Chapter 9 Aromatic compounds 芳香化合物. Text 1: chapter 16, 17 Text 2: 第七章. Contents. 1. The structure of benzene 2. Aromaticity 3. Nomenclature of benzene derivatives 4. Physical properties 5. Reactions of benzene and its derivatives
Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.
Chapter 9 Aromatic compounds芳香化合物 Text 1: chapter 16, 17 Text 2: 第七章
Contents • 1. The structure of benzene • 2. Aromaticity • 3. Nomenclature of benzene derivatives • 4. Physical properties • 5. Reactions of benzene and its derivatives • Electrophilic aromatic substitution ----Effects of substituents: Reactivity and orientation • Nucleophilic aromatic substitution • Addition reactions of benzene derivatives • Side-chain reactions of benzene derivatives • Reactions of phenols
Discovery of Benzene • Isolated in 1825 by Michael Faraday who determined C:H ratio to be 1:1. • Synthesized in 1834 by Eilhard Mitscherlich who determined molecular formula to be C6H6. • Other related compounds with low C:H ratios had a pleasant smell, so they were classified as aromatic(芳香性的).
1. The Structure of benzene (16-2, 3) The Kekule Structure for benzene 苯的克库勒结构 • Proposed in 1866 by Friedrich Kekulé, shortly after multiple bonds were suggested. • Failed to explain existence of only one isomer of 1,2-dichlorobenzene. Same compund
The unusual stability of benzene Hydrogenation of just one double bond in benzene is endothermic（吸热）!
2. Aromaticity芳香性 Aromatic Requirements • Structure must be cyclic with conjugated pi bonds. • Each atom in the ring must have an unhybridized p orbital. • The p orbitals must overlap continuously around the ring. (Usually planar structure) • Compound is more stable than its open-chain counterpart
Anti- and Nonaromatic反芳香性和非芳香性 • Antiaromatic（反芳香性的） compounds are cyclic, conjugated, with overlapping p orbitals around the ring, but the energy of the compound is greater than its open-chain counterpart. • Nonaromatic（非芳香性的） compounds do not have a continuous ring of overlapping p orbitals and may be nonplanar.
Hückel’s Rule （休克尔规则） The (4n+2) pi-Electron Rule • If the compound has a continuous ring of overlapping p orbitals and has 4N + 2 pielectrons, it is aromatic. • If the compound has a continuous ring of overlapping p orbitals and has 4N electrons, it is antiaromatic.
Other aromatic compounds (1) polynuclear aromatic hydrocarbons (PAH, 多核芳烃,16-10) Fused aromatic compounds稠环芳烃
As the number of aromatic rings increases, the resonance energy per ring decreases, so larger PAH’s will add Br2. (mixture of cisand trans isomers)
(2) [N]annulenes (轮烯) • Annulene is antiaromatic (4N e-’s) • Annulene would be antiaromatic, but it’s not planar, so it’s nonaromatic. • Annulene is aromatic except for the isomers that are not planar. • Larger 4N annulenes are not antiaromatic because they are flexible enough to become nonplanar.
Nonaromatic 非芳香性 annulene annulene nonaromatic annulene nonaromatic annulene Antiaromatic if planar annulene Aromatic if planar
Problem: Which molecule is more stable? Azulene 奥 The dipole moment is 1.0D ( aromatic 具有芳香性) (aromatic) Answer: (A)
=> MO Derivation of Hückel’s Rule • Lowest energy MO has 2 electrons. • Each filled shell has 4 electrons.
ring Number of πelectrons E π* n π* The Polygon Rule多边形规则
Aromatic ions Cyclopentadienyl Ions 环戊二烯离子 4 electrons, antiaromatic. 6 electrons, aromatic. Acidity
Tropylium Ion 卓翁离子 6 p electrons and an empty p orbital. Aromatic: more stable than open chain ion Aromatic ions Cycloheptatrienyl cation 环庚三烯阳离子
Dianion of Annulene 轮烯二价阴离子 10 p electrons, aromatic. Aromatic ions Cyclooctatetraenyl dianion 环辛四烯二价阴离子
(3) Heterocyclic aromatic compounds （16-9） 杂环芳香化合物（中文下册）
Pyridine 吡啶 Nonbonding pair of electrons in sp2 orbital, so weak base, pKb = 8.8.
Pyrrole 吡咯 Also aromatic, but lone pair of electrons is delocalized, so much weaker base.
Basic or Nonbasic? Pyrimidine（嘧啶） has two basic nitrogens. Imidazole（咪唑） has one basic nitrogen and one nonbasic. Purine（嘌呤） ?
3. Nomenclature of benzene derivatives • Monosubstituted benzenes • “XXbenzene” ， XX苯 Fluorobenzene (氟苯） Chlorobenzene （氯苯） Bromobenzene （溴苯） Nitrobenzene (硝基苯）
Methylbenzene Toluene (甲苯） Ethylbenzene （乙苯） Hydroxybenzene Phenol （苯酚） Aminobenzene Aniline （氨基苯， 苯胺） Benzenesulfonic acid 苯磺酸 Benzoic acid 苯甲酸 Acetophenone 苯乙酮 Anisole 苯甲醚
(2) disubstituted benzenes 1,2-; 1,3-; 1,4- o- (邻), m- (间), p- (对) 1,2-Dibromobenzene o-Dibromobenzene 邻二溴苯 1,3-Dibromobenzene m-Dibromobenzene 间二溴苯 1,4-Dibromobenzene p-Dibromobenzene 对二溴苯
1,2-Dimethylbenzene 1,3-Dimethylbenzne 1,4-Dimethylbenzene o-xylenem-xylenep-xylene 2-Nitrobenzoic acid 3-Nitrobenzic acid 4-Nitrobenzic acid o-Nitrobenzoic acid m-Nitrobenzoic acid p-Nitrobenzoic acid
(3) benzene as substituent: phenyl group (苯基)， Ph- Butylbenzene 2-Phenyl-2-butene 2-Phenylheptane Benzyl group (苄基),Bn- Aryl group (芳基),Ar- Arene = Aromatic hydrocarbon
4. Physical properties and spectroscopy of aromatic compounds • Melting points: More symmetrical than corresponding alkane, pack better into crystals, so higher melting points. • Boiling points: Dependent on dipole moment, so ortho > meta > para, for disubstituted benzenes. • Density: More dense than nonaromatics, less dense than water. • Solubility: Generally insoluble in water.
spectroscopy of aromatic compounds UV: 254 nm (B 带) IR: 3030 (C-H), 1600 (skeleton), 1500 cm-1 1H NMR: 7~9ppm MS:
Assignments: T-1: 16-27, 28, 30, 33, 36 Chapter 9 Aromatic compounds
Chapter 9 Aromatic compounds 5. Reactions of benzene and its derivatives (Chapter 17)
阿司匹林， 消炎止痛 布洛芬， 止痛 芬氟拉明， 减肥 沙丁胺醇， 哮喘 对乙酰氨基酚， 扑热息痛 萘普生， 关节炎 普鲁卡因， 局部麻醉 Manybenzene and naphthalene(萘) derivatives are useful drugs
17-2 17-3 17-4 17-10 17-11 1) Electrophilic aromatic substitution reactions
Electrophilic addition-elimination mechanism 亲电加成-消除机理 Step 1: addition Arenium ion (芳基正离子) intermediate σ- complex (σ-络合物） Step 2: elimination
E TS 1 rate-limiting step TS 2 Eact2 Eact1 σ-complex reactants product reaction coordination The reaction-energy diagram (反应能线图)
Halogenation of benzene: X2, FeX3 Reactivity: F2 > Cl2 > Br2> I2
Nitration of benzene: HNO3 + H2SO4 Nitronium ion srong electrophile
Sulfonation of benzene: fuming sulfuric acid (发烟硫酸) 7% SO3 in H2SO4
δ+ δ- R······X······AlCl3 The Friedel-Crafts alkylation of benzene: RX, AlCl3( or ether Lewis acid) E+: C=C, H+ Other carbocation source C-C-OH, acid Limitations: • R+ can rearrangement; • Polyalkylations often occur.
Problems: Predict products for the reactions of benzene with the following reagents.
The Friedel-Crafts Acylation of benzene: RCOCl, AlCl3 (Lewis acid) E+: Acylium ion 酰基正离子
The Gatterman-Koch formylation: ——synthesis of genzaldehyde
The Clemmensen reduction: ——synthesis of alkylbenzenes. The Clemmensen reduction How to shnthesize n-propylbenzene from benzene?
o- m- p- The effects of substituents on the electrophilic aromatic substitution: Reactivity and Orientation • Reactivity: • Electron donating group (给电子): Activiting (活化) • Electron withdrawing (拉电子): deactivating (钝化) • (2) Orientation:o-, m-, p-, which is major?
Nitration of toluene Yield: 60% 4% 36% • Toluene is about 25 times faster than benzene. Why?