Reactions of Benzene and its Derivatives Chapter 22 Chapter 22

Reactions of Benzene The most characteristic reaction of aromatic compounds is substitution at a ring carbon. This is Electrophilic Aromatic Substitution (EAS).

Reactions of Benzene + Benzenesulfonic acid Sulfonation: H S O 3 2 4 An alkylbenzene Alkylation: R X A l Acylation: An acylbenzene C

22.1 Electrophilic Aromatic Substitution Electrophilic aromatic substitution (EAS): a reaction in which a hydrogen atom of an aromatic ring is replaced by an electrophile. To study several common types of electrophiles. how each is generated. the mechanism by which each replaces hydrogen.

A. Chlorination of Benzene Step 1: formation of a chloronium ion. Step 2: attack of the chloronium ion on the ring.

Chlorination Step 3: proton transfer regenerates the aromatic character of the ring.

EAS: General Mechanism A general mechanism: General question: what is the electrophile and how is it generated ?

Bromination of Benzene Figure 22.1: Energy diagram for the bromination of benzene.

B. Formation of the Nitronium Ion Generation of the nitronium ion, NO2+ : Step 1: proton transfer to nitric acid. Step 2: loss of H2O gives the nitronium ion, a very strong electrophile.

Nitration of Benzene Step 1: attack of the nitronium ion (an electrophile) on the aromatic ring (a nucleophile). Step 2: proton transfer regenerates the aromatic ring.

Reduction of the Nitro Group A particular value of nitration is that the nitro group can be reduced to a 1° amino group. Reduction occurs with other reagents such as an active metal (Fe, Sn or Zn) in HCl.

Sulfonation of Benzene Carried out using concentrated sulfuric acid containing dissolved sulfur trioxide. Concentrated sulfuric acid containing dissolved sulfur trioxide is fuming sulfuric acid. The sulfonation reaction is reversible whereas the halogenation and nitration reactions are not. H 2 S O 4 + S O 3 S O 3 H Benzene B enzenesulfonic acid

C. Friedel-Crafts Alkylation of Benzene Friedel-Crafts alkylation forms a new C-C bond between an aromatic ring and an alkyl group.

Friedel-Crafts Alkylation Step 1: formation of an alkyl cation as an ion pair. Step 2: attack of the alkyl cation on the ring. Step 3: proton transfer regenerates aromaticity.

Limitations on Friedel-Crafts Alkylation There are three major limitations on Friedel-Crafts alkylations. 1. carbocation rearrangements are common. A l C 3 + C l + H C l B enzene Isobutyl chloride tert- Butylbenzene C H 3 C H 3 C H 3 + - C H 3 2 - l + A l C 3 C H 3 - 2 l A C H 3 + A l 4 - H C H 3 I s o b u t y l c h r i d e a m o l e c u r p x a n i o p r

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

Limitations on Friedel-Crafts Alkylation 3. Polyalkylation: An alkyl group added to the ring activates the ring and further alkylation occurs. Limitations 1 & 3 do not apply to Friedel-Crafts Acylation reactions. CH3 A l C 3 x + CH3 C l CH3 HCl + CH3 B e n z

Friedel-Crafts Acylation of Benzene Friedel-Crafts acylation forms a new C-C bond between a benzene ring and an acyl group.

Friedel-Crafts Acylation The electrophile is an acylium ion. R - C l O A + 4 Aluminum chloride An acyl A molecular complex with a positive charge charge on chlorine n ion pair containing an acylium ion •• (1) (2)

Friedel-Crafts Acylation an acylium ion is a resonance hybrid of two major contributing structures. F-C acylations are free of two major limitation of F-C alkylations; acylium ions do not rearrange nor do they polyacylate. complete valence shells + + R - C O : R - C O : : The more important contributing structure

Friedel-Crafts Acylation A special value of F-C acylations is preparation of unrearranged alkylbenzenes. Wolff-Kishner reduction, pg 623

D. Other Aromatic Alkylations Carbocations are also generated from alkenes and alcohols: by treatment of an alkene with a protic acid, most commonly H2SO4, H3PO4, or HF/BF3,

Other Aromatic Alkylations by treating an alkene with a Lewis acid, and by treating an alcohol with H2SO4 or H3PO4. A l C 3 + Benzene Cyclohexene Phenylcyclohexane + Benzene H 3 P O 4 2 2-Methyl-2-propanol ( tert- Butyl alcohol) 2-Methyl-2- phenylpropane Butylbenzene

Di- and Polysubstitution of Benzene 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. 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.

Di- and Polysubstitution -OCH3 is ortho-para directing. -CO2H is meta directing.

Di- and Polysubstitution, Table 22.2

Di- and Polysubstitution From the information in Table 21.1, we can make these generalizations: alkyl, phenyl, and all other 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. all ortho-para directing groups except the halogens are activating toward further substitution; the halogens are weakly deactivating.

22.2 A. Di- and Polysubstitution, Table 22.1 Orientation on nitration of monosubstituted benzenes.

Di- and Polysubstitution the sequence of reactions is important.

B. 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.

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.

Theory of Directing Effects -OCH3 : events during an unfavored meta attack. Only three resonance structures and the cation never appears on oxygen.

Theory of Directing Effects -OCH3 : events during a favored ortho-para attack. Four resonance structures here and the cation does appear on oxygen.

Theory of Directing Effects -CO2H : events during a favored meta attack. The cation never appears adjacent to the (+) carbon of C=O.

Theory of Directing Effects -CO2H : events during an unfavored ortho-para attack. The cation appears adjacent to a (+) carbon of C=O.

C. Activating-Deactivating Effects Any resonance effect, such as that of -NH2, -OH, and -OR, that delocalizes the positive charge on the cation intermediate lowers the activation energy for its formation, and has an activating effect toward further EAS. Any resonance or inductive effect, such as that of -NO2, -CN, -CO, and -SO3H, that decreases electron density on the ring deactivates the ring toward further EAS.

Activating-Deactivating Any inductive effect, such as that of -CH3 or other alkyl group, that releases electron density toward the ring activates the ring toward further EAS. Any inductive effect, such as that of halogen, -NR3+, -CCl3, or -CF3, that decreases electron density on the ring deactivates the ring toward further EAS.

Activating-Deactivating for the halogens, the inductive and resonance effects run counter to each other, but the former is somewhat stronger with respect to deactivation. the net effect is that halogens are deactivating but ortho-para directing.

Relative rates of EAS Relative rates of reaction for substituted benzenes compared to unsubstituted benzene. rel. rate Aniline 106 strongly activating NH2 Toluene 25 weakly activating CH3 Benzene 1 neutral Chlorobenzene 0.03 weakly deactivating Cl Nitrobenzene 10-6 strongly deactivating NO2

22.3 Nucleophilic Aromatic Substitution Aryl halides do not undergo nucleophilic aromatic substitution (NAS) by either SN1 or SN2. They do undergo nucleophilic substitutions, but by mechanisms quite different from those of nucleophilic aliphatic substitution. There are two common mechanisms: The benzyne mechanism. The addition-elimination mechanism. Nucleophilic aromatic substitutions are far less common than electrophilic aromatic substitutions.

A. Benzyne Intermediates When heated under pressure with aqueous NaOH, chlorobenzene is converted to sodium phenoxide. neutralization with HCl gives phenol.

Benzyne Intermediates the same reaction with 2-chlorotoluene gives a mixture of ortho- and meta-cresol. the same type of reaction can be brought about using of sodium amide in liquid ammonia.

Benzyne Intermediates -elimination of HX gives a benzyne intermediate, that then adds the nucleophile to give products. Benzyne is unstable due to poor orbital overlap, brackets mean that this is a transient intermediate.

B. Addition-Elimination when an aryl halide contains electron-withdrawing NO2 groups ortho and/or para to X, nucleophilic aromatic substitution takes place more readily. neutralization with HCl gives the phenol.

Meisenheimer Complex reaction involves a Meisenheimer complex intermediate.

Benzene and its Derivatives Reaction of Benzene and its Derivatives End Chapter 22