Chapter 17 Reactions of Aromatic Compounds
Electrophilic Aromatic Substitution Although benzene s pi electrons are in a stable aromatic system, they are available to attack a strong electrophile to give a carbocation. This resonance-stabilized carbocation is called a sigma complex because the electrophile is joined to the benzene ring by a new sigma bond. Aromaticity is regained by loss of a proton. Chapter 17 2
Mechanism of Electrophilic Aromatic Substitution Chapter 17 3
Bromination of Benzene Chapter 17 4
Mechanism for the Bromination of Benzene: Step 1 + - B r B r F e B r 3 B r B r F e B r 3 (stronger electrophile than Br 2 ) Before the electrophilic aromatic substitution can take place, the electrophile must be activated. A strong Lewis acid catalyst, such as FeBr 3, should be used. Chapter 17 5
Mechanism for the Bromination of Benzene: Steps 2 and 3 Step 2: Electrophilic attack and formation of the sigma complex. Step 3: Loss of a proton to give the products. Chapter 17 6
Energy Diagram for Bromination Chapter 17 7
Chlorination and Iodination Chlorination is similar to bromination. AlCl 3 is most often used as catalyst, but FeCl 3 will also work. Iodination requires an acidic oxidizing agent, like nitric acid, to produce iodide cation. H + + HNO 3 + ½ I 2 I + + NO 2 + H 2 O Chapter 17 8
Nitration of Benzene Sulfuric acid acts as a catalyst, allowing the reaction to be faster and at lower temperatures. HNO 3 and H 2 SO 4 react together to form the electrophile of the reaction: nitronium ion (NO 2 + ). Chapter 17 9
Mechanism for the Nitration of Benzene Chapter 17 10
Reduction of the Nitro Group Treatment with zinc, tin, or iron in dilute acid will reduce the nitro to an amino group. This is the best method for adding an amino group to the ring. Chapter 17 11
Sulfonation of Benzene Sulfur trioxide (SO 3 ) is the electrophile in the reaction. A 7% mixture of SO 3 and H 2 SO 4 is commonly referred to as fuming sulfuric acid. The SO 3 H groups is called a sulfonic acid. Chapter 17 12
Mechanism of Sulfonation Benzene attacks sulfur trioxide, forming a sigma complex. Loss of a proton on the tetrahedral carbon and reprotonation of oxygen gives benzenesulfonic acid. Chapter 17 13
Nitration of Toluene Toluene reacts 25 times faster than benzene. The methyl group is an activator. The product mix contains mostly ortho and para substituted molecules. Chapter 17 14
Ortho and Para Substitution Ortho and para attacks are preferred because their resonance structures include one tertiary carbocation. Chapter 17 15
Energy Diagram Chapter 17 16
Meta Substitution When substitution occurs at the meta position, the positive charge is not delocalized onto the tertiary carbon, and the methyl groups has a smaller effect on the stability of the sigma complex. Chapter 17 17
Alkyl Group Stabilization Alkyl groups are activating substituents and ortho, para-directors. This effect is called the inductive effect because alkyl groups can donate electron density to the ring through the sigma bond, making them more active. Chapter 17 18
Substituents with Nonbonding Electrons Resonance stabilization is provided by a pi bond between the OCH 3 substituent and the ring. Chapter 17 19
Meta Attack on Anisole Resonance forms show that the methoxy group cannot stabilize the sigma complex in the meta substitution. Chapter 17 20
Bromination of Anisole A methoxy group is so strongly activating that anisole is quickly tribrominated without a catalyst. Chapter 17 21
Summary of Activators Chapter 17 22
Activators and Deactivators If the substituent on the ring is electron donating, the ortho and para positions will be activated. If the group is electron withdrawing, the ortho and para positions will be deactivated. Chapter 17 23
Nitration of Nitrobenzene Electrophilic substitution reactions for nitrobenzene are 100,000 times slower than for benzene. The product mix contains mostly the meta isomer, only small amounts of the ortho and para isomers. Chapter 17 24
Ortho Substitution on Nitrobenzene The nitro group is a strongly deactivating group when considering its resonance forms. The nitrogen always has a formal positive charge. Ortho or para addition will create an especially unstable intermediate. Chapter 17 25
Meta Substitution on Nitrobenzene Meta substitution will not put the positive charge on the same carbon that bears the nitro group. Chapter 17 26
Energy Diagram Chapter 17 27
Other Deactivators Chapter 17 28
Nitration of Chlorobenzene When chlorobenzene is nitrated the main substitution products are ortho and para. The meta substitution product is only obtained in 1% yield. Chapter 17 29
Halogens Are Deactivators Inductive Effect: Halogens are deactivating because they are electronegative and can withdraw electron density from the ring along the sigma bond. Chapter 17 30
Halogens Are Ortho, Para- Directors Resonance Effect: The lone pairs on the halogen can be used to stabilize the sigma complex by resonance. Chapter 17 31
Energy Diagram Chapter 17 32
Summary of Directing Effects Chapter 17 33
Friedel Crafts Alkylation Synthesis of alkyl benzenes from alkyl halides and a Lewis acid, usually AlCl 3. Reactions of alkyl halide with Lewis acid produces a carbocation, which is the electrophile. Chapter 17 34
Mechanism of the Friedel Crafts Reaction Step 1 Step 2 Step 3 Chapter 17 35
Friedel Crafts Acylation Acyl chloride is used in place of alkyl chloride. The product is a phenyl ketone that is less reactive than benzene. Chapter 17 36
Mechanism of Acylation Step 1: Formation of the acylium ion. Step 2: Electrophilic attack to form the sigma complex. Chapter 17 37
Nucleophilic Aromatic Substitution A nucleophile replaces a leaving group on the aromatic ring. This is an addition elimination reaction. Electron-withdrawing substituents activate the ring for nucleophilic substitution. Chapter 17 38
Mechanism of Nucleophilic Aromatic Substitution Step 1: Attack by hydroxide gives a resonance-stabilized complex. Step 2: Loss of chloride gives the product. Step 3: Excess base deprotonates the product. Chapter 17 39
Activated Positions Nitro groups ortho and para to the halogen stabilize the intermediate (and the transition state leading to it). Electron-withdrawing groups are essential for the reaction to occur. Chapter 17 40
S N 1 Reactions Benzylic carbocations are resonancestabilized, easily formed. Benzyl halides undergo S N 1 reactions. C H 2 B r C H 3 C H 2 O H, h e a t C H 2 O C H 2 C H 3 Chapter 17 41
S N 2 Reactions Benzylic halides are 100 times more reactive than primary halides via S N 2. The transition state is stabilized by a ring. Chapter 17 42