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Ch 16 Synthetic Strategies

Ch 16 Synthetic Strategies. Reactions of Disubstituted Benzenes The strongest activator wins Ortho/para directors generally activate the ring, so these substituents should control the reactivity instead of a deactivation meta director

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Ch 16 Synthetic Strategies

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  1. Ch 16 Synthetic Strategies • Reactions of Disubstituted Benzenes • The strongest activator wins • Ortho/para directors generally activate the ring, so these substituents should control the reactivity instead of a deactivation meta director • Steric considerations also play a role, especially when more than one location is equivalent by activation • Guidelines for predicting where a third substituent will go • The most powerful activator has primary control

  2. Substituent groups have been ranked • Group I = NR2, OR = most powerful activators • Group II = X, R • Group III = all of the other deactivators • I ~ I > II ~ II > III ~III 3) Ortho attack to a bulky group, or attack between 2 substituents is unlikely

  3. The same rules apply for higher substituted benzenes • Synthetic Strategies for Substituted Benzenes • Reversibly Interchanging meta and ortho/para directors • Nitro (meta) and Amino (ortho/para) Interconversions • How could we prepare m-bromoaniline?

  4. Use the NH2/NO2 interconversion: • Alkanoyl (meta) and Alkyl (ortho/para) Interconversion

  5. How can we prepare m-chloroethylbenzene? • Use reduction of alkanoyl groups to give monosubstituted alkylbenzenes • Friedel-Crafts Electrophiles Don’t Attack Deactivated Benzenes NO2 deactivates ring; electrophile not strong enough to react

  6. Use of Reversible Sulfonation for Ortho Disubstitution • Para products are usually major when substituting a o,p directed benzene • We can block the para position with a sulfonate group. Steric bulk will lead to the para product as major • Nitration will then occur only at the original ortho site (meta to NO2) and deprotection yields the desired ortho product

  7. Moderation of Highly Activating Groups • NH2 and OH substituents “overactivate” benzene to multiple substitutions • The NH2 and OH groups can also react with electrophiles themselves • Amine moderation can be had through an acetyl protecting group • Phenols can be protected as the methyl ether • Di- or polysubstitution won’t occur with the less-activated derivatives

  8. Polycyclic Aromatic Hydrocarbon Reactions • Naphthalene • Naphthalene undergoes electrophilic aromatic substitution very easily • Substitution is selective for C-1 • Resonance forms determine higher reactivity at C-1 • C-1 attack has 2 resonance structures with benzene rings • C-2 attack has only 1 resonance structure with a benzene ring • The most stable intermediate (C-1 attack) gives faster reaction

  9. Electrophilic Attack of Substituted Naphthalenes • The ring carrying the substituent is most affected • Activating group puts the next substituent on the same ring • Deactivating group puts the next substituent on the other ring

  10. C-5 and C-8 are the preferred sites for deactivating groups • Larger PAH’s • Use resonance structures to predict substitution • Phenanthrene is monosubstituted preferentially at C-9

  11. Carcinogenic PAH’s • Benzo[a]pyrene is a carcinogen • Many carcinogens are alkylating groups that alkylate DNA. This can lead to proliferating (cancerous) cells

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