5. Benzene and Aromaticity

1. 5. Benzene and Aromaticity

2. Aromatic Compounds • The term “Aromatic” is used to refer to the class of compounds structurally related to Benzene. • The first discovered of these compounds were fragrant substances but the term aromatic, though still used, is not applicable to the vast majority of these compounds

3. The common names of some substituted aromatics are so firmly entrenched in the literature that they must be memorized =>

4. 15.2 Naming Aromatic Compounds • Monosubstituted benzenes are named by first naming the substituent and following this with the word benzene

5. Naming Alkyl Substituted Benzenes • Alkyl benzenes are named in one of two different ways: • If the alkyl group contains 6 or fewer carbons, then the compound is named as an alkyl substituted benzene • If the alkyl group contains more than 6 carbons then the compound is named as phenyl substituted alkane

6. Naming Benzenes With More Than Two Substituents • Choose numbers to get lowest possible values • List substituents alphabetically with hyphenated numbers • Common names, such as “toluene” can serve as root name

7. Naming Disubstituted Benzenes • Relative positions on a benzene ring are indicated by the following prefixes • ortho- (o) on adjacent carbons (1,2) • meta- (m) separated by one carbon (1,3) • para- (p) separated by two carbons (1,4) • Also used to describe reaction patterns (“reaction occurs at the para position”)

8. Complete the Following Examples

9. Structure of Benzene • The actual structure of benzene lies somewhere between the two resonance forms pictured below

10. Experimental Observations That Lead To This Resonance Picture of Benzene • All its C-C bonds are the same length: 139 pm — between single (154 pm) and double (134 pm) bonds • Electron density in all six C-C bonds is identical • Structure is planar, hexagonal

11. => Molecular Orbital Description of the Resonance in Benzene • Each C is sp2 hybridized and has a p orbital perpendicular to the plane of the six-membered ring. Each p orbital has one electron in it. This makes it impossible to identify 3 localized double bonds in benzene .

12. Consequence of Resonance Stability • The resonance stability of benzene is so very substantial that benzene shows none of the characteristic chemical behavior of other alkenes • Alkene + Br2/CCl4  dibromide (addition product)Benzene + Br2/CCl4  no reaction. • Alkene + HBr  Bromoalkane (addition product)Benzene + HBr  no reaction. • The reason that benzene does not take part in any electrophilic addition rxns. is that, to do so, would destroy it’s stable conjugated system. An energetically unfavorable situation.

13. Please Recall the General Mechanism for Aromatic Substition + - Br

14. Heats of Hydrogenation as Indicators of Resonance Stability of Benzene • The addition of H2 to C=C normally gives off about 118 kJ/mol ; 3 double bonds should give off 356kJ/mol • Benzene has 3 double bonds but gives off only 206 kJ/mol on reacting with 3 H2 molecules • Therefore it is about 150 kJ more “stability” than a regular alkene having s set of three double bonds

15. Reactions of Aromatic Compounds • Electrophilic addition reactions, • so common amongst normal alkenes, do not occur with aromatics, in spite of the fact that each aromatic ring contains three double bonds. • The reason for this is that these reactions break the double bond and this would mean that the very stable aromatic system would be disrupted. • Instead, the characteristic reactions of aromatics are electrophilic substitution reactions rather that addition because these retain the very stable cyclic aromatic system

16. Electrophilic Addition and Electrophilic Substitution E++base- ElectrophilicAddition Electrophilic Substitution base:-

17. Br+Br- Aromatic Addition Compared to Aromatic Substition

18. FeBr4- + Br+ A stronger electrophile than Br2 All Electrophilic Aromatic Substitution Reactions take place by the same General Mechanism. • Aromatics (benzene) are less reactive towards electrophiles then are normal alkenes. • Consequently, a catalyst is usually needed to convert the “electrophile containing reagent” into a stronger electrophile. • The catalyst needed to react molecular bromine (Br2) with benzene is ferric bromide. FeBr3 basically turns the weaker electrophile, Br2, into the stronger electrophile, Br+ • FeBr3 is a Lewis Acid and accepts an electron pair from Br2 and thereby puts a strong positive charge on the end Bromine atom.

19. Generalized Mechanism for Electrophilic Aromatic Substitution cont. • Once generated. the stronger electrophile gets attacked by the pi electrons of the aromatic system, forming an intermediate, resonance stabilized, carbocation. • Finally, the carbocation stabilizes itself by loosing a ring H+ and regenerating the stable cyclic conjugated system, with the electrophile on the ring where the H+ used to be. See next slide.

20. A stronger Electrophile than Br2 + FeBr4- + Br+ + - Br +

21. A stronger Electrophile than Br2 + FeBr4- + Br+ + - Br +

22. Aromatic Chlorination • Chlorine and iodine (but not fluorine, which is too reactive) can substitute on an aromatic ring. Each requires a special catalyst or promoter to generate a sufficiently strong electrophile • Chlorination follows the same mechanism as bromination and requires FeCl3 catalyst

23. Aromatic Iodination • Iodine (I2) must be oxidized with Cu+2 or peroxide to form the more powerful electrophile, I+

24. Aromatic Nitration • The combination of nitric acid and sulfuric acid produces the electrophile NO2+ (nitronium ion) • It reacts with benzene to produce nitrobenzene HNO3

25. Nitroaromatics are Important for Two Reasons • Nitroaromatics are important in themselves and also the nitro group can be converted into other functional groups that couldn’t be placed on the aromatic ring directly • For example, reduction of the nitro group by stannous chloride yields the corresponding amine

26. Aromatic Sulfonation • The combination of sulfuric acid and sulfur trioxide (SO3) produces the electrophile HSO3+ • Its reaction with benzene produces benzenesulfonic acid SO3

27. Importance of Aromatic Sulfonic Acids • Aromatic Sulfonic Acids are valuable intermediates in the preparation of dyes and pharmaceuticals. • Aromatic Sulfonic Acids are the precursors needed for the synthesis of Sulfa Drugs such as sulfanilamide.These were among the first useful antibiotics known and credited with saving countless lives during W.W.II

28. Aromatic Sulfonic Acids are also important for the further chemistry that they can undergo • When sulfonic acids are mixed with sodium hydroxide at elevated temperatures a net replacement of the sulfonic group by the hydroxyl group results. • This constitutes one of the few methods for preparing phenols.

29. 16.3 Alkylation of Aromatic Rings: The Friedel–Crafts Reaction • Aromatic substitution of a R+ for an aromatic proton (H+) • Aluminum trichloride, a Lewis Acid catalyst, promotes the formation of the (R+) carbocation

30. Limitations of the Friedel-Crafts Alkylation • Only alkyl halides can be used (F, Cl, I, Br) • Aryl halides and vinylic halides do not react (their carbocations are too hard to form) • This rxn will not work with rings containing an amino group or a strongly electron-withdrawing deactivating group

31. Control Problems with F/C Alkylations • Unwanted multiple alkylations can occur because the first alkylation is activating. That is to say, once the first alkyl group substitutes on the ring; the monosubstituted benzene is more reactive than benzene itself and consequently more likely to be substituted with another alkyl group

32. Carbocation Rearrangements During Alkylation • The last problem associated with F/C Alkylation is the possible rearrangement of the intermediate carbocation to a more stable carbocation • These rearrangements usually involve hydride (H-) or alkide (R-) shifts

33. 16.4 Acylation of Aromatic Rings • Reaction of an acid chloride (RCOCl) in the presence of AlCl3 catalyst with an aromatic ring substitutes an acyl group, COR , on to the aromatic ring • Benzene with acetyl chloride yields acetophenone

34. Mechanism of Friedel-Crafts Acylation • Similar to alkylation • Reactive electrophile: resonance-stabilized acyl cation • An acyl cation does not rearrange

35. Electrophilic Aromatic Substitution of a Monosubstituted Benzene • What effects does a substituent already present on a benzene ring have on the electrophilic substitution of a second group? • Reactivity: Some monosubstituted benzenes are morereactive that benzene towards further electrophilic aromatic substitution (activating substituents); some monosubstituted benzenes are less reactive (deactivating substituents) • Orientation: A substituent that is already on a benzene ring directs the position of any incoming groups

36. Reactivity: Activating Substituents • Activating Substituents – these activate a benzene ring towards further substitution by donating electron density into the aromatic ring. Donating electon density into the ring increases the reaction rate by stabilizing the intermediate carbocation.

37. Reactivity: Deactivating Substituents • Deactivating Substituents – these deactivate a benzene ring towards further substitution by withdrawing electron density from the aromatic ring. Withdrawing electon density from the ring decreases the reaction rate by destabilizing the intermediate carbocation

38. Orientation • The second effect that the substituent of a monosubstituted benzene can have on further electrophilic aromatic substitution is to direct incoming electrophiles to particular positions on the aromatic ring. Substituents are either ortho – para directors or they are meta directors. Combining this information with the reactivity characteristics of a substituent we find that all substituents can be classified into one of three groups; • Ortho – Para Activators • Meta Deactivators • Ortho – Para Deactivators

39. Ortho-Para Activating Groups • Please recall that activating groups increase the e- density of the aromatic ring. These substituents also direct incoming groups to the ortho and para positions as only these positions afford a resonance structure for the intermediate carbocation in which the positive charge is on the ring carbon to which the e- donating group is bonded – a very stable situation. The increased stability of this resonance structure favors substitution in these positions. The electron donating substituents may stabilize the positive charge by the inductive effect or by resonance. See Next Slide for Example

41. +

42. Meta Deactivators • Recall that deactivating groups withdraw e- density from the aromatic ring. All members of this group except for the halogens direct incoming groups to the meta position for it is only in this position that resonance structures for the intermediate carbocation do not place the positive charge on the ring carbon to which the e- withdrawing group is bonded (an unstable situation). Avoiding this extremely unstable situation is what makes the meta position the most highly favored (most stable).

44. Ortho-Para Deactivating Groups • Recall that halogens deactivate aromatic rings by inductive withdrawal of e- density. In addition to this ability, all halogens possess nonbonded e-’s that can be used to resonance-stabilize a positive charge on an adjacent carbon. It is this ability that make halogens ortho-para directors. If the incoming group attaches to either the ortho or para position, one of the resonance structures for the intermediate carbocation places the positive charge on a ring carbon to which the halogen is bonded. This allows the halogens to resonance-stabilize the positive charge.

46. 16.5 Substituent Effects in Aromatic Rings: Summarized • Substituents already present on an aromatic ring can cause the aromatic compound to be (much) more or (much) less reactive than benzene • Substituents also direct the orientation of incoming groups on to the aromatic ring • ortho- and para-directing activators, ortho- and para-directing deactivators, and meta-directing deactivators

47. 16.7 Trisubstituted Benzenes: Additivity of Effects • How does one predict the orientation of a third group coming in to a disubstituted benzene • If the directing effects of the two groups are the same, the result is additive

48. Substituents with Opposite Effects • If the directing effects of two groups oppose each other, the more powerful activating group decides the principal outcome

49. Meta-Disubstituted Compounds • Substitution between two groups in a meta-disubstituted compound rarely occurs because the site is too sterically hindered • To make aromatic rings with three adjacent substituents, it is best to start with an ortho-disubstituted compound

50. 16.10 Oxidation of Aromatic Compounds • Alkyl side chains can be oxidized to CO2H by strong reagents such as KMnO4 and Na2Cr2O7 if they have a C-H next to the ring • Converts an alkylbenzene into a benzoic acid, ArR  ArCO2H