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To be classified as aromatic, a compound must meet both of the following criteria: It must have an un-interrupted cyclic p cloud above & below the plane of the molecule. The p cloud must contain (4n + 2) p electrons (n = 0, 1, 2 ). | ORGANIC CHEMISTRY Dr Nam T. S. Phan Faculty of Chemical Engineering HCMC University of Technology Office: room 211, B2 Building Phone: 8647256 ext. 5681 Email: ptsnam@hcmut.edu.vn Chapter 8: ARENES Benzene CRITERIA FOR AROMATICITY To be classified as aromatic, a compound must meet both of the following criteria: It must have an un-interrupted cyclic p cloud above & below the plane of the molecule The p cloud must contain (4n + 2) p electrons (n = 0, 1, 2 ) un-interrupted cyclic p cloud 6 p e = 4 x 1 + 2 Aromatic: NOT aromatic: 4 p e 5 p e 7 p e 8 p e NOT (4n + 2) p e Interrupted cyclic p cloud NOMENCLATURE OF MONOSUBSTITUTED BENZENES Name of substituent + benzene Names have to be memorized: NOMENCLATURE OF DI- & POLYSUBSTITUTED BENZENES Alphabetical order, 1-position for the 1st stated substituent 1 of the substituents can be incorporated into a name: Names incorporating 2 substituents Alphabetical order Lowest possible numbers PREPARATION OF BENZENE Petroleum Bezene REACTIONS OF BENZENE Halogenations of benzene Reaction mechanism: electrophilic substitution Catalyst regeneration Nitration of benzene Sulfonation of benzene Reversible reaction Reaction mechanism: electrophilic substitution Friedel-Crafts Alkylations of benzene Carbocation rearrangement Friedel-Crafts Acylations of benzene Reaction mechanism: electrophilic substitution Rearrangement REACTIONS OF SUBSTITUTED BENZENES Relative rates of electrophilic substitutions: Electron-donating Electron-withdrawing Relative reactivity Electron-donating Eelectron-withdrawing Activating Deactivating ortho/para-directing meta-direcring +I, +C, +H - I, -C, EFFECTS OF SUBSTITUENTS ON ORIENTATION E/D group Only for halogen group E/W group E/D group E/W group THE ORTHO-PARA RATIO Increase in the size of substituents Decrease in the o/p ratio ADDITIONAL CONSIDERATIONS More deactivating than halogen, the ring is too unreactive for (only) Friedel-Crafts alkylations & acylations Aniline & N-substituted anilines do NOT undergo Friedel-Crafts reactions: More deactivating than halogen Phenol & anisole do undergo Friedel-Crafts reactions, orienting ortho & para – oxygen does NOT complex with the Lewis acid Also can NOT undergo nitration – primary amines are easily oxidized SYNTHESIS OF TRISUBSTITUTED BENZENES More activating substituent controls the regioselectivity HALOGENATIONS OF ALKYL SUBSTITUENTS NOT Lewis acid Can undergo E1 & E2, SN1 & SN2 reactions as usual OXIDATIONS OF ALKYL SUBSTITUENTS NUCLEOPHILIC AROMATIC SUBSTITUTION REACTIONS E/W groups must be positioned ortho / para to the halogen The greater the number of E/W substituents, the easier it is to carry out the reaction
