Chapter 17 reactions of aromatic compounds
Download
1 / 52

Chapter 17 - PowerPoint PPT Presentation


  • 439 Views
  • Updated On :

Organic Chemistry , 5 th Edition L. G. Wade, Jr. Chapter 17 Reactions of Aromatic Compounds. Jo Blackburn Richland College, Dallas, TX Dallas County Community College District ã 2003, Prentice Hall. Part 1 - Electrophilic Aromatic Substitution.

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'Chapter 17' - Patman


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
Chapter 17 reactions of aromatic compounds l.jpg

Organic Chemistry, 5th EditionL. G. Wade, Jr.

Chapter 17Reactions of Aromatic Compounds

Jo Blackburn

Richland College, Dallas, TX

Dallas County Community College District

ã 2003,Prentice Hall


Part 1 electrophilic aromatic substitution l.jpg
Part 1 - Electrophilic Aromatic Substitution

Electrophile substitutes for a hydrogen on the benzene ring.

Chapter 17


Mechanism l.jpg
Mechanism

Chapter 17


Slide4 l.jpg

E

+E

E

Energy Diagramfor aromatic electrophilic addition

Chapter 17


Slide5 l.jpg

Reactions

  • Nitration HNO3 and H2SO4

  • Bromination Br2 and FeBr3

  • Chlorination Cl2 and AlCl3

  • Iodination I2 and HNO3

  • Sulfonation SO3 (fuming sulfuric acid)

  • Alkylation R-Cl and AlCl3

  • Acylation RCOCl and AlCl3

Chapter 17


Nitration of benzene l.jpg
Nitration of Benzene

Use sulfuric acid with nitric acid to form the nitronium ion electrophile.

NO2+ then forms a

sigma complex with

benzene

Chapter 17


Nitration of toluene l.jpg
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 so methyl group is ortho, para directing.

Chapter 17


Sigma complex l.jpg
Sigma Complex

Intermediate is more stable if nitration occurs at the orthoor para position.

Chapter 17


Energy diagram l.jpg
Energy Diagram

Chapter 17


Activating o p directing substituents l.jpg
Activating, O-, P-Directing Substituents

  • Alkyl groups stabilize the sigma complex by induction, donating electron density through the sigma bond.

  • Other kinds of groups with a lone pair of electrons stabilize the sigma complex by resonance.

Chapter 17


The amino group l.jpg
The Amino Group

Aniline reacts with bromine water (without a catalyst) to yield the tribromide. Sodium bicarbonate is added to neutralize the HBr that’s also formed.

Chapter 17


Summary of activators l.jpg
Summary ofActivators

Chapter 17


Ortho substitution on nitrobenzene l.jpg
Ortho Substitutionon Nitrobenzene

Chapter 17


Para substitution on nitrobenzene l.jpg
Para Substitution on Nitrobenzene

Chapter 17


Meta substitution on nitrobenzene l.jpg
Meta Substitutionon Nitrobenzene

Chapter 17


Deactivating meta directing substituents l.jpg
Deactivating Meta-Directing Substituents

  • 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 orthoand para isomers.

  • Meta-directors deactivate all positions on the ring, but the meta position is less deactivated.

Chapter 17


Energy diagram17 l.jpg
Energy Diagram

Chapter 17


Structure of meta directing deactivators l.jpg
Structure of Meta-Directing Deactivators

  • The atom attached to the aromatic ring will have a partial positive charge.

  • Electron density is withdrawn inductively along the sigma bond, so the ring is less electron-rich than benzene.

Chapter 17



More deactivators l.jpg
More Deactivators

Chapter 17


Summary of directing effects l.jpg
Summary of Directing Effects

Chapter 17


Multiple substituents l.jpg
Multiple Substituents

The most strongly activating substituent will determine the position of the next substitution. May have mixtures.

Chapter 17


Bromination of benzene l.jpg
Bromination of Benzene

  • Requires a stronger electrophile than Br2.

  • Use a strong Lewis acid catalyst, FeBr3.

Chapter 17


Chlorination and iodination l.jpg
Chlorination and Iodination

  • Chlorination uses AlCl3 as the Lewis acid catalyst.

  • Iodination requires an acidic oxidizing agent, like nitric acid, which oxidizes the iodine to an iodonium ion.

Chapter 17


Halobenzenes l.jpg
Halobenzenes

  • Halogens are deactivating toward electrophilic substitution, but are ortho, para-directing!

  • Since halogens are very electronegative, they withdraw electron density from the ring inductively along the sigma bond.

  • But halogens have lone pairs of electrons that can stabilize the sigma complex by resonance.

Chapter 17


Sigma complex for bromobenzene l.jpg

Ortho and para attacks produce a bromonium ionand other resonance structures.

No bromonium ion possible with meta attack.

Sigma Complexfor Bromobenzene

Chapter 17


Energy diagram27 l.jpg
Energy Diagram

Chapter 17


Sulfonation l.jpg
Sulfonation

Sulfur trioxide, SO3, in fuming sulfuric acid is the electrophile.

Chapter 17


Desulfonation l.jpg

Benzene-d6

Desulfonation

  • All steps are reversible, so sulfonic acid group can be removed by heating in dilute sulfuric acid.

  • This process is used to place deuterium in place of hydrogen on benzene ring.

Chapter 17



Formation of alkyl benzene l.jpg

-

+

Formation of Alkyl Benzene

Chapter 17


Limitations of friedel crafts l.jpg
Limitations ofFriedel-Crafts

  • Reaction fails if benzene has a substituent that is more deactivating than halogen.

  • Carbocations rearrange. Reaction of benzene with n-propyl chloride and AlCl3 produces isopropylbenzene.

  • The alkylbenzene product is more reactive than benzene, so polyalkylation occurs.

Chapter 17



Gatterman koch formylation l.jpg
Gatterman-KochFormylation

  • Formyl chloride is unstable. Use a high pressure mixture of CO, HCl, and catalyst.

  • Product is benzaldehyde.

Chapter 17


Clemmensen reduction l.jpg
Clemmensen Reduction

Acylbenzenes can be converted to alkylbenzenes by treatment with aqueous HCl and amalgamated zinc.

Chapter 17


Part 2 nucleophilic aromatic substitution l.jpg
Part 2- NucleophilicAromatic Substitution

  • A nucleophile replaces a leaving group on the aromatic ring.

  • Electron-withdrawing substituents activate the ring for nucleophilic substitution.

Chapter 17


Examples of nucleophilic substitution l.jpg
Examples ofNucleophilic Substitution

Chapter 17


Addition elimination mechanism l.jpg
Addition-EliminationMechanism

Chapter 17


Benzyne mechanism l.jpg
Benzyne Mechanism

  • Reactant is halobenzene with no electron-withdrawing groups on the ring.

  • Use a very strong base like NaNH2.

Chapter 17



Part 3 addition reactions chlorination of benzene l.jpg
Part 3- Addition ReactionsChlorination of Benzene

  • Addition to the benzene ring may occur with high heat and pressure (or light).

  • The first Cl2 addition is difficult, but the next 2 moles add rapidly.

  • The product, benzene hexachloride, is an insecticide.

Chapter 17


Catalytic hydrogenation l.jpg
Catalytic Hydrogenation

  • Elevated heat and pressure is required.

  • Possible catalysts: Pt, Pd, Ni, Ru, Rh.

  • Reduction cannot be stopped at an intermediate stage.

Chapter 17


Part 4 side chain reactions birch reduction regiospecific l.jpg
Part 4 – Side Chain ReactionsBirch Reduction: Regiospecific

  • A carbon with an e--withdrawing group

  • is reduced.

  • A carbon with an e--releasing group

  • is not reduced.

Chapter 17


Birch mechanism l.jpg
Birch Mechanism

Chapter 17


Side chain oxidation l.jpg
Side-Chain Oxidation

Alkylbenzenes are oxidized to benzoic acid by hot KMnO4 or Na2Cr2O7/H2SO4.

Chapter 17


Side chain halogenation l.jpg
Side-Chain Halogenation

  • Benzylic position is the most reactive.

  • Chlorination is not as selective as bromination, results in mixtures.

  • Br2 reacts only at the benzylic position.

Chapter 17


S n 1 reactions l.jpg
SN1 Reactions

  • Benzylic carbocations are resonance-stabilized, easily formed.

  • Benzyl halides undergo SN1 reactions.

Chapter 17


S n 2 reactions l.jpg
SN2 Reactions

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

  • Transition state is stabilized by ring.

Chapter 17


Part 5 reactions of phenols l.jpg
Part 5 - Reactions of Phenols

  • Some reactions like aliphatic alcohols:

    • phenol + carboxylic acid  ester

    • phenol + aq. NaOH  phenoxide ion

  • Oxidation to quinones: 1,4-diketones.

Chapter 17


Quinones l.jpg
Quinones

  • Hydroquinone is used as a developer for film. It reacts with light-sensitized AgBr grains, converting it to black Ag.

  • Coenzyme Q is an oxidizing agent found in the mitochondria of cells.

Chapter 17


Electrophilic substitution of phenols l.jpg
ElectrophilicSubstitution of Phenols

  • Phenols and phenoxides are highly reactive.

  • Only a weak catalyst (HF) required for Friedel-Crafts reaction.

  • Tribromination occurs without catalyst.

  • Even reacts with CO2.

Chapter 17


End of chapter 17 l.jpg
End of Chapter 17

Chapter 17


ad