Department of Chemistry AROMATIC COMPOUNDS by Parinya Theramongkol Department of Chemistry Khon Kaen University 1/3/2019

ORGANIC COMPOUNDS Aliphatic Aromatic (fatty) (fragrant) Open-chain + cyclic compounds Benzene + others that resemble benzene in chemical behavior AROMATIC CHARACTER 1/3/2019

Aromatic character : The HÜckel 4n+2 rule An aromatic compound must have a molecule that contains cyclic clouds of delocalized π electrons above and below the plane of the molecule and the π clouds must contain a total of ( 4n+2 ) π electrons. Benzene : C6H6 August Kekulé I II III Kekulé structures Resonance hybrid 1/3/2019

Aromatic Aromatic 1/3/2019

Stability of the benzene ring : resonance energy Potential energy 49.8 (obs) 85.8 (calc) Cyclohexatriene + 3H2 Benzene + 3H2 Resonance Energy 36 kcal Cyclohexadiene + 2H2 55.4 (obs) 57.2 (calc) Cyclohexene + H2 28.6 (obs) Cyclohexane Heat of hydrogenation Benzene contains 36 kcal less energy than predicted ! Benzene is more stable by 36 kcal than we would have expected ! 1/3/2019

Resonance structure of benzene I II III Kekulé structures Resonance hybrid Resonance structures : structures that differ only in the arrangement of electrons Benzene is a resonance hybrid of I and II 1/3/2019

Orbital picture of benzene Benzene is… a flat molecule very symmetrical C-C bond length = 1.39 A ( intermediate between the length of single and double bond) o 1/3/2019

Nomenclature Simply prefix the name of the substituent group to the word -benzene 1/3/2019

2. Special names 1/3/2019

3. Disubstituted benzene : Tell group names + relative positions 3.1 Same groups 1/3/2019

3.2.1 neither is a special group…end the name with -benzene 3.2 Different groups 3.2.1 neither is a special group…end the name with -benzene 1/3/2019

3.2.2 a special group present…name as a derivative of that special compound. 1/3/2019

4. Polysubstituted benzene Use numbers to indicate their relative positions 1/3/2019

Polynuclear aromatic hydrocarbons Aromatic rings share a pair of carbon atoms = fused - ring 1/3/2019

C60 buckminsterfullerene (1985) 20 + 12 20 + 12 1/3/2019

Reactivity : benzene vs. alkene 1/3/2019

Reaction Typical reactions of benzene = Electrophilic substitution addition substitution 1/3/2019

The first substitution 1. Bromination : Lewis acid needed e.g., FeBr3 , AlBr3 1/3/2019

Benzenonium ion is stable via resonance = 1/3/2019

2. Nitration Nitronium ion nitrobenzene 1/3/2019

3. Sulfonation Benzenesulfonic acid 1/3/2019

Page out of Friedel's notebook 4. Friedel-Crafts Alkylation (1877) Charles Friedel 1832-1899 Page out of Friedel's notebook James Mason Crafts 1839-1917 Alkylation of an aromatic ring with an alkyl halide and a trace of AlCl3. 1/3/2019

But there are problems with this alkylation method ! 1. The electrophile may undergo rearrangement because it is a carbocation. 70% 30% 1/3/2019

No rearrangement 1/3/2019

Rearrangement 1,2-H-shift 2o carbocation 1/3/2019

2. The alkyl substituent activates the ring so that a second substitution may also occur. 1/3/2019

5. Friedel-Crafts Acylation RCO- or ArCO- group is called an acyl group. This is a method of choice to prepare an aryl ketone. Linear alkyl benzene obtained without rearrangement ! 1/3/2019

Mechanism of Friedel-Crafts Acylation 1/3/2019

The second substitution The first substituent may affect the second substitution in 2 ways. 1. Reactivity : the second substitution may be easier = Activating group the second substitution may be harder = Deactivating group 2. Orientation : ortho ortho , para - director meta - director meta para 1/3/2019

Increasing activation Increasing deactivation Effect of the 1st substituent on the 2nd substitution o,p-Directors m-Directors (all deactivating) Increasing activation Increasing deactivation 1/3/2019

Mechanism of the 2nd substitution with an o,p-director A ring is activated by increasing electron density. Electron density can be increased by an electron donating group. A group can donate electrons by a) inductive effect b) resonance effect 1/3/2019

No catalyst needed ! a benzenonium ion 1/3/2019

p-intermediate gets extra stabilization by resonance effect extra resonance form 1/3/2019

Alkyl group is electron donating by the inductive effect ortho para Extra stabilization + next to e- donating group meta 1/3/2019

Mechanism of the 2nd substitution with a m-director A meta-director is : an e – withdrawing group deactivates all positions deactivates meta-positions less than the other positions 1/3/2019

Favored : no adjacent + charges ortho para Destabilization + next to +ve center meta Favored : no adjacent + charges 1/3/2019

Summary of substituent effects A substituent that is electron-donating activates a benzene ring and is an o,p-director. A halogen is an o,p-director (donating electrons by resonance), but deactivating by its electron – withdrawing inductive effect. 1/3/2019

A meta director deactivates all positions on the ring by electron withdrawal and deactivates the o,p-positions especially by resonance-destabilization. 1/3/2019

Aromatic substitution as a synthetic tool To prepare substituted benzene compounds the order of substitution reactions is of importance. Example : to prepare o- and p-chloronitrobenzene Nitration or chlorination in the first place ? 1/3/2019

How about m-chloronitrobenzene ? Now try to prepare m-bromoaniline !!!! 1/3/2019

This one needs the conversion of one group to another ( reduction of nitro to amino). 1/3/2019

Notes about Friedel-Crafts alkylation and acylation Aniline does not undergo Friedel-Crafts reactions because an amino group (a basic group) reacts with Lewis acids ! Nitrobenzene does not undergo Friedel-Crafts reactions because the ring is deactivated by the strong e-withdrawing nitro group. No aromatic substitution 1/3/2019

Alkylbenzenes : The benzylic position Benzylic position is a site of attack in many reactions because the benzyl cation, a benzyl radical, and benzyl carbanion are all resonance-stabilized by the benzene ring ! Oxidation 1/3/2019

Free radical halogenation Takes place on the alkyl sidechain preferentially ! 1/3/2019

Problems 1. ถ้านำสารต่อไปนี้มาทำ monobromination ที่วง จะได้ผลิตภัณฑ์หลักมีโครงสร้างและชื่ออะไร? ให้ระบุด้วยว่าปฏิกริยาจะเกิดเร็วหรือช้ากว่าเบ็นซีน a) acetanilide(C6H5NHCOCH3) b) iodobenzene c) sec-butylbenzene d) N-methylaniline(C6H5NHCH3) e) ethyl benzoate(C6H5COOC2H5) f) acetophenone(C6H5COCH3) 2. จงแสดงโครงสร้างและบอกชื่อที่ถูกต้องของผลิตภัณฑ์หลักที่ได้จากปฏิกริยาmononitration ของสารต่อไปนี้: a) o-nitrotoluene b) m-dibromobenzene c) m-cresol(m-CH3C6H4OH) d) o-cresol e) terephthalic acid(p-C6H4(COOH)2 f) anilinium hydrogen sulfate (C6H5NH3+HSO4-) 3. จงบอกว่าสารต่อไปนี้ เมื่อทำnitration จะเกิดขึ้นที่วงใด ? โครงสร้างของผลิตภัณฑ์หลักเป็นเช่นไร a) b) 1/3/2019

cyclooctatetraene dianion 4. จงเรียงลำดับสารประกอบในแต่ละชุดตามลำดับความว่องไวต่อปฏิกริยา electrophilic substitution พร้อมระบุด้วยว่าสารใดในแต่ละชุดจะให้ผลิตภัณฑ์ที่เป็น m-isomer มากที่สุด และชนิดใดให้น้อยที่สุด a) C6H5N(CH3)3+, C6H5CH2N(CH3)3+, C6H5CH2 CH2 N(CH3)3+, b) C6H5NO2 , C6H5 CH2 NO2 , C6H5 CH2 CH2 NO2 c) C6H5CH3 , C6H5CH2COOC2H5 , C6H5CH(COOC2H5)2 , C6H5C(COOC2H5)3 5. จงบอกผลิตภัณฑ์หลักที่ควรได้จากปฏิกริยาต่อไปนี้ ... a) ethylbenzene + Cl2 b) ethylbenzene + Br2 c) toluene + 1-chloropropane d) toluene + propene FeCl3 hv AlCl3 AlCl3 HCl 6. สารต่อไปนี้มีชนิดใดที่จัดว่าเป็นสารประเภท aromatic : a) b) c) d) cyclooctatetraene dianion 1/3/2019