1. Aromatic Compounds

2. Reactions of Benzene
Even though benzene is highly unsaturated it does not undergo any of the regular reactions of alkenes such as addition or oxidation
Benzene can be induced to react with bromine if a Lewis acid catalyst is present however the reaction is a substitution and not an addition
Benzene produces only one monobrominated compound, which indicates that all 6 carbon-hydrogen bonds are equivalent in benzene
Chapter 14

3. The Kekule Structure for Benzene
Kekule was the first to formulate a reasonable representation of benzene
The Kekule structure suggests alternating double and single carbon-carbon bonds
Based on the Kekule structure one would expect there to be two different 1,2-dibromobenzenes but there is only one
Kekule suggested an equilibrium between these compounds to explain this observation but it is now known no such equilibrium exists
Chapter 14

4. The Stability of Benzene
Benzene is much more stable than would be expected based on calculations for “cyclohexatriene”
A reasonable prediction for the heat of hydrogenation of hypothetical cyclohexatriene is -360 kJ mol-1 (3 times that of cyclohexene, -120 kJ mol-1 )
The experimentally determined heat of hydrogenation for benzene is -280 mol-1, 152 kJ mol-1 more stable than hypothetical cyclohexatriene
This difference is called the resonance energy
Chapter 14

5. Modern Theories of the Structure of Benzene
The Resonance Explanation of the Structure of Benzene
Structures I and II are equal resonance contributors to the real structure of benzene
Benzene is particularly stable because it has two equivalent and important resonance structures
Each carbon-carbon bond is 1.39 Å, which is between the length of a carbon-carbon single bond between sp2 carbons (1.47Å) and a carbon-carbon double bond (1.33 Å)
Often the hybrid is represented by a circle in a hexagon (III)
Chapter 14

6. The Molecular Orbital Explanation of the Structure of Benzene
The carbons in benzene are sp2 hybridized with p orbitals on all 6 carbons (a)
The p orbitals overlap around the ring (b) to form a bonding molecular orbital with electron density above and below the plane of the ring (c)
There are six p molecular orbitals for benzene
Chapter 14

7. Huckel’s Rule: The 4n+2p Electron Rule
Planar monocyclic rings with a continuous system of p orbitals and 4n + 2p electrons are aromatic (n = 0, 1, 2, 3 etc)
Aromatic compounds have substantial resonance stabilization
Benzene is aromatic: it is planar, cyclic, has a p orbital at every carbon, and 6 p electrons (n=1)
There is a polygon-and-circle method for deriving the relative energies of orbitals of a system with a cyclic continuous array of p orbitals
A polygon corresponding to the ring is inscribed in a circle with one point of the polygon pointing directly down
A horizontal line is drawn where vertices of the polygon touch the circle - each line corresponds to the energy level of the p MOs at those atoms
A dashed horizontal line half way up the circle indicates the separation of bonding and antibonding orbitals
Benzene has 3 bonding and 3 antibonding orbitals
All the bonding orbitals are full and there are no electrons in antibonding orbitals; benzene has a closed shell of delocalized electrons and is very stable
Chapter 14

8. Cyclooctatetraene has two nonbonding orbitals each with one electron
This is an unstable configuration; cyclooctatetraene adopts a nonplanar conformation with localized p bonds to avoid this instability
Chapter 14

9. Aromatic Ions
Cyclopentadiene is unusually acidic (pKa = 16) because it becomes the aromatic cyclopentadienyl anion when a proton is removed
Cyclopentadienyl anion has 6 p electrons in a cyclic, continuous p-electron system, and hence follows the 4n + 2 rule for aromaticity
Cycloheptatriene is not aromatic because its p electrons are not delocalized around the ring (the sp3-hybridized CH2 group is an “insulator”)
Lose of hydride produces the aromatic cycloheptatrienyl cation (tropylium cation)
Chapter 14

10. Other Aromatic Compounds
Benzenoid Aromatic Compounds
Polycyclic benzenoid aromatic compounds have two or more benzene rings fused together
Chapter 14

11. Naphthalene can be represented by three resonance structures
The most important resonance structure is shown below
Calculations show that the 10 p electrons of napthalene are delocalized and that it has substantial resonance energy
Pyrene has 16 p electrons, a non-Huckel number, yet is known to be aromatic
Ignoring the central double bond, the periphery of pyrene has 14 p electrons, a Huckel number, and on this basis it resembles the aromatic [14]annulene
Chapter 14

12. Fullerenes
Buckminsterfullerene is a C60 compound shaped like a soccer ball with interconnecting pentagons and hexagons
Each carbon is sp2 hybridized and has bonds to 3 other carbons
Buckminsterfullerene is aromatic
Analogs of “Buckyballs” have been synthesized (e.g. C70)
Chapter 14

13. Heterocyclic Aromatic Compounds
Heterocyclic compounds have an element other than carbon as a member of the ring
Example of aromatic heterocyclic compounds are shown below
Numbering always starts at the heteroatom
Pyridine has an sp2 hybridized nitrogen
The p orbital on nitrogen is part of the aromatic p system of the ring
The nitrogen lone pair is in an sp2 orbital orthogonal to the p orbitals of the ring; these electrons are not part of the aromatic system
The lone pair on nitrogen is available to react with protons and so pyridine is basic
Chapter 14

14. The nitrogen in pyrrole is sp2 hybridized and the lone pair resides in the p orbital
This p orbital contains two electrons and participates in the aromatic system
The lone pair of pyrrole is part of the aromatic system and not available for protonation; pyrrole is therefore not basic
In furan and thiophene an electron pair on the heteroatom is also in a p orbital which is part of the aromatic system
Chapter 14

15. Nomenclature of Benzene Derivatives
Benzene is the parent name for some monosubstituted benzenes; the substituent name is added as a prefix
For other monosubstituted benzenes, the presence of the substituent results in a new parent name
Chapter 14

16. Dimethyl substituted benzenes are called xylenes
When two substituents are present their position may be indicated by the prefixes ortho, meta, and para (o, m and p) or by the corresponding numerical positions
Dimethyl substituted benzenes are called xylenes
Chapter 14

17. Numbers must be used as locants when more than two substituents are present
The lowest possible set of numbers should be given to the substituents
The substituents should be listed in alphabetical order
If one of the substituents defines a parent other than benzene, this substituent defines the parent name and should be designated position 1
Chapter 14

18. The C6H5- group is called phenyl when it is a substituent
Phenyl is abbreviated Ph or F
A hydrocarbon with a saturated chain and a benzene ring is named by choosing the larger structural unit as the parent
If the chain is unsaturated then it must be the parent and the benzene is then a phenyl substituent
The phenylmethyl group is called a benyl (abbreviated Bz)
Chapter 14

19. Reactions of Aromatic Compounds

20. Electrophilic Aromatic Substitution
Arene (Ar-H) is the generic term for an aromatic hydrocarbon
The aryl group (Ar) is derived by removal of a hydrogen atom from an arene
Aromatic compounds undergo electrophilic aromatic substitution (EAS)
The electrophile has a full or partial positive charge
Chapter 14

21. A General Mechanism for Electrophilic Aromatic Substitution: Arenium Ion Intermediates
Benzene reacts with an electrophile using two of its p electrons
This first step is like an addition to an ordinary double bond
Unlike an addition reaction, the benzene ring reacts further so that it may regenerate the very stable aromatic system
In step 1 of the mechanism, the electrophile reacts with two p electrons from the aromatic ring to form an arenium ion
The arenium ion is stabilized by resonance which delocalizes the charge
In step 2, a proton is removed and the aromatic system is regenerated
Chapter 14

22. The second step is highly exothermic and has a small DG‡ (2)
The energy diagram of this reaction shows that the first step is highly endothermic and has a large DG‡ (1)
The first step requires the loss of aromaticity of the very stable benzene ring, which is highly unfavorable
The first step is rate-determining
The second step is highly exothermic and has a small DG‡ (2)
The ring regains its aromatic stabilization, which is a highly favorable process
Chapter 14

23. Halogenation of Benzene
Halogenation of benzene requires the presence of a Lewis acid
Fluorination occurs so rapidly it is hard to stop at monofluorination of the ring
A special apparatus is used to perform this reaction
Iodine is so unreactive that an alternative method must be used
Chapter 14

24. In the step 1 of the mechanism, bromine reacts with ferric bromide to generate an electrophilic bromine species
In step 2, the highly electrophilic bromine reacts with p electrons of the benzene ring, forming an arenium ion
In step 3, a proton is removed from the arenium ion and aromaticity is regenerated
The FeBr3 catalyst is regenerated
Chapter 14

25. Nitration of Benzene
Nitration of benzene occurs with a mixture of concentrated nitric and sulfuric acids
The electrophile for the reaction is the nitronium ion (NO2+)
Chapter 14

26. Sulfonation of Benzene
Sulfonation occurs most rapidly using fuming sulfuric acid (concentrated sulfuric acid that contains SO3)
The reaction also occurs in conc. sulfuric acid, which generates small quantities of SO3, as shown in step 1 below
Chapter 14

27. Sulfonation is an equilibrium reaction; all steps involved are equilibria
The sulfonation product is favored by use of concentrated or fuming sulfuric acid
Desulfonation can be accomplished using dilute sulfuric acid (i.e. with a high concentration of water), or by passing steam through the reaction and collecting the volatile desulfonated compound as it distils with the steam
Chapter 14

28. Friedel-Crafts Alkylation
An aromatic ring can be alkylated by an alkyl halide in the presence of a Lewis acid
The Lewis acid serves to generate a carbocation electrophile
Chapter 14

29. Primary alkyl halides probably do not form discreet carbocations but the primary carbon in the complex develops considerable positive charge
Any compound that can form a carbocation can be used to alkylate an aromatic ring
Chapter 14

30. Friedel-Crafts Acylation
An acyl group has a carbonyl attached to some R group
Friedel-Crafts acylation requires reaction of an acid chloride or acid anhydride with a Lewis acid such as aluminium chloride
Chapter 14

31. Acid chlorides are made from carboxylic acids
Chapter 14

32. Effects of Substituents on Reactivity and Orientation
The nature of groups already on an aromatic ring affect both the reactivity and orientation of future substitution
Activating groups cause the aromatic ring to be more reactive than benzene
Deactivating groups cause the aromatic ring to be less reactive than benzene
Ortho-para directors direct future substitution to the ortho and para positions
Meta directors direct future substitution to the meta position
Activating Groups: Ortho-Para Directors
All activating groups are also ortho-para directors
The halides are also ortho-para directors but are mildly deactivating
The methyl group of toluene is an ortho-para director
Toluene reacts more readily than benzene, e.g. at a lower temperatures than benzene
Chapter 14

33. The methyl group of toluene is an ortho-para director
Amino and hydroxyl groups are also activating and ortho-para directors
These groups are so activating that catalysts are often not necessary
Alkyl groups and heteroatoms with one or more unshared electron pairs directly bonded to the aromatic ring will be ortho-para directors (see chart on slide 22)
Chapter 14

34. Deactivating Groups: Meta Directors
Strong electron-withdrawing groups such as nitro, carboxyl, and sulfonate are deactivators and meta directors
Halo Substitutents: Deactivating Ortho-Para Directors
Chloro and bromo groups are weakly deactivating but are also ortho, para directors
In electrophilic substitution of chlorobenzene, the ortho and para products are major:
Chapter 14

35. Classification of Substitutents
Chapter 14

36. Theory of Substituent Effects on Electrophilic Substitution
Reactivity: The Effect of Electron-Releasing and Electron-Withdrawing Groups
Electron-releasing groups activate the ring toward further reaction
Electron-releasing groups stabilize the transition state of the first step of substitution and lead to lower DG‡ and faster rates of reaction
Electron-withdrawing groups deactivate the ring toward further reaction
Electron-withdrawing groups destabilize the transition state and lead to higher DG‡ and slower rates of reaction
Chapter 14

37. The following free-energy profiles compare the stability of the first transition state in electrophilic substitution when various types of substitutents are already on the ring
These substitutents are electron-withdrawing, neutral (e.g., H), and electron-donating
Chapter 14

38. Inductive and Resonance Effects: Theory of Orientation
The inductive effect of some substituent Q arises from the interaction of the polarized bond to Q with the developing positive charge in the ring as an electrophile reacts with it
If Q is an electron-withdrawing group then attack on the ring is slowed because this leads to additional positive charge on the ring
The following are some other groups that have an electron- withdrawing effect because the atom directly attached to the ring has a partial or full positive charge
Chapter 14

39. Electron-donating resonance ability is summarized below
The resonance effect of Q refers to its ability to increase or decrease the resonance stabilization of the arenium ion
When Q has a lone pair on the atom directly attached to the ring it can stabilize the arenium by contributing a fourth resonance form
Electron-donating resonance ability is summarized below
Chapter 14

40. Ortho-Para Directing Groups
Many ortho-para directors are groups that have a lone pair of electrons on the atom directly attached to the ring
Chapter 14

41. Meta-directing Groups
All meta-directing groups have either a partial or full positive charge on the atom directly attached to the aromatic ring
The trifluoromethyl group destabilizes the arenium ion intermediate in ortho and para substitution pathways
The arenium ion resulting from meta substitution is not so destabilized and therefore meta substitution is favored
Chapter 14

42. Activating groups having unshared electrons on the atom bonded to the ring exert primarily a resonance effect
The aromatic ring is activated because of the resonance effect of these groups
They are ortho-para directors because they contribute a fourth important resonance form which stabilizes the arenium ion in the cases of ortho and para substitution only
The fourth resonance form that involves the heteroatom is particularly important because the octet rule is satisfied for all atoms in the arenium ion
Chapter 14

43. Halo groups are ortho-para directors but are also deactivating
The electron-withdrawing inductive effect of the halide is the primary influence that deactivates haloaromatic compounds toward electrophilic aromatic substitution
The electron-donating resonance effect of the halogen’s unshared electron pairs is the primary ortho-para directing influence
Chapter 14

44. Ortho-Para Direction and Reactivity of Alkylbenzenes
Alkyl groups activate aromatic rings by inductively stabilizing the transition state leading to the arenium ion
Alkyl groups are ortho-para directors because they inductively stabilize one of the resonance forms of the arenium ion in ortho and para substitution
Chapter 14

45. Oxidation of the Side Chain
Alkyl and unsaturated side chains of aromatic rings can be oxidized to the carboxylic acid using hot KMnO4
Chapter 14

46. Heteroaromatic compounds

47. Heterocyclic aromatic compounds – five membered rings

48. Pyrrole is not a base

49. The dipole-moments of pyrrole and pyrrolydine

50. indole
benzofurane
benzothiophene

51. Heterocyclic aromatic compounds six membered rings
Chapter 14

52. The pyridinium ion is a stronger acid than the piperidinium
ion, therefore itself is a weaker base

53. Quinoline and isoquinoline are also known as benzopyridines
Chapter 14

54. Imidazole
Chapter 14

57. Porphirin ring

58. Nitrogen containing heterocyclic aromatic compounds