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REACTIONS OF AROMATIC COMPOUNDS

REACTIONS OF AROMATIC COMPOUNDS. Electrophilic Aromatic Substitution : The mechanism of many reactions of aromatic compounds are explained by minor variations of electrophilic aromatic substitution.

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REACTIONS OF AROMATIC COMPOUNDS

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  1. REACTIONS OF AROMATIC COMPOUNDS Electrophilic Aromatic Substitution: The mechanism of many reactions of aromatic compounds are explained by minor variations of electrophilic aromatic substitution. Recall that there are clouds of pi electrons above and below the sigma bonds of a benzene ring. These pi electrons are in a stable aromatic system. These pi electrons make the aromatic ring an electron rich materials. These pi electron are available to attach by strong electrophiles to give a carbocation intermediate.

  2. REACTIONS OF AROMATIC COMPOUNDS

  3. REACTIONS OF AROMATIC COMPOUNDS Step 1: Attack on the electrophile forms the sigma complex. Step 2: Loss of a proton gives the substitution product.

  4. REACTIONS OF AROMATIC COMPOUNDS Show the mechanism for the following reaction. Hint: AlCL3 is a Lewis acid (an electron pair acceptor). Many substitution reactions involving an aromatic ring require a Lewis acid catalyst.

  5. REACTIONS OF AROMATIC COMPOUNDS

  6. REACTIONS OF AROMATIC COMPOUNDS There are two things that must be considered when predicting the products of an electrophilic aromatic substitution reaction. 1. Identify the electrophilic species (what is the electrophile)? 2. What are the substituents groups that are already on the aromatic ring? (substituent effects)

  7. REACTIONS OF AROMATIC COMPOUNDS halogenation

  8. REACTIONS OF AROMATIC COMPOUNDS halogenation Iodination requires an acidic oxidizing agent, like nitric acid, which oxidizes the iodine to an iodonium ion.

  9. REACTIONS OF AROMATIC COMPOUNDS Nitration The nitration of benzene is conveniently done using a mixture of nitric acid and sulfuric acid. The sulfuric acid is a catalyst which reacts with nitric acid to generate the nitronium ion (NO2+). The use of sulfuric acid allows the reaction to take place at a faster rate and at a lower temperature. 85%

  10. REACTIONS OF AROMATIC COMPOUNDS Nitration Draw the mechanism for the nitration of benzene.

  11. REACTIONS OF AROMATIC COMPOUNDS Reduction of nitroaromatics Nitration of an aromatic ring is often the first step in a two step process that is used to add an amine group to an aromatic ring. The reduction of the nitro group is easily accomplished by treatment with a metal and dilute acid. It is common in organic synthesis to add a functional group to a substrate and then to convert the group to the the desired group.

  12. REACTIONS OF AROMATIC COMPOUNDS Sulfonation of benzene This reaction is done using fuming sulfuric acid (7% SO3 in H2SO4).

  13. REACTIONS OF AROMATIC COMPOUNDS desulfonation of benzene Under what conditions do you think this reaction would be run?

  14. REACTIONS OF AROMATIC COMPOUNDS Exchange reactions of benzene Write a mechanism for this reaction.

  15. REACTIONS OF AROMATIC COMPOUNDS Effect of ring substituents Mono-nitration of benzene gives only a single product, while mono-nitration of toluene can give three different products.

  16. REACTIONS OF AROMATIC COMPOUNDS Effect of ring substituents There are two interesting observations that can be made when comparing the nitration of benzene vs toluene. The first is that the reaction rate for toluene is ~25 times faster then benzene. The methyl group activated the ring toward electrophilic substitution. Methyl is an activating group. The second is the distribution of products. If all of the positions on the ring were equivalent you would expect a 2:2:1 ratio of the ortho, metha and para products.

  17. REACTIONS OF AROMATIC COMPOUNDS Effect of ring substituents There are two ortho positions, two meta positions and one para position.

  18. REACTIONS OF AROMATIC COMPOUNDS Effect of ring substituents Nitration of toluene preferentially occurs at the positions ortho and para to the methyl group. The methyl group is referred to as being an ortho, para-director. The presences of the methyl group on the aromatic ring has two effects. It affects reaction rates and where substitution occurs at on the ring.

  19. REACTIONS OF AROMATIC COMPOUNDS You must be able to do the following: 1. Know how different functional groups are added to an aromatic ring. 2. Know how different substituent groups affect the reactivity of an aromatic ring. 3. Know how different substituent groups affect where substitution will occur.

  20. REACTIONS OF AROMATIC COMPOUNDS

  21. REACTIONS OF AROMATIC COMPOUNDS

  22. REACTIONS OF AROMATIC COMPOUNDS The results seen here for toluene (methylbenzene) are general for all mono-alkylbenzenes when undergoing electrophilic aromatic substitution reactions. The sigma complexes formed ortho and para to the alkyl group are more stable then the meta complex because the ortho and para complex have resonance forms with tertiary carbocations. This effect is called inductive stabilization because the alkyl group is donating electron density to the intermediate through the sigma bond.

  23. REACTIONS OF AROMATIC COMPOUNDS Effect of substituents with non-bonding electrons When a substituent group has a non-bonding pair of electrons on the atom directly bonded to the ring, the sigma complex initially formed during an electrophilic substitution reaction can be resonance stabilized by the non-bonding electrons.

  24. REACTIONS OF AROMATIC COMPOUNDS Effect of substituents with non-bonding electrons

  25. REACTIONS OF AROMATIC COMPOUNDS Effect of substituents with non-bonding electrons The affect of resonance stabilization by substituents with non-bonding electrons on reaction rates can be very large. In the case of anisole the rate of nitration is ~10,000 time faster than benzene and ~ 400 times faster then toluene. This type of stabilization is also called resonance donating and pi-donating. Substituents with non-bonding electrons are ortho/para directors. They may be either activating or deactivating.

  26. REACTIONS OF AROMATIC COMPOUNDS Effect of substituents with non-bonding electrons Recall that the bromination of benzene required a Lewis catalyst. However, with strong activating substituent like the amino group in aniline the reaction occurs with multiply additions of bromine without a catalyst. Where did substitution occur at and what happens if you don’t have the bicarbonate in the reaction?

  27. REACTIONS OF AROMATIC COMPOUNDS Activating Ortho/Para directors

  28. REACTIONS OF AROMATIC COMPOUNDS Deactivating meta-directing substituents We have seem how the presence of some substituents can greatly enhance the reactivity of an aromatic ring compared to benzene. We will now look at substituents that deactivate the aromatic ring toward electrophilic attack. In electrophilic aromatic substitution reactions nitrobenzene is ~100,000 less reactive than benzene. In addition to deactivation of the ring the substitution occurs at the meta position.

  29. REACTIONS OF AROMATIC COMPOUNDS Deactivating meta-directing substituents In electrophilic aromatic substitution reactions nitrobenzene is ~100,000 less reactive than benzene. In addition to deactivation of the ring the substitution occurs at the meta position.

  30. REACTIONS OF AROMATIC COMPOUNDS Deactivating meta-directing substituents Why does the nitro group deactivate the ring in electrophilic aromatic substitution reactions? Why is the nitro group a meta director? To answer these questions we need to look at the intermediates that are formed during the reaction.

  31. REACTIONS OF AROMATIC COMPOUNDS Deactivating meta-directing substituents

  32. REACTIONS OF AROMATIC COMPOUNDS Deactivating meta-directing substituents

  33. REACTIONS OF AROMATIC COMPOUNDS Deactivating meta-directing substituents

  34. REACTIONS OF AROMATIC COMPOUNDS Deactivating meta-directing substituents

  35. REACTIONS OF AROMATIC COMPOUNDS Deactivating meta-directing substituents Structural characteristics of Meta-Directing Deactivators The atom attached to the aromatic ring will have a formal positive charge or a partial positive charge. Electron density is withdrawn inductively along the sigma bond, so the ring is less electron-rich than benzene. Destabilizes the sigma complex.

  36. REACTIONS OF AROMATIC COMPOUNDS Deactivating meta-directing substituents

  37. REACTIONS OF AROMATIC COMPOUNDS Deactivating meta-directing substituents

  38. REACTIONS OF AROMATIC COMPOUNDS Halogenated aromatics Halogenated aromatic compounds under go electrophile substitution ortho and para to the halogen. This is an expected result since halogens have non-bonding electrons that can resonance stabilize the intermediate sigma complex . Halogens are orhto/para directors but unlike other ortho/para directors, halogens deactivate the aromatic ring toward electrophilic substitution reactions. Why are halogens deactivators?`

  39. REACTIONS OF AROMATIC COMPOUNDS Halogenated aromatics Ortho and para attacks produce a bromonium ionand other resonance structures.

  40. REACTIONS OF AROMATIC COMPOUNDS Halogenated aromatics In the meta position there is no stabilization of the sigma complex.

  41. REACTIONS OF AROMATIC COMPOUNDS Halogenated aromatics

  42. REACTIONS OF AROMATIC COMPOUNDS Summary of substituent effects What about multiple substituents?

  43. REACTIONS OF AROMATIC COMPOUNDS Effects of multiple substituents • When two or more substituents are present on an aromatic ring a combined effect is observed in subsequent reactions. • In many cases it is easy to predict the effects of multiple substituent groups because the individual effects are mutually supporting of each other. • In cases were there is a conflict in the directing effects of the substituent groups it can more difficult to predict what products will be produced.

  44. REACTIONS OF AROMATIC COMPOUNDS Effects of multiple substituents • When dealing with multiple substituents activating groups are generally stronger directors than deactivating groups. • Strong activating ortho, para-directors that stabilize the transition state through resonance. i.e. –OH, –OR • Activating ortho, para-directors. i.e. alkyl groups and halogens • Deactivating meta directors.

  45. REACTIONS OF AROMATIC COMPOUNDS Friedel-Crafts Alkylation What is the mechanism? Hint: Think about halogenation of an aromatic ring.

  46. REACTIONS OF AROMATIC COMPOUNDS Friedel-Crafts Alkylation

  47. REACTIONS OF AROMATIC COMPOUNDS Friedel-Crafts Alkylation

  48. REACTIONS OF AROMATIC COMPOUNDS Friedel-Crafts Alkylation Rearrangement of the alkylating agent is possible and is limitation of Friedel-Crafts alkylation. As a result, only certain alkylbenzenes can be made using the Friedel-Crafts alkylation.

  49. REACTIONS OF AROMATIC COMPOUNDS Friedel-Crafts Alkylation Multiple alkylation is a limitation and as a result mixtures of products are common.

  50. REACTIONS OF AROMATIC COMPOUNDS Friedel-Crafts Alkylation Limitations of Friedel-Crafts Alkylation: 1. Only works with benzene and activated benzene derivatives. Fails with strong deactivating groups on the ring. 2. Rearrangement of the alkylating agent can occur, limiting the types of alkyl benzenes that can be produced. 3. Multiple alkylation's can occur resulting in undesired side products.

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