbitals combine to give Three bonding orbitals with 6 T electrons Two nonbonding and two antibonding orbitals Antibonding 目++++++ nonbonding Six p atomic orbitals 中23 计k Six benzene molecular orbitals Thomson· Brooks Coli
Molecular Orbital Description of Benzene The 6 p-orbitals combine to give: – Three bonding orbitals with 6 p electrons, – Two nonbonding and two antibonding orbitals p.155
5.2 Stability of benzene From high temperature distillation of coal tar Heating petroleum at high temperature and pressure over a catalyst KMnO4 HO HX
5.2 Stability of Benzene • From high temperature distillation of coal tar • Heating petroleum at high temperature and pressure over a catalyst KMnO4 HX H3O +
Heats of hydrogenation as Indicators of Stability E (kJ/ mol 假想的环己三烯 实际存在的苯 +3H2 2H2+ +3H 36 232 H2+ 209 120
H2 + 209 232 120 336 2H2 3H2 ( kJ / mol ) + + + 3H2 假想的环己三烯 实际存在的苯 E Heats of Hydrogenation as Indicators of Stability
5.3 Structure of benzene The resonance proposal ……④
5.3 Structure of Benzene: The Resonance Proposal + -
157 ** Aromaticity and the Huckel 4n+2 rule 芳香性和 Huckel4n+2规如 a molecule is aromatic only if it has a planar, monocyclic system of conjugation with a total of 4n+2 I electrons where n is an integer 在一个平面单环热軛体系中,含有4n+2电 子的体系具有与惰性气体相类似的闭壳层结构,从而 显示出芳香性
**Aromaticity and the Huckel 4n+2 rule 芳香性和 Huckel 4n+2规则 p.157 “A molecule is aromatic only if it has a planar, monocyclic system of conjugation with a total of 4n+2 p electrons, where n is an integer. 在一个平面 单环 共軛 体系中, 含有4n+2 p电 子的体系具有与惰性气体相类似的闭壳层结构, 从而 显示出芳香性
aromaticity and the 4n+2 Rule For n=1 4n+2=6 benzene is stable and the electrons are delocalized Benzene Three double bonds six electrons Thomson.Brooks Cole
Aromaticity and the 4n + 2 Rule For n=1: 4n+2 = 6; benzene is stable and the electrons are delocalized
