C h a p t e r N i n e t e e n Aromatics II: Reactions of Benzene & Its Derivatives Arenium ion from addition of tert-butyl cation to benzene (blue is δ+and red δ-)
Note: Problems with italicized numbers are more challenging. You may want to try them last. Copyright 2012 by Martin Hulce. All rights reserved. Except as permitted under the United States Copyright Act of 1976, no part of this publication may be reproduced or distributed in any form or by any means, or stored in a data base or retrieval system, without the prior permission of the copyright holder
CHM 323: Summary of Important Concepts YConcepts for Chapter 19: Aromatics II: Reactions of Benzene and Its Derivatives I. Typical Mechanism of Electrophilic Aromatic Substitution H E E + E + + + H + H E H E + + resonance-stabilized arenium ion II. Typical Reactions A. Electrophilic Aromatic Halogenation 1. E + is X + 2. Reaction conditions: AlX 3 + X 2 B. Electrophilic Aromatic Nitration 1. E + is N 2 + 2. Reaction conditions: H 2 S 4 + HN 3 C. Electrophilic Aromatic Sulfonation 1. E + is S 3 2. Reaction conditions: H 2 S 4 3. Reaction is very reversible D. Electrophilic Aromatic Alkylation: Friedel-Crafts Alkylation 1. E + is a carbocation 2. Reaction conditions: a. AlX 3 + RX b. alkene + H 2 S 4 c. RH + anhydrous HF 3. Carbocations that can rearrange will reagrrange E. Electrophilic Aromatic Acylation: Friedel-Crafts Acylation 1. E + is an acylium cation, R C/ + 2. Reaction conditions: a. RCCl + AlX 3 b. RC 2 CR + AlX 3 139
III. 3. Acylium cations do not rearrange 4. Friedel-Crafts Acylation + Reduction = Friedel-Crafts Alkylation without rearrangement a. Use i. Wolff-Kishner Reduction (cf. chapter 19.9): H 2 NNH 2, KH, ii. Clemmenson Reduction : Zn(Hg), HCl, Rate and rientation A. E vs RXN coordinate diagram: E H E + E a G G + E + E + H + G reaction B. Rate 1. When group G causes rate of reaction to be faster than benzene (E a < E a of 140
benzene), G is said to be activating a. All electron donating G are activating 2. When group G causes rate of reaction to be slower than benzene (E a > E a of benzene), G is said to be deactivating a. All electron withdrawing G are deactivating C. rientation 1. When group G causes ortho and para products to predominate, G is said to be an o,p-director a. All G that are o,p-directors stabilize the arenium ion intermediate that results from o- or p-attack of E + to a greater extent than they stabilize the arenium ion intermediate that results from m-attack of E + b. If G is large, steric hinderance may cause percent of o-products to diminish 2. When group G causes meta products to predominate, G is said to be a m- director a. All G that are m-directors cause the arenium ion intermediate that results from m-attack of E + to be more stable than those arenium ion intermediates that result from o- or p-attack of E + D. Multiple Substituent Effects 1. Effects of substiuents are additive 2. If there are conflicting effects of directing groups, a. o,p is always preferred over m b. The more activating o,p director is always preferred 3. Steric effects many block one or more competing substitution positions IV. Nucleophilic Aromatic Substitution A. ortho- and para-substituted haloarenes undergo substitution of the halogen by good nucleophiles when the substituent is electron-withdrawing V. Benzyne A. Forms from haloarenes in presence of strong bases by elimination of HX 1. Useful for syntheses of phenols and anilines 2. Excellent dienophilesin Diels-Ader reactions VI. Reactions of the side chain A. Reduction of benzylic carbonyl groups by hydrogenation 1. Another example of Friedel-Crafts Acylation + Reduction = Friedel-Crafts Alkylation without rearrangement 2. nly for benzylic carbonyls: others do not work B. xidation of alkyl benzenes 1. Aqueous KMn 4 oxidizes alkylbenzenes to benzoic acids 2. So does chromic acid, H 2 Cr 4 a. In ether case, alkyl group must have at least one benzylic H for oxidation to proceed C. Halogenation of the benzylic position 1. NBS and NCS cause benzylic radical halogenation 141
1. Activated aromatic compounds, such as toluene, can undergo electrophilic aromatic chlorination using hypochlorous acid, HCl. The reaction is catalyzed by a proton source, such as HCl or H 2 S 4. Provide a clear, concise mechanism for the electrophilic aromatic chlorination of toluene using these reaction conditions. 2. Write the structure(s) of the major product(s) expected in each of the following reactions. If no reaction is expected, write "NR": a. HBr dark, 20 EC CH 3 Cl b. Cl 2 H AlCl 3 142
c. H H 2 S 4 d. AlCl 3 e. CH 3 1 : 1 H 2 S 4 : HN 3 2 N f. para-isopropylaniline AlCl 3 g. CN + HF 0 EC 143
3. Write a synthetic route for a reasonable laboratory preparation of the following compounds, starting with the reactant shown and any other needed organic or inorganic reagents: a. 4-bromo-2-nitrobenzoic acid N 2 b. Br 144
c. H d. Benzene )))) m-dipropylbenzene 145
4. Dinitroaromatic compound A (C 14 H 12 N 2 4 ) is oxidized by hot aqueous basic KMn 4 ; aqueous acid workup gives two moles of p-nitrobenzoic acid per mole of starting material A. Reaction of A with Br 2 at 127EC gives only one monobromo derivative, B, and reaction of A with Br 2 /FeBr 3 gives only one monobromo derivative, C. Given that B and C are not the same compound, provide correct structures for A, B, and C. Be sure to show how you arrived at your answers. 5. Provide short answers to each of the following questions: a. Draw the isomer of dibromobenzene that produces only one tribromobenzene product upon bromination with Br 2 /FeBr 3. 146
b. Circle the more stable arenium ion: Cl H N 2 N 2 HN 2 + + 6. Provide a reasonable, clear, complete mechanism for the following transformation, and answer the questions following it: a. Provide a mechanism for the reaction: CH 3 + CH 2 =C(CH 3 ) 2 HF CH 3 b. If the HF in the above reaction were replaced by AlBr 3, would the reaction still proceed as written? Briefly explain the chemical reasoning behind your answer. 147
7. Provide a clear and reasonable mechanism which accounts for the observation that t- butylarenes often are cleaved under electrophilic aromatic substitution conditions, providing unanticipated products: + Cl 2 AlCl 3 Cl + CH 2 =C(CH 3 ) 2 148
8. Functional groups other than hydrogen can be displaced by electrophilic aromatic substitution reactions. For example, the (CH 3 ) 3 Si group of the substrate below is easily substituted for an H using HF; the rate of the reaction is quite rapid. n the other hand, the carbon analog is inert under identical reaction conditions: H 3 C CH 3 Si CH 3 + HF K > 100 H + H 3 C CH 3 Si H 3 C F C H 3 CH 3 CH 3 + HF K < 0.1 H + H 3 C CH 3 C H 3 F a. Provide a clear, detailed, reasonable mechanism for the first substitution reaction. b. Explain why the carbon analog is unreactive. (The bond strength table below may be useful.) Bond Strength, kcal/mol C Si 108 Si F 132 C C 145 C F 108 C H 110 H F 136 149
9. A not-so-bright organic chemistry professor wanted to prepare p-(bromomethyl)nitrobenzene from benzene. He performed the following synthetic sequence: Br Br CH 3 I AlCl 3 AlCl 3 H 2 S 4 Br 2 HN 3 N 2 When he checked the product he obtained by 1 H NMR, he got the following spectrum: δ 8.40 (s, 1H) 8.07 (d, J=9 Hz, 1H) 7.43 (d, J=9 Hz, 1H) 2.56 (s, 3H) He decided he d made an error in planning his synthesis. (a) What is the structure of the product the professor actually made? (b) Using a sequence of chemical transformations you have learned in your study of organic chemistry, provide a synthesis that will produce p-(bromomethyl)nitrobenzene from benzene. 150
10. Write the structure(s) of the major product(s) expected in each of the following reactions. If no reaction is expected, write "NR": a. Ethylbenzene Cl 2 b. + Br 2 CCl 4, dark c. NBS hν, CCl 4 d. H 2 Cr 4, H 2 N 2 e. Br H 2 Cr 4 H 2, ) f. Cl NaH H 2, 23 EC 151