310_19 - Organic Organic Lecture Series CH 310 N LECTURE 19...

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Unformatted text preview: Organic Organic Lecture Series CH 310 N LECTURE 19 Textbook Assignment: Chapter 22 Reactions of Benzene Homework (for credit): Problem Set – 6 Posted Today’s Topics: Acidity of phenols; e- donating & withdrawing groups Notice & Announcements: EXAM 2: Pick up after class 1 Organic Lecture Series Reactions of Benzene & Its Derivatives Chapter 22 2 Organic Organic Lecture Series Reactions of Benzene The most characteristic reaction of aromatic compounds is substitution at a ring carbon: 3 Organic Lecture Series Reactions of Benzene Sulfonation: H + SO3 H2 SO4 SO3 H Benzenesulfonic acid Alkylation: H + RX A lX 3 R + HX An alkylbenzene Acylation: O H + RC X A lX 3 O CR + H X An acylbenzene 4 Organic Organic Lecture Series Carbon-Carbon Bond Formations: 5 Organic Lecture Series Electrophilic Aromatic Substitution • Electrophilic aromatic substitution: a reaction in which a hydrogen atom of an aromatic ring is replaced by an electrophile • In this section: – several common types of electrophiles – how each is generated – the mechanism by which each replaces hydrogen 6 Organic Organic Lecture Series EAS: General Mechanism • A general mechanism • Key question: What is the electrophile and how is it generated? 7 Organic Lecture Series 8 Organic Organic Lecture Series Chlorination Step 1: formation of a chloronium ion Step 2: attack of the chloronium ion on the ring 9 Organic Lecture Series Chlorination Step 3: proton transfer regenerates the aromatic character of the ring + H Cl Cation intermediate + Cl-FeCl3 f ast Cl + HCl + FeCl3 Chlorobenzene 10 Organic Organic Lecture Series Bromination H + Br 2 F eBr3 Br + HBr Bromobenzene This is the general method for Substitution of halogen onto a benzene ring (CANNOT be halogenated by Free Radical Mechanism) 11 Organic Lecture Series Bromination-Why not addn of Br2? Regains Aromatic Energy 12 Organic Organic Lecture Series Nitration • Generation of the nitronium ion, NO2+ – Step 1: proton transfer to nitric acid pKa= -3 pKa= -1.4 – Step 2: loss of H2O gives the nitronium ion, a very strong electrophile 13 Nitration Organic Lecture Series Step 1: attack of the nitronium ion (an electrophile) on the aromatic ring (a nucleophile) Step 2: proton transfer regenerates the aromatic ring 14 Organic Organic Lecture Series Nitration • A particular value of nitration is that the nitro group can be reduced to a 1°amino group 15 Organic Lecture Series Sulfonation • Carried out using concentrated sulfuric acid containing dissolved sulfur trioxide + Benzene SO3 H2 SO4 SO3 H B enzenesulfonic acid (SO3 in H2SO4 is sometimes called “fuming” sulfuric acid.) 16 Organic Organic Lecture Series Friedel-Crafts Alkylation • Friedel-Crafts alkylation forms a new C-C bond between an aromatic ring and an alkyl group The electrophilic partner is a carbocation; it will arrange to the most stable ion: allylic>3o>2o>1o 17 Friedel-Crafts Alkylation Organic Lecture Series Step 1: formation of an alkyl cation as an ion pair Step 2: attack of the alkyl cation on the aromatic ring Step 3: proton transfer regenerates the aromatic ring 18 Organic Organic Lecture Series Friedel-Crafts Alkylation There are two major limitations on Friedel-Crafts alkylations: 1. carbocation rearrangements are common: Cl + Benzene CH3 CH3 CHCH2 -Cl Isobutyl chloride + AlCl3 I sobutyl chloride AlCl3 + HCl tert-Butylbenzene CH3 + CH3 C-CH2 -Cl-AlCl3 H a molecular complex CH3 CH3 C+ AlCl4 CH3 an ion pair 19 Organic Lecture Series Friedel-Crafts Alkylation 2. F-C alkylation fails on benzene rings bearing one or more of these strongly electronwithdrawing groups 20 Organic Organic Lecture Series 21 Organic Lecture Series The “De-activation” of Aromatic Systems Note: deactivation refers to the rate of EAS 22 Organic Organic Lecture Series Friedel-Crafts Acylation • Friedel-Crafts acylation forms a new C-C bond between a benzene ring and an acyl group: 23 Friedel-Crafts Acylation Organic Lecture Series • The electrophile is an acylium ion •• O •• R -C C l •• An acyl chloride Cl + (1) A l-C l Cl Aluminum chloride O R -C + •• Cl Cl •• Al Cl - Cl A molecular complex with a positive charge charge on chlorine (2) O R -C + A lC l 4 - A n ion pair containing an acylium ion 24 Friedel-Crafts Acylation Organic Organic Lecture Series – an acylium ion is a resonance hybrid of two major contributing structures O: : + R-C complete valence shells + R-C O: The more important contributing structure • F-C acylations are free of a major limitation of F-C alkylations; acylium ions do not rearrange. 25 Friedel-Crafts Acylation Organic Lecture Series A special value of F-C acylations is preparation of unrearranged alkylbenzenes: 26 Organic Organic Lecture Series Other Aromatic Alkylations • Carbocations are also generated by: – treatment of an alkene with a proton acid, most commonly H2SO4, H3PO4, or HF/BF3 – by treating an alkene with a Lewis acid AlCl3 + Benzene Cyclohexene Phenylcyclohexane 27 Organic Lecture Series Other Aromatic Alkylations – and by treating an alcohol with H2SO4 or H3PO4 + Benzene HO H 3 PO 4 2-Methyl-2-propanol (tert- Butyl alcohol) + H2 O 2-Methyl-2phenylpropane ( tert- Butylbenzene 28 Organic Organic Lecture Series Di- and Polysubstitution Only a trace 29 Organic Lecture Series Di- and Polysubstitution Orientation on nitration of monosubstituted benzenes: 30 ...
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