Chapter 22 Reaction of Benzene and its Derivatives

Chapter 22 Reaction of Benzene and its Derivatives 22 -1

Reactions of Benzene u Substitution at a ring carbon. Contrast to radical mechanism for benzylic hyrdogens 22 -2

Reactions of Benzene Friedel Crafts 22 -3

Electrophilic Aromatic Substitution u Electrophilic u We aromatic substitution: study several common electrophiles • how each is generated. • the mechanism by which each replaces hydrogen. 22 -4

EAS: General Mechanism u. A general mechanism u General question: What are the electrophiles and how are they generated? Look at particular reactions. 22 -5

Chlorination Step 1: Generation of the electrophile: a chloronium ion. Step 2: Attack of the chloronium ion on the ring. 22 -6

Chlorination Step 3: Proton ejection regenerates the aromatic character of the ring. 22 -7

Addition vs Substitution u Energy diagram for the bromination of benzene. 22 -8

Nitration (Nitric and Sulfuric Acids) u Generation of the nitronium ion, NO 2+ • Step 1: Proton transfer to nitric acid. • Step 2: Loss of H 2 O gives the nitronium ion, a very strong electrophile. Dehydrated nitric acid. 22 -9

Nitration, Attack of electrophile as before…. . Step 1: Attack of the nitronium ion) on the aromatic ring. Step 2: Proton transfer regenerates the aromatic ring. 22 -10

Synthesis, Nitro Amines u The nitro group can be reduced to a 1° amino group. Notice the carboxylic was untouched. 22 -11

Sulfonation u Carried out using concentrated sulfuric acid containing dissolved sulfur trioxide. 22 -12

Friedel-Crafts Alkylation u Friedel-Crafts alkylation forms a new C-C bond between an aromatic ring and an alkyl group. 22 -13

Friedel-Crafts Alkylation Step 1: Formation of an alkyl cation as an ion pair. Step 2: Attack of the alkyl cation. Step 3: Proton transfer regenerates the aromatic ring. 22 -14

Friedel-Crafts Alkylation u There are four major limitations on Friedel-Crafts alkylations: 1. Carbocation rearrangements are common 22 -15

Friedel-Crafts Alkylation 2. F-C alkylation fails on benzene rings bearing one or more of these strongly electron-withdrawing groups. 22 -16

Friedel-Crafts Alkylation 3. F-C multiple alkylation can occur more rapidly than monoalkylation. The first alkyl group activates the ring to the second substitution. 4. The steps in the Friedel Crafts Alkylation are reversible and rearrangments may occur. 22 -17

Friedel-Crafts Acylation u Friedel-Crafts acylation forms a new C-C bond between a benzene ring and an acyl group. 22 -18

Friedel-Crafts Acylation u The electrophile is an acylium ion. 22 -19

Friedel-Crafts Acylation • An acylium ion is represented as a resonance hybrid of two major contributing structures. u Friedel-Crafts acylations are free of major limitation of Friedel-Crafts alkylations; acylium ions do not rearrange, do not polyacylate (why? ), do not rearrange. 22 -20

Synthesis, Friedel-Crafts Acylation u preparation of unrearranged alkylbenzenes. What else could be used here? 22 -21

Other Aromatic Alkylations u Carbocations are generated by • treatment of an alkene with a proton acid, most commonly H 2 SO 4, H 3 PO 4, or HF/BF 3. • treating an alkene with a Lewis acid. 22 -22

Other Aromatic Alkylations • and by treating an alcohol with H 2 SO 4 or H 3 PO 4. 22 -23

Di- and Polysubstitution u Orientation on nitration of monosubstituted benzenes. Favor ortho/para substitution Favor meta substitution 22 -24

Directivity of substituents 22 -25

Di- and Polysubstitution u Two ways to characterize the substituent • Orientation: • Some 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. • Rate • Some 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. 22 -26

Di- and Polysubstitution • -OCH 3 is ortho-para directing. • -COOH is meta directing. 22 -27

Di- and Polysubstitution Recall the polysubstitution in FC alkylation. 22 -28

Di- and Polysubstitution u Generalizations: • Directivity: Alkyl, phenyl, and all substituents in which the atom bonded to the ring has an unshared pair of electrons are ortho-para directing. All other substituents are meta directing. • Activation: All ortho-para directing groups except the halogens are activating toward further substitution. The halogens are weakly deactivating. 22 -29

Di- and Polysubstitution • The order of steps is important. 22 -30

Theory of Directing Effects u The rate of EAS is limited by the slowest step in the reaction. u For almost every EAS, the rate-determining step is attack of E+ on the aromatic ring to give a resonance-stabilized cation intermediate. u The more stable this cation intermediate, the faster the rate-determining step and the faster the overall reaction. 22 -31

Theory of Directing Effects u The orientation is controlled by the stability of the carbocation being formed by attack of the electrophile. u Products are formed under kinetic control. 22 -32

Theory of Directing Effects, ortho-para director. • -OCH 3: assume ortho-para attack. Here only para attack is shown. o, p director Very stable resonance structure. Why? 22 -33

Theory of Directing Effects , ortho-para director. • -OCH 3; look at meta attack. o, p director No corresponding resonance structure putting positive charge on oxygen. 22 -34

Theory of Directing Effects, meta director. • -CO 2 H : assume ortho-para attack. Meta director Disfavored because CO 2 H is electron withdrawing 22 -35

Theory of Directing Effects, meta director. • -CO 2 H; assume meta attack. Meta director 22 -36

Activating-Deactivating Resonance Effects u Any resonance effect, effect such as that of -NH 2, -OH, and -OR, that delocalizes the positive charge on the cation by has an activating effect toward further EAS. u Any resonance effect, effect such as that of -NO 2, -CN, C=O, and -SO 3 H, that decreases electron density on the ring deactivates the ring toward further EAS. 22 -37

Activating-Deactivating Inductive Effects u Any inductive effect, effect such as that of -CH 3 or other alkyl group, that releases electron density toward the ring activates the ring toward further EAS. u Any inductive effect, effect such as that of halogen, -NR 3+, -CCl 3, or -CF 3, that decreases electron density on the ring deactivates the ring toward further EAS. 22 -38

Activating-Deactivating: Halogens • For the halogens, the inductive and resonance effects oppose each other. Inductive is somewhat stronger. • Result: halogens are deactivating but ortho-para directing. 22 -39

Nucleophilic Aromatic Substitution u Aryl halides do not undergo nucleophilic substitution by either SN 1 or SN 2 pathways. u They do undergo nucleophilic substitutions, but by two mechanisms. • Benzyne using strong base. • Addition/elimination typically with nitro activating groups. 22 -40

Benzyne Intermediates u When heated under pressure with aqueous Na. OH, chlorobenzene is converted to sodium phenoxide. • Neutralization with HCl gives phenol. 22 -41

Benzyne Intermediates (strong base) • The same reaction with 2 -chlorotoluene gives a mixture of ortho- and meta-cresol. • The same type of reaction can be brought about using sodium amide in liquid ammonia. 22 -42

Benzyne Intermediates • -elimination of HX gives a benzyne intermediate, that then adds the nucleophile to give products. 22 -43

Benzyne Intermediates But wait, do we believe this crazy idea? We need some evidence…. A B 22 -44

Benzyne Intermediates The deuterated fluoride below exchanges the D with solvent ammonia although the deuterated bromide does not. This indicates a relatively rapid exchange process for the fluoro compound. C next 22 -45

Benzyne Intermediates explanation 22 -46

Orientation u The methyl group is essentially just a marker to allow the observation of the mixture of products. u Consider the methoxy group, -OCH 3, stabilizing of positive charge via resonance but also inductively withdrawing. u The methoxy group is not in resonance with the negative charge of the anion, Inductive Effect dominates. Next slide. 22 -47

Benzyne Intermediates D Get same product Explation next 22 -48

Benzyne Intermediates explanation 22 -49

Addition-Elimination (nitro groups) • When an aryl halide contains electron-withdrawing NO 2 groups ortho and/or para to X, nucleophilic aromatic substitution takes place readily. • Neutralization with HCl gives the phenol. 22 -50

Meisenheimer Complex • Reaction involves formation of reactive intermediate called a Meisenheimer complex. Similar to nucleophilic subsititution on carboxylic acid derivatives. 22 -51