Week+7+Slides (2)
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Organic Chemistry II UC-Berkeley Extension x36B Spring 2012 March 12, 2012 Silence/Turn off your Cell Phones Week 7: Benzene and the Concept of Aromaticity: nomenclature, acidity of phenols, Alkyl-aryl ether synthesis, Kolbe carboxylation, benzylic oxidation and halogenation, benzyl ethers. Reactions of Benzene and its Derivatives: electrophilic aromatic substitution, chlorination, bromination, nitration, sulfonation, Friedel-Crafts alkylation, Friedel-Crafts acylation, acylium ion.
Transcript of Week+7+Slides (2)
Organic Chemistry IIUC-Berkeley Extension
x36BSpring 2012
March 12, 2012
Silence/Turn off your Cell Phones
Week 7: Benzene and the Concept of Aromaticity: nomenclature, acidity of phenols, Alkyl-aryl ether synthesis, Kolbe carboxylation, benzylic oxidation and halogenation, benzyl ethers.
Reactions of Benzene and its Derivatives: electrophilic aromatic substitution, chlorination, bromination, nitration, sulfonation, Friedel-Crafts alkylation, Friedel-Crafts acylation, acylium ion.
Week 7
Nomenclature
• Monosubstituted alkylbenzenes are named as derivatives of benzene.
• Many common names are retained.
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Nomenclature
• Benzyl and phenyl groups
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Disubstituted Benzenes
• Locate two groups by numbers or by the locators orthoortho (1,2-), metameta (1,3-), and parapara (1,4-).
• Where one group imparts a special name, name the compound as a derivative of that molecule.
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Disubstituted Benzenes
• Where neither group imparts a special name, locate the groups and list them in alphabetical order.
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Polysubstituted Derivatives
• If one group imparts a special name, name the molecule as a derivative of that compound.
• If no group imparts a special name, list them in alphabetical order, giving them the lowest set of numbers.
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Phenols
• The functional group of a phenol is an -OH group bonded to a benzene ring.
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Phenols
• Hexylresorcinol is a mild antiseptic and disinfectant.• Eugenol is used as a dental antiseptic and analgesic.• Urushiol is the main component of the oil of poison
ivy.
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Acidity of Phenols
• Phenols are significantly more acidic than alcohols, compounds that also contain the OH group.
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Acidity of Phenols
• The greater acidity of phenols compared with alcohols is due to the greater stability of the phenoxide ion relative to an alkoxide ion.
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Acidity of Phenols
• Alkyl and halogen substituents affect acidities by inductive effects:
• Alkyl groups are electron-releasing.• Halogens are electron-withdrawing.
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Acidity of Phenols
• Nitro groups increase the acidity of phenols by both an electron-withdrawing inductive effect and a resonance effect.
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Acidity of Phenols
• Part of the acid-strengthening effect of -NO2 is due to its electron-withdrawing inductive effect.
• In addition, -NO2 substituents in the ortho and para positions help to delocalize the negative charge.
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Acidity of Phenols
• Phenols are weak acids and react with strong bases to form water-soluble salts.
• Water-insoluble phenols dissolve in NaOH(aq).
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Acidity of Phenols
• Separation of water-insoluble phenols from water-insoluble alcohols.
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Alkyl-Aryl Ethers
• Alkyl-aryl ethers can be prepared by the Williamson ether synthesis:
• but only using phenoxide salts and haloalkanes.
• haloarenes cannot be used because they are unreactive to SN2 reactions.
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Alkyl-Aryl Ethers
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Kolbe Carboxylation
• Phenoxide ions react with carbon dioxide to give a carboxylate salt.
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Kolbe Carboxylation
• The mechanism begins by nucleophilic addition of the phenoxide ion to a carbonyl group of CO2.
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Benzylic Oxidation
• Benzene is unaffected by strong oxidizing agents such as H2CrO4 and KMnO4
• Halogen and nitro substituents are also unaffected by these reagents.
• An alkyl group with at least one hydrogen on its benzylic carbon is oxidized to a carboxyl group.
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Benzylic Chlorination
• Chlorination and bromination occur by a radical chain mechanism.
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Benzylic Reactions
• Benzylic radicals (and cations also) are easily formed because of the resonance stabilization of these intermediates.
• The benzyl radical is a hybrid of five contributing structures.
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Hydrogenolysis
• Hydrogenolysis:Hydrogenolysis: Cleavage of a single bond by H2
• Among ethers, benzylic ethers are unique in that they are cleaved under conditions of catalytic hydrogenation.
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Benzyl Ethers
• The value of benzyl ethers is as protecting groups for the OH groups of alcohols and phenols.
• To carry out hydroboration/oxidation of this alkene, the phenolic -OH must first be protected; it is acidic enough to react with BH3 and destroy the reagent.
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Reactions of Benzene
• The most characteristic reaction of aromatic compounds is substitution at a ring carbon.
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Reactions of Benzene
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Electrophilic Aromatic Substitution
• Electrophilic aromatic substitution:Electrophilic aromatic substitution: A reaction in which a hydrogen atom of an aromatic ring is replaced by an electrophile.
• We study several common electrophiles• how each is generated.• the mechanism by which each replaces hydrogen.
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Chlorination
Step 1: Formation of a chloronium ion.
Step 2: Attack of the chloronium ion on the ring.
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Chlorination
Step 3: Proton transfer regenerates the aromatic character of the ring.
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EAS: General Mechanism
• A general mechanism
• General question: What is the electrophile and how is it generated?
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Bromination
• Energy diagram for the bromination of benzene.
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Nitration
• 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.
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Nitration
Step 1: Attack of the nitronium ion (an electrophile) on the aromatic ring (a nucleophile).
Step 2: Proton transfer regenerates the aromatic ring.
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Nitration
• A particular value of nitration is that the nitro group can be reduced to a 1° amino group.
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Sulfonation
• Carried out using concentrated sulfuric acid containing dissolved sulfur trioxide.
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Friedel-Crafts Alkylation
• Friedel-Crafts alkylation forms a new C-C bond between an aromatic ring and an alkyl group.
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Friedel-Crafts Alkylation
Step 1: Formation of an alkyl cation as an ion pair.
Step 2: Attack of the alkyl cation on the aromatic ring.
Step 3: Proton transfer regenerates the aromatic ring.
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Friedel-Crafts Alkylation
• There are two major limitations on Friedel-Crafts alkylations:
1. Carbocation rearrangements are common
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Friedel-Crafts Alkylation
2. F-C alkylation fails on benzene rings bearing one or more of these strongly electron-withdrawing groups.
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Friedel-Crafts Acylation
• Friedel-Crafts acylation forms a new C-C bond between a benzene ring and an acyl group.
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Friedel-Crafts Acylation
• The electrophile is an acylium ion.
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Friedel-Crafts Acylation
• An acylium ion is represented as a resonance hybrid of two major contributing structures.
• Friedel-Crafts acylations are free of a major limitation of Friedel-Crafts alkylations; acylium ions do not rearrange
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Friedel-Crafts Acylation
• A special value of F-C acylations is preparation of unrearranged alkylbenzenes.
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Di- and Polysubstitution
• Orientation on nitration of monosubstituted benzenes.
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Di- and Polysubstitution
• Orientation:• Certain substituents direct preferentially to ortho & para
positions; others to meta positions.• Substituents are classified as either ortho-para directingortho-para directing or
meta directing 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 activatingactivating or deactivatingdeactivating toward further substitution.
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Di- and Polysubstitution
• -OCH3 is ortho-para directing.
• -COOH is meta directing.
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Di- and Polysubstitution
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Di- and Polysubstitution
• Ortho-para directing groups have an unshared pair of electrons adjacent to the phenyl ring.
• Alkyl and phenyl are also orth-para directing.• All other substituents are meta directing.
• All ortho-para directing groups are activating.• Except the halogens, which are weakly deactivating.• All meta directing groups are deactivating.
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Di- and Polysubstitution
• The order of steps is important.
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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.
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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.
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Theory of Directing Effects
• -OCH3; assume meta attack.
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Theory of Directing Effects
• -OCH3: assume ortho-para attack. Here only para attack is shown.
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Theory of Directing Effects
• -NO2; assume meta attack.
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Theory of Directing Effects
• -NO2: assume ortho-para attack.
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Week 7 56
This week…
• Study for Quiz 6• OWL homework due (Chap. 21)
• Due before 6 PM 3/19/12
• Check Bspace for class announcements• Read Chapter 29: Organic Polymer Chemistry
