1. (Aromatic hydrocarbons)
ARENES
(Aromatic hydrocarbons)
18 April 2018

2. All symbols are equivalent and used for description
Benzene structure
All symbols are equivalent and used for description
of benzene molecule

3. Heats of Hydrogenation as Indicators of Stability
The addition of H2 to C=C normally gives off about 118 kJ/mol – 3 double bonds would give off 356 kJ/mol
Two conjugated double bonds in cyclohexadiene add 2 H2 to give off 230 kJ/mol
Benzene has 3 unsaturation sites but gives off only 206 kJ/mol on reacting with 3 H2 molecules
Therefore it has about 150 kJ more “stability” than an isolated set of three double bonds

4. Energy diagram for benzene hydrogenation
Hypothetical
cyclohexatriene
Difference = 150 kJ/mol
G0 expected
Benzene
G0 observed
Cyclohexane
REACTION PROGRESS

5. Benzene’s Unusual Structure
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
C–C–C bond angles 120°
Each C is sp2 and has a p orbital perpendicular to the plane of the six-membered ring

6. Molecular Orbital Description of Benzene
The 6 p-orbitals combine to give 3 bonding orbitals with 6  electrons and 3 antibonding with no electrons
Orbitals with the same energy are degenerate

7. Benzene molecular models
C-C bond = 1.39 Angstroem
All bond angles = 120

8. Other aromatic hydrocarbons

9. When molecule is aromatic?
Cyclic, planar molecule with conjugated system of p orbitals
containing 4n + 2 p electrons
where n = 0, 1, 2, 3… (Hückel rule)
10 p electrons - aromatic

10. Why 4n +2?
When electrons fill the various molecular orbitals, it takes two electrons (one pair) to fill the lowest-lying orbital and four electrons (two pairs) to fill each of n succeeding energy levels
This is a total of 4n + 2

11. When molecule is aromatic?
14 p electrons - aromatic

12. When molecule is aromatic?
6 p electrons - aromatic

13. When molecule is aromatic?
6 p electrons - aromatic

14. When molecule is aromatic?
Nonplanar
8 p electrons
nonaromatic

15. When molecule is aromatic?
Cation is planar but
only 4 p electrons
- nonaromatic
Anion is planar,
Contains 6 p electrons –
aromatic

16. When molecule is aromatic?
6 p electrons but nonplanar - nonaromatic
6 p electrons, planar - aromatic

17. Electrophilic aromatic substitution
Halogenation
Acylation
Nitration
Alkylation
Sulfonation

18. Halogenation of benzene

19. Mechanism of the electrophilic bromination
of benzene
The product is formed by loss of a proton, which is replaced by bromine
FeBr3 is added as a catalyst to polarize the bromine reagent
In the first step the  electrons act as a nucleophile toward Br2 (in a complex with FeBr3)
This forms a cationic addition intermediate from benzene and a bromine cation
The intermediate is not aromatic and therefore high in energy

20. Formation of Product from Intermediate
The cationic addition intermediate transfers a proton to FeBr4- (from Br- and FeBr3)
This restores aromaticity (in contrast with addition in alkenes)

21. Other Aromatic Halogenations
Chlorine and iodine (but not fluorine, which is too reactive) can produce aromatic substitution with the addition of other reagents to promote the reaction
Chlorination requires FeCl3
Iodine must be oxidized to form a more powerful I+ species (with Cu2+ from CuCl2)

22. Aromatic Nitration
The combination of nitric acid and sulfuric acid produces NO2+ (nitronium ion)
The reaction with benzene produces nitrobenzene

23. The Nitro group can be reduced to an Amino group if needed

24. Aromatic Sulfonation Substitution of H by SO3H (sulfonation)
Reaction with a mixture of sulfuric acid and SO3 (“Fuming” H2SO4)
Reactive species is sulfur trioxide or its conjugate acid

25. Sulfonamides are “sulfa drugs”

26. Alkylation of Aromatic Rings: The Friedel–Crafts Reaction
Alkylation among most useful electrophilic aromatic substitution reactions
Aromatic substitution of R for H
Aluminum trichloride promotes the formation of the carbocation

27. Limitations of the Friedel-Crafts Alkylation
Only alkyl halides can be used
Aryl halides and vinylic halides do not react (their carbocations are too hard to form)
Will not work with rings containing
an amino group substituent or a strongly
electron-withdrawing group

28. Other Problems with Alkylation
Multiple alkylations can occur because the first alkylation is activating
Carbocation rearrangements occur during alkylation

29. Acylation of Aromatic Rings
Reaction of an acid chloride (RCOCl) and an aromatic ring
in the presence of AlCl3 introduces acyl group, COR
Avoids many of the problems of alkylation
Only substitutes once, because acyl group is deactivating
No rearrangement because of resonance stabilized cation

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

31. Can reduce carbonyl to get alkyl product
Propiophenone Propylbenzene

32. Substituent Effects in Aromatic Rings
Substituents can cause a compound to be (much) more or (much) less reactive than benzene
Substituents affect the orientation of the reaction – the positional relationship is controlled

33. Three Kinds of Substituent Effects
ortho- and para-directing activators,
ortho- and para-directing deactivators,
meta-directing deactivators.

34. Reactivity in electrophilic substitution
Y – electron donor
ACTIVATOR
Y – electron acceptor
DEACTIVATOR

35. Origins of Substituent Effects
An interplay of inductive effects and resonance effects
Inductive effect - withdrawal or donation of electrons through a σ bond = Polar Covalent Bonds
Resonance effect - withdrawal or donation of electrons through a  bond due to the overlap of a p orbital on the substituent with a p orbital on the aromatic ring

36. Inductive Effects
Controlled by electronegativity and the polarity of bonds in functional groups
Halogens, C=O, CN, and NO2 withdraw electrons through σ bond connected to ring
Alkyl groups donate electrons

37. Resonance Effects – Electron Withdrawal
C=O, CN, NO2 substituents withdraw electrons from the aromatic ring by resonance
 electrons flow from the rings to the substituents
Look for a double (or triple) bond connected to the ring by a single bond

38. Resonance Effects – Electron Donation
Halogen, OH, alkoxyl (OR), and amino substituents donate electrons
 electrons flow from the substituents to the ring
Effect is greatest at ortho and para positions
Look for a lone pair on an atom attached to the ring

39. An Explanation of Substituent Effects
Activating groups donate electrons to the ring, stabilizing the carbocation intermediate
Deactivating groups withdraw electrons from the ring, destabilizing carbocation intermediate

40. Ortho/Para-Directing Activators: Alkyl Groups
Alkyl groups activate by induction: direct further substitution to positions ortho and para to themselves
Alkyl group has most effect on the ortho and para positions

41. Ortho/Para-Directing Activators: OH and NH2
Hydroxyl, alkoxyl, and amino groups have a strong, electron-donating resonance effect
Most pronounced at the ortho and para positions

42. Ortho/Para-Directing Deactivators: Halogens
Electron-withdrawing inductive effect outweighs weaker electron-donating resonance effect
Resonance effect is only at the ortho and para positions, stabilizing carbocation intermediate

43. Meta-Directing Deactivators
Inductive and resonance effects reinforce each other
Ortho and para intermediates destabilized by deactivation of carbocation intermediate
Resonance cannot produce stabilization

44. Summary Table

45. Summary

46. Nitration of substituted benzenes
halogenobenzene
halogenonitrobenzene X
ortho
meta
para -F 13 1 86
-Cl 35 64 Br 43 56 I 45 54

47. Nitration of substituted benzenesX
ortho
meta
para
-CH3 63 3 34
-OH 50
-NHCOCH3 19 2 79

48. Nitration of substituted benzenesX
ortho
meta
para
-N+(CH3)3 2 87 11
-NO2 7 91
-COOH 22 77 1
-CN 17 81
-COOC2H5 28 66 6
-COCH3 26 72
-CHO 19 9

49. Oxidation of Aromatic Compounds
Alkyl side chains can be oxidized to CO2H by strong reagents such as KMnO4 if they have a C-H next to the ring
Converts an alkylbenzene into a benzoic acid
A benzylic C-H bond is required, or no reaction takes place

50. Bromination of Alkylbenzene Side Chains
Reaction of an alkylbenzene with N-bromo-succinimide (NBS) and benzoyl peroxide (radical initiator) introduces Br into the side chain only at benzylic position

51. Radical Mechanism
Resonance stabilization of benzylic radical

52. Reduction of Aromatic Compounds
Aromatic rings are inert to catalytic hydrogenation under conditions that reduce alkene double bonds
Can selectively reduce an alkene double bond in the presence of an aromatic ring
Reduction of an aromatic ring requires more powerful reducing conditions (high pressure or rhodium catalysts)