1. Chapter 5 Aromatic Compounds
Suggested Problems: 22,24,26b-f,27,32-35,37-8,41-2,47-8,55,58

2. Aromatic Compounds
Aromatic was once used to described some fragrant compounds in early 19th century
Not correct: later they were grouped by chemical behavior (unsaturated compounds that undergo substitution rather than addition)
Current: distinguished from aliphatic compounds by electronic configuration

3. 5.1 Structure of Benzene
Benzene reacts only very slowly with Br2 to give bromobenzene (where Br replaces H)
This is substitution rather than the rapid addition reaction common to compounds with C=C, suggesting that, in benzene, there is a higher energy barrier to reaction

4. 5.2 Naming Aromatic Compounds
Many common names (toluene = methylbenzene; aniline = aminobenzene; phenol = hydroxybenzene)
Monosubstituted benzenes have systematic names as hydrocarbons with –benzene
C6H5Br = bromobenzene
C6H5NO2 = nitrobenzene, and C6H5CH2CH2CH3 is propylbenzene

5. The Phenyl Group
When a benzene ring is a substituent, the term phenyl is used (for C6H5—)
You may also see “Ph” or “f” in place of “C6H5”
“Benzyl” refers to “C6H5CH2—”

6. Disubstituted Benzenes
Relative positions on a benzene ring
ortho- (o) substituents on adjacent carbons (1,2)
meta- (m) substituents separated by one carbon (1,3)
para- (p) substituents separated by two carbons (1,4)
Describes reaction patterns (“occurs at the para position”)

7. 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

8. 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 (as in TNT)

9. 5.3 Electrophilic Aromatic Substitution Reactions: Bromination
Benzene is aromatic: a cyclic conjugated compound with 6  electrons
Reactions of benzene lead to the retention of the aromatic core

10. 5.3 Electrophilic Aromatic Substitution Reactions: Bromination
Benzene’s  electrons participate in a nucleophilic fashion as a Lewis base in reactions with Lewis acids
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 (AlBr3 behaves similarly)

11. Addition Intermediate in Bromination
The addition of bromine occurs in two steps
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

12. Electrophilic Bromination: Energy Diagram

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

14. 5.4 Other Aromatic Substitutions
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 or AlCl3
Iodine must be oxidized to form a more powerful I+ species (with Cu+ or peroxide)

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

16. Aromatic Sulfonation Substitution of H by SO3H (sulfonation)
Reaction with a mixture of sulfuric acid and SO3
Reactive species is sulfur trioxide or its conjugate acid
This process is an equilibrium

17. 5.5 The Friedel–Crafts Alkylation and Acylation Reactions
Alkylation is among the most useful electrophilic aromatic subsitution reactions
Aromatic substitution of R+ for H+
Aluminum chloride promotes the formation of the carbocation

18. 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)
Will not work with rings containing an amino group substituent or a strongly electron-withdrawing group

19. Acylation of Aromatic Rings
Reaction of an acid chloride (RCOCl) and an aromatic ring in the presence of AlCl3 introduces an acyl group, COR
Benzene with acetyl chloride yields acetophenone

20. 5.6 Substituent Effects in Electrophilic Aromatic Substitution
Substituents can cause a compound to be (much) more or (much) less reactive than benzene
Substituents affect the orientation of the reaction as well – the positional relationship is controlled
ortho- and para-directing activators, ortho- and para-directing deactivators, and meta-directing deactivators

21. 5.7 An Explanation of Substituent Effects
Activating groups donate electrons into the ring, stabilizing the intermediate carbocation
Deactivating groups withdraw electrons from the ring, destabilizing the carbocation intermediate

22. Ortho- and Para-Directing Activators: Alkyl Groups
Alkyl groups activate: direct further substitution to positions ortho and para to themselves
Alkyl group is most effective in the ortho and para positions

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

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

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

26. Summary Table: Effect of Substituents in Aromatic Substitution

27. Summary Graph

28. 5.8 Oxidation and Reduction 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

29. 5.8 Oxidation and Reduction of Aromatic Compounds
Just as aromatic rings are inert to oxidation,they are also inert to reduction under typical alkene conditions
High temperatures and pressures are required in order to reduce aromatic rings.

30. 5.9 Other Aromatic Compounds
Aromatic compounds can have rings that share a set of carbon atoms (fused rings)
Compounds from fused benzene or aromatic heterocycle rings are themselves aromatic

31. Naphthalene Orbitals
Three resonance forms and delocalized electrons

32. 5.9 Other Aromatic Compounds
Heterocyclic compounds contain elements other than carbon in a ring, such as N,S,O,P
Aromatic compounds can have elements other than carbon in the ring
There are many heterocyclic aromatic compounds and many are very common
Cyclic compounds that contain only carbon are called carbocycles
Nomenclature is specialized

33. Pyridine A six-membered heterocycle with a nitrogen atom in its ring
 electron structure resembles benzene (6 electrons)
The nitrogen lone pair electrons are not part of the aromatic system (perpendicular orbital)
Pyridine is a relatively weak base compared to normal amines but protonation does not affect aromaticity

34. Pyrrole A five-membered heterocycle with one nitrogen
Four sp2-hybridized carbons with 4 p orbitals perpendicular to the ring and 4 p electrons
Nitrogen atom is sp2-hybridized, and lone pair of electrons occupies a p orbital (6  electrons)
Since lone pair electrons are in the aromatic ring, protonation destroys aromaticity, making pyrrole a very weak base

35. 5.10 Organic Synthesis Organic synthesis creates molecules by design
Synthesis can produce new molecules that are needed as drugs or materials
Syntheses can be designed and tested to improve efficiency and safety for making known molecules
Highly advanced synthesis is used to test ideas and methods, answering challenges
Chemists who engage in synthesis may see some work as elegant or beautiful when it uses novel ideas or combinations of steps – this is very subjective and not part of an introductory course

36. Synthesis as a Tool for Learning Organic Chemistry
In order to propose a synthesis you must be familiar with reactions
What they begin with
What they lead to
How they are accomplished
What the limitations are
A synthesis combines a series of proposed steps to go from a defined set of reactants to a specified product
Questions related to synthesis can include partial information about a reaction of series that the student completes

37. Strategies for Synthesis
Compare the target and the starting material
Consider reactions that efficiently produce the outcome. Look at the product and think of what can lead to it
Read the practice problems in the text

38. Synthesizing a Substituted Aromatic Compound
Synthesize p-bromobenzoic acid starting from benzene.
Strategy:
Work backward by first asking, “What is an immediate precursor of p-bromobenzoic acid?”.
Continue this process until benzene is the starting material.
Note: There is usually more than one synthetic route possible.

39. Synthesizing a Substituted Aromatic Compound: Solution
Two workable routes are possible.