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Reactions of Benzene & Its Derivatives

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Organic Lecture Series ReactionsReactions ofof BenzeneBenzene && ItsIts DerivativesDerivatives Chapter 22 1 Organic Lecture Series Reactions of Benzene The most characteristic reaction of aromatic compounds is substitution at a ring carbon: Halogenation: FeCl3 H + Cl2 Cl + HCl Chlorobenzene Nitration: H2 SO4 HNO+ HNO3 2 + H2 O Nitrobenzene 2 Organic Lecture Series Reactions of Benzene Sulfonation: H 2 SO4 HSO+ SO3 3 H Benzenesulfonic acid Alkylation: AlX3 H + RX R + HX An alkylbenzene Acylation: O O AlX H + RCX 3 CR + HX An acylbenzene 3 Organic Lecture Series Carbon-Carbon Bond Formations: R RCl AlCl3 Arenes Alkylbenzenes 4 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 H E + + + E + H • In this section: – several common types of electrophiles – how each is generated – the mechanism by which each replaces hydrogen 5 Organic Lecture Series EAS: General Mechanism • A general mechanism slow, rate + determining H Step 1: H + E+ E El e ctro - Resonance-stabilized phile cation intermediate + H fast Step 2: E + H+ E • Key question: What is the electrophile and how is it generated? 6 Organic Lecture Series + + 7 Organic Lecture Series Chlorination Step 1: formation of a chloronium ion Cl Cl + + - - Cl Cl+ Fe Cl Cl Cl Fe Cl Cl Fe Cl4 Cl Cl Chlorine Ferric chloride A molecular complex An ion pair (a Lewis (a Lewis with a positive charge containing a base) acid) on ch lorine ch loronium ion Step 2: attack of the chloronium ion on the ring slow, rate determining + Cl + H H H + Cl Cl Cl + Resonance-stabilized cation intermediate; the positive charge is delocalized onto three atoms of the ring 8 Organic Lecture Series Chlorination Step 3: proton transfer regenerates the aromatic character of the ring + H - + fast Cl-FeCl3 Cl + HCl + FeCl3 Cl Cation Chlorobenzene intermediate 9 Organic Lecture Series Bromination FeBr 3 + H + Br 2 Br HBr Bromobenzene This is the general method for Substitution of halogen onto a benzene ring (CANNOT be halogenated by Free Radical Mechanism) 10 Organic Lecture Series Bromination-Why not addn of Br2? Regains Aromatic Energy 11 Organic Lecture Series Nitration + • Generation of the nitronium ion, NO2 – Step 1: proton transfer to nitric acid O H O HSO3 OH+ HON HSO4 + ON O H O Sulfuric Nitric Co nju gate acid acid pKa= -3 acid pKa= -1.4 of nitric acid – Step 2: loss of H2O gives the nitronium ion, a very strong electrophile H O H ON O + ONO H H O The nitronium ion 12 Nitration Organic Lecture Series Step 1: attack of the nitronium ion (an electrophile) on the aromatic ring (a nucleophile) HNO 2 H NO2 HNO2 + + + + ONO + Resonance-stabilized cation intermediate Step 2: proton transfer regenerates the aromatic ring HHNO NO2 H 2 H O + + + O H H H 13 Organic Lecture Series Nitration • A particular value of nitration is that the nitro group can be reduced to a 1° amino group COOH COOH Ni + 3H + 2H O 2 (3 atm) 2 NO2 NH2 4-Nitrobenzoic acid 4-Aminobenzoic acid 14 Organic Lecture Series Sulfonation • Carried out using concentrated sulfuric acid containing dissolved sulfur trioxide H SO + 2 4 SO3 SO3 H Benzene B enzenesulfonic acid (SO3 in H2SO4 is sometimes called “fuming” sulfuric acid.) 15 Organic Lecture Series Friedel-Crafts Alkylation • Friedel-Crafts alkylation forms a new C-C bond between an aromatic ring and an alkyl group AlCl3 + Cl + HCl Benzene 2-Chloropropane Cumene (Isopropyl chloride) (Isopropylbenzene) The electrophilic partner is a carbocation; it will arrange to the most stable ion: allylic>3o>2o>1o 16 Friedel-Crafts Alkylation Organic Lecture Series Step 1: formation of an alkyl cation as an ion pair Cl + Cl - + - RCl+ Al Cl RClAl Cl R AlCl4 Cl Cl A molecular An ion pair containing comp lex a carbocation Step 2: attack of the alkyl cation on the aromatic ring + H H H + R+ + R R R + A resonance-stabilized cation Step 3: proton transfer regenerates the aromatic ring H + Cl AlCl3 R ++AlCl3 HCl R 17 Friedel-Crafts Alkylation Organic Lecture Series There are two major limitations on Friedel-Crafts alkylations: 1. carbocation rearrangements are common: AlCl3 + Cl +HCl Benzene Isobutyl tert-Butylbenzene chloride CH CH 3 CH3 + 3 - + - CH3 CHCH2 -Cl + AlCl3 CH 3 C- CH2 -Cl-AlCl3 CH3 C AlCl4 H CH 3 I sobutyl chloride amolecular an ion pair complex 18 Organic Lecture Series Friedel-Crafts Alkylation 2. F-C alkylation fails on benzene rings bearing one or more of these strongly electron- withdrawing groups Y AlCl + RX 3 No reacti on When Y Equals Any of These Groups, the Benzene Ring Does Not Undergo Friedel-Crafts Alkylation O O O O O CH CR COH COR CNH2 + SO3 HNOCN 2 NR3 CF3 CCl3 19 Organic Lecture Series 20 Organic Lecture Series The “De-activation” of Aromatic Systems Note: deactivation refers to the rate of EAS 21 Organic Lecture Series Friedel-Crafts Acylation • Friedel-Crafts acylation forms a new C-C bond between a benzene ring and an acyl group: O O AlCl3 + CH3 CCl + HCl BenzeneAcetyl Acetophenone ch loride O Cl O AlCl 3 + HCl 4-Phenylbutanoyl α-Tetralone chlorid e 22 Organic Lecture Series Friedel-Crafts Acylation • The electrophile is an acylium ion O Cl (1) •• •• R-C Cl + Al-Cl •• Cl An acyl Aluminum chloride chloride O O + Cl (2) •• - R-C Cl Al Cl R-C + AlCl - •• 4 Cl A molecular complex A n ion pair with a positive charge containing an charge on chlorine acylium ion 23 Friedel-Crafts Acylation Organic Lecture Series – an acylium ion is a resonance hybrid of two major contributing structures complete valence shells + + : R-C O: : 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. 24 Organic Lecture Series Friedel-Crafts Acylation A special value of F-C acylations is preparation of unrearranged alkylbenzenes: O AlCl + Cl 3 2-Methylpropanoyl chloride O N 2H 4, KOH diethylene 2-Methyl-1- glycol Isobutylbenzene phenyl-1-propanone 25 Organic Lecture Series Di- and Polysubstitution Only a trace 26 Organic Lecture Series Di- and Polysubstitution Orientation on nitration of monosubstituted benzenes: ortho + Substituent ortho meta para para meta - OCH3 44 55 99 trace CH 3 58 4 38 96 4 Cl 70 - 30 100 trace Br 37 1 62 99 1 COOH 18 80 2 20 80 CN 19 80 1 20 80 NO2 6.4 93.2 0.3 6.7 93.2 27 Organic Lecture Series Di- and Polysubstitution • Orientation: –certain substituents direct preferentially to ortho & para positions; others to meta positions –substituents are classified as either ortho-para directing or meta directing toward further substitution 28 Di- and Polysubstitution Organic Lecture Series • Rate –certain substituents cause the rate of a second substitution to be greater than that for benzene itself; others cause the rate to be lower –substituents are classified as activating or deactivating toward further substitution 29 Organic Lecture Series 30 Di- and Polysubstitution Organic Lecture Series – -OCH3 is ortho-para directing: OCH3 OCH3 OCH3 NO2 + HNO3 + + H2 O CH 3 COOH NO2 Anisole o-Nitroanisole p-Nitroanisole (44%) (55%) –-CO2H is meta directing COOH COOH COOH COOH NO H2 SO4 2 ++HNO3 + 100°C NO2 Benzoic NO2 acid o-Nitro- m-Nitro- p-Nitro- benzoic benzoic benzoic acid acid acid (18%) (80%) (2%) 31 Organic Lecture Series Di- and Polysubstitution : : : Strongly : : NH NHR NR OH OR: activating 2 2 : OOOO: : Moderately : : NHCR NHCAr OCR: OCAr activating : Weakly activating R :: :: :: : Ortho-para Directing Weakly F: : : : deactivating Cl: Br I OOO O CH CR COH COR Moderately deactivating O CNH2 SO3 H CN Strongly + Meta Directing deactivating NO2 NH3 CF3 CCl3 32 Di- and Polysubstitution Organic Lecture Series the order of steps is important: CH3 COOH HNO 3 K2 Cr2O7 H SO 2 4 H2SO4 CH 3 NO2 NO2 p-Nitrobenzoic acid COOH COOH HNO K2 Cr 2 O7 3 H SO H2 SO4 2 4 NO2 m-Nitrobenzoic acid 33 Organic Lecture Series Theory of Directing Effects • The rate of EAS is limited by the slowest step in the reaction • For almost every EAS, the rate- determining step is attack of E+ on the aromatic ring to give a resonance- stabilized cation intermediate •The more stable this cation intermediate, the faster the rate- determining step and the faster the overall reaction 34 Organic Lecture Series Theory of Directing Effects • For ortho-para directors, ortho-para attack forms a more stable cation than meta attack – ortho-para products are formed faster than meta products • For meta directors, meta attack forms a more stable cation than ortho-para attack – meta products are formed faster than ortho-para products 35 Theory of Directing Effects Organic Lecture Series Nitration of anisole -OCH3; examine the meta attack: OCH3 slow + + NO2 OCH3 OCH3 OCH3 OCH3 + + fast H H H -H+ NO2 NO2 NO2 + NO2 (a) (b) (c) 36 Organic Lecture Series Nitration of anisole -OCH3: examine the ortho-para attack: OCH3 OCH3 + slow + NO2 : +: : : NO : 2 OCH3 ::OCH3 OCH3 OCH3 fast + -H+ + + NO H 2 HNO2 H NO2 H NO2 (d) (e) (f) (g) This resonance structure accounts for the selectivity 37 Organic Lecture Series Theory of Directing Effects Nitration of benzoic acid -NO2; examine the meta attack: COOH + slow + NO2 COOH COOH COOH COOH fast H H H -H+ NO2 NO2 NO2 NO2 (a) (b) (c) 38 Organic Lecture Series Nitration of benzoic acid -NO2: assume ortho-para attack: COOH + slow + NO2 COOH COOH COOH COOH fast -H+ H NO 2 H NO2 H NO2 NO2 (d) (e) (f) The most disfavored contributing structure This resonance structure accounts for the selectivity 39 Organic Lecture Series Activating-Deactivating • Any resonance effect,effect such as that of - NH2,
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  • Summary of Gas Cylinder and Permeation Tube Standard Reference Materials Issued by the National Bureau of Standards

    Summary of Gas Cylinder and Permeation Tube Standard Reference Materials Issued by the National Bureau of Standards

    A111D3 TTbS?? o z C/J NBS SPECIAL PUBLICATION 260-108 o ^EAU U.S. DEPARTMENT OF COMMERCE/National Bureau of Standards Standard Reference Materials: Summary of Gas Cylinder and Permeation Tube Standard Reference Materials Issued by the National Bureau of Standards QC 100 U57 R. Mavrodineanu and T. E. Gills 260-108 1987 m he National Bureau of Standards' was established by an act of Congress on March 3, 1901. The Bureau's overall goal i s t0 strengthen and advance the nation's science and technology and facilitate their effective application for public benefit. To this end, the Bureau conducts research to assure international competitiveness and leadership of U.S. industry, science arid technology. NBS work involves development and transfer of measurements, standards and related science and technology, in support of continually improving U.S. productivity, product quality and reliability, innovation and underlying science and engineering. The Bureau's technical work is performed by the National Measurement Laboratory, the National Engineering Laboratory, the Institute for Computer Sciences and Technology, and the Institute for Materials Science and Engineering. The National Measurement Laboratory Provides the national system of physical and chemical measurement; • Basic Standards 2 coordinates the system with measurement systems of other nations and • Radiation Research furnishes essential services leading to accurate and uniform physical and • Chemical Physics chemical measurement throughout the Nation's scientific community, • Analytical Chemistry industry, and commerce; provides advisory and research services to other Government agencies; conducts physical and chemical research; develops, produces, and distributes Standard Reference Materials; provides calibration services; and manages the National Standard Reference Data System.