Chapter 17 Reactions of Aromatic Compounds

Chapter 17 2

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 3

Mechanism of Electrophilic Aromatic Substitution

Chapter 17 4

Bromination of Benzene

Chapter 17 5

Mechanism for the Bromination of Benzene: Step 1

  Before the electrophilic aromatic substitution can take place, the electrophile must be activated.

  A strong Lewis acid catalyst, such as FeBr3, should be used.

B r B r F e B r 3 B r B r F e B r 3 + -

(stronger electrophile than Br2)

Chapter 17 6

Step 2: Electrophilic attack and formation of the sigma complex.

Step 3: Loss of a proton to give the products.

Mechanism for the Bromination of Benzene: Steps 2 and 3

Chapter 17 7

Energy Diagram for Bromination

Chapter 17 8

Chlorination and Iodination

  Chlorination is similar to bromination. AlCl3 is most often used as catalyst, but FeCl3 will also work.

  Iodination requires an acidic oxidizing agent, like nitric acid, to produce iodide cation.

H+ + HNO3 + ½ I2 I+ + NO2 + H2O

Chapter 17 9

Nitration of Benzene

  Sulfuric acid acts as a catalyst, allowing the reaction to be faster and at lower temperatures.

  HNO3 and H2SO4 react together to form the electrophile of the reaction: nitronium ion (NO2+).

Chapter 17 10

Mechanism for the Nitration of Benzene

Chapter 17 11

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 12

Sulfonation of Benzene

  Sulfur trioxide (SO3) is the electrophile in the reaction.   A 7% mixture of SO3 and H2SO4 is commonly

referred to as “fuming sulfuric acid”.   The —SO3H groups is called a sulfonic acid.

Chapter 17 13

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 14

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 15

Ortho and Para Substitution

  Ortho and para attacks are preferred because their resonance structures include one tertiary carbocation.

Chapter 17 16

Energy Diagram

Chapter 17 17

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 18

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 19

Substituents with Nonbonding Electrons

Resonance stabilization is provided by a pi bond between the —OCH3 substituent and the ring.

Chapter 17 20

Meta Attack on Anisole

  Resonance forms show that the methoxy group cannot stabilize the sigma complex in the meta substitution.

Chapter 17 21

Bromination of Anisole

  A methoxy group is so strongly activating that anisole is quickly tribrominated without a catalyst.

Chapter 17 22

Summary of Activators

Chapter 17 23

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 24

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 25

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 26

Meta Substitution on Nitrobenzene

  Meta substitution will not put the positive charge on the same carbon that bears the nitro group.

Chapter 17 27

Energy Diagram

Chapter 17 28

Other Deactivators

Chapter 17 29

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 30

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 31

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 32

Energy Diagram

Chapter 17 33

Summary of Directing Effects

Chapter 17 34

Friedel–Crafts Alkylation

  Synthesis of alkyl benzenes from alkyl halides and a Lewis acid, usually AlCl3.

  Reactions of alkyl halide with Lewis acid produces a carbocation, which is the electrophile.

Chapter 17 35

Mechanism of the Friedel–Crafts Reaction

Step 1

Step 2

Step 3

Chapter 17 36

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 37

Mechanism of Acylation Step 1: Formation of the acylium ion.

Step 2: Electrophilic attack to form the sigma complex.

Chapter 17 38

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 39

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 40

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 41

SN1 Reactions

  Benzylic carbocations are resonance-stabilized, easily formed.

  Benzyl halides undergo SN1 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 42

SN2 Reactions

  Benzylic halides are 100 times more reactive than primary halides via SN2.

  The transition state is stabilized by a ring.