bromination of benzene
DESCRIPTION
Bromination of Benzene. Mechanism for the Bromination of Benzene: Preliminary Step. 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. - PowerPoint PPT PresentationTRANSCRIPT
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Bromination of Benzene
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Mechanism for the Bromination of Benzene: Preliminary Step
• Before the electrophilic aromatic substitution can take place, the electrophile must be activated.
• A strong Lewis acid catalyst, such as FeBr3, should be used.
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Step 1: Electrophilic attack and formation of the sigma complex.
Step 2: Loss of a proton to give the products.
Mechanism for the Bromination of Benzene: Steps 1 and 2
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Chlorination of Benzene
• Chlorination is similar to bromination. AlCl3 is most often used as catalyst, but FeCl3 will also work.
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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
+).
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Mechanism for the Nitration of Benzene: Preliminary Step
• Formation of the nitronium ion is the preliminary step of the reaction.
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Mechanism for the EAS Nitration of Benzene
Step 1: Formation of the sigma complex.
Step 2: Loss of a proton gives nitrobenzene.
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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.
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Mechanism of the Friedel–Crafts Reaction
Step 1
Step 2
Step 3
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Rearrangements
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Protonation of Alkenes
• An alkene can be protonated by HF. • This weak acid is preferred because the fluoride
ion is a weak nucleophile and will not attack the carbocation.
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Alcohols and Lewis Acids
• Alcohols can be treated with BF3 to form the carbocation.
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Limitations of Friedel–Crafts
• Reaction fails if benzene has a substituent that is more deactivating than halogens.
• Rearrangements are possible.• The alkylbenzene product is more reactive
than benzene, so polyalkylation occurs.
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Friedel–Crafts Acylation
• Acyl chloride is used in place of alkyl chloride.• The product is a phenyl ketone that is less reactive
than benzene.
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Mechanism of Acylation
Step 1: Formation of the acylium ion.
Step 2: Electrophilic attack to form the sigma complex.
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Mechanism of Acylation (Continued)
Step 3: Loss of a proton to form the product.
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The Gattermann-Koch Reaction
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HINT
Friedel–Crafts acylations are generally free from rearrangements and multiple substitution. They do not go on strongly
deactivated rings, however.
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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 group is called a sulfonic acid.
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Sulfur Trioxide
• Sulfur trioxide is a strong electrophile, with three sulfonyl bonds drawing electron density away from the sulfur atom.
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Desulfonation Reaction
• Sulfonation is reversible. • The sulfonic acid group may be removed from an
aromatic ring by heating in dilute sulfuric acid.
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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.
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Ortho and Para Substitution
• Ortho and para attacks are preferred because their resonance structures include one tertiary carbocation.
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Energy Diagram
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Meta Substitution
• When substitution occurs at the meta position, the positive charge is not delocalized onto the tertiary carbon, and the methyl group has a smaller effect on the stability of the sigma complex.
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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.
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Anisole
• Anisole undergoes nitration about 10,000 times faster than benzene and about 400 times faster than toluene.
• This result seems curious because oxygen is a strongly electronegative group, yet it donates electron density to stabilize the transition state and the sigma complex.
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Substituents with Nonbonding Electrons
Resonance stabilization is provided by a pi bond between the —OCH3 substituent and the ring.
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Meta Attack on Anisole
• Resonance forms show that the methoxy group cannot stabilize the sigma complex in the meta substitution.
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Bromination of Anisole
• A methoxy group is so strongly activating that anisole is quickly tribrominated without a catalyst.
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Summary of Activators
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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.
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Nitration of Nitrobenzene
• Electrophilic substitution reactions for nitrobenzene are 100,000 times slower than for benzene.
• The product mix contains mostly the meta isomer, and only small amounts of the ortho and para isomers.
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Ortho Substitution of 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.
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Meta Substitution on Nitrobenzene
• Meta substitution will not put the positive charge on the same carbon that bears the nitro group.
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Energy Diagram
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Deactivators and Meta-Directors
• Most electron-withdrawing groups are deactivators and meta-directors.
• The atom attached to the aromatic ring has a positive or partial positive charge.
• Electron density is withdrawn inductively along the sigma bond, so the ring has less electron density than benzene, and thus it will be slower to react.
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Other Deactivators
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Halogens
• Halogens are deactivators since they react slower than benzene.
• Halogens are ortho, para-directors because the halogen can stabilize the sigma complex.
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Halogens Are Deactivators
• Inductive effect: Halogens are deactivating because they are electronegative and can withdraw electron density from the ring along the sigma bond.
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Halogens Are Ortho, Para-Directors
• Resonance effect: The lone pairs on the halogen can be used to stabilize the sigma complex by resonance.
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Energy Diagram
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Summary of Directing Effects
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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.
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Clemmensen Reduction
• The Clemmensen reduction is a way to convert acylbenzenes to alkylbenzenes by treatment with aqueous HCl and amalgamated zinc.
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Wolff–Kishner Reduction
• Forms hydrazone, then heat with strong base like KOH or potassium tert-butoxide
• Use a high-boiling solvent: ethylene glycol, diethylene glycol, or DMSO.
• A molecule of nitrogen is lost in the last steps of the reaction.
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Side-Chain Oxidation
• Alkylbenzenes are oxidized to benzoic acid by heating in basic KMnO4 or heating in Na2Cr2O7/H2SO4.
• The benzylic carbon will be oxidized to the carboxylic acid.
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Side-Chain Halogenation
• The benzylic position is the most reactive.• Br2 reacts only at the benzylic position.
• Cl2 is not as selective as bromination, so results in mixtures.