1. Heterocyclic Aromatics
Heterocyclic compound: A compound that contains more than one kind of atom in a ring.
In organic chemistry, the term refers to a ring with one or more atoms that differ from carbon.
Pyridine and pyrimidine are heterocyclic analogs of benzene; each is aromatic.

2. Database for unknown compounds

3. Pyridine The nitrogen atom of pyridine is sp2 hybridized.
The unshared pair of electrons lies in an sp2 hybrid orbital and is not a part of the six pi electrons of the aromatic system (the aromatic sextet).
Resonance energy of pyridine is134 kJ (32 kcal)/mol.

4. Furan and Pyrrole The oxygen atom of furan is sp2 hybridized.
one unshared pairs of electrons on oxygen lies in an unhybridized 2p orbital and is a part of the aromatic sextet.
The other unshared pair lies in an sp2 hybrid orbital and is not a part of the aromatic system.
The resonance energy of furan is 67 kJ (16 kcal)/mol.

5. Other Heterocyclics

6. Aromatic Hydrocarbon Ions
Any neutral, monocyclic, unsaturated hydrocarbon with an odd number of carbons must have at least one CH2 group and, therefore, cannot be aromatic.
Cyclopropene, for example, has the correct number of pi electrons to be aromatic, 4(0) + 2 = 2, but does not have a closed loop of 2p orbitals.

7. Cyclopropenyl Cation
If, however, the CH2 group of cyclopropene is transformed into a CH+ group in which carbon is sp2 hybridized and has a vacant 2p orbital, the overlap of orbitals is continuous and the cation is aromatic.

8. Cyclopropenyl Cation
When 3-chlorocyclopropene is treated with SbCl5, it forms a stable salt.
This chemical behavior is to be contrasted with that of 5-chloro-1,3-cyclopentadiene, which cannot be made to form a stable salt.

9. Cyclopentadienyl Cation
If planar cyclopentadienyl cation were to exist, it would have 4 pi electrons and be antiaromatic.
Note that we can draw five equivalent contributing structures for the cyclopentadienyl cation. Yet this cation is not aromatic because it has only 4 pi electrons.

10. Cyclopentadienyl Anion, C5H5-
To convert cyclopentadiene to an aromatic ion, it is necessary to convert the CH2 group to a CH group in which carbon becomes sp2 hybridized and has 2 electrons in its unhybridized 2p orbital.
n = 1

11. Cyclopentadienyl Anion, C5H5-
As seen in the Frost circle, the six pi electrons of cyclopentadienyl anion occupy the p1, p2, and p3 molecular orbitals, all of which are bonding.

12. Cyclopentadienyl Anion, C5H5-
The pKa of cyclopentadiene is 16.
In aqueous NaOH, it is in equilibrium with its sodium salt.
It is converted completely to its anion by very strong bases such as NaNH2 , NaH, and LDA.

13. Cycloheptatrienyl Cation, C7H7+
Cycloheptatriene forms an aromatic cation by conversion of its CH2 group to a CH+ group with its sp2 carbon having a vacant 2p orbital.

14. Nomenclature
Monosubstituted alkylbenzenes are named as derivatives of benzene.
Many common names are retained.

15. Nomenclature
Benzyl and phenyl groups

16. Disubstituted Benzenes
Locate two groups by numbers or by the locators ortho (1,2-), meta (1,3-), and para (1,4-).
Where one group imparts a special name, name the compound as a derivative of that molecule.

17. Disubstituted Benzenes
Where neither group imparts a special name, locate the groups and list them in alphabetical order.

18. Polysubstituted Derivatives
If one group imparts a special name, name the molecule as a derivative of that compound.
If no group imparts a special name, list them in alphabetical order, giving them the lowest set of numbers.

19. Phenols
The functional group of a phenol is an -OH group bonded to a benzene ring.

20. Phenols Hexylresorcinol is a mild antiseptic and disinfectant.
Eugenol is used as a dental antiseptic and analgesic.
Urushiol is the main component of the oil of poison ivy.

21. Acidity of Phenols
Phenols are significantly more acidic than alcohols.

22. Acidity of Phenols
Separation of water-insoluble phenols from water-insoluble alcohols.

23. Acidity of Phenols (Resonance)
The greater acidity of phenols compared with alcohols is due to the greater stability of the phenoxide ion relative to an alkoxide ion.

24. Phenol Subsitituents (Inductive Effect)
Alkyl and halogen substituents effect acidities by inductive effects:
Alkyl groups are electron-releasing.
Halogens are electron-withdrawing.

25. Phenol Subsitituents(Resonance, Inductiion)
Nitro groups increase the acidity of phenols by both an electron-withdrawing inductive effect and a resonance effect.

26. Acidity of Phenols
Part of the acid-strengthening effect of -NO2 is due to its electron-withdrawing inductive effect.
In addition, -NO2 substituents in the ortho and para positions help to delocalize the negative charge.

27. Acidity of Phenols
Phenols are weak acids and react with strong bases to form water-soluble salts.
Water-insoluble phenols dissolve in NaOH(aq).

28. Acidity of Phenols
Most phenols do not react with weak bases such as NaHCO3; they do not dissolve in aqueous NaHCO3.
Carbonic acid is a stronger acid than phenol. Therefore, the position of this equilibrium lies far to the left.

29. Synthesis: Alkyl-Aryl Ethers
Alkyl-aryl ethers can be prepared by the Williamson ether synthesis:
but only using phenoxide salts and haloalkanes.
haloarenes cannot be used because they are unreactive to SN2 reactions.

30. Synthesis: Alkyl-Aryl Ethers

31. Synthesis: Kolbe Carboxylation
Phenoxide ions react with carbon dioxide to give a carboxylate salt.

32. Mechanism: Kolbe Carboxylation
The mechanism begins by nucleophilic addition of the phenoxide ion to a carbonyl group of CO2.
Go back to aromatic structure

33. Synthesis: Quinones
Because of the presence of the electron-donating -OH group, phenols are susceptible to oxidation by a variety of strong oxidizing agents.

34. Quinones

35. Quinones
Readily reduced to hydroquinones.

36. Coenzyme Q
Coenzyme Q is a carrier of electrons in the respiratory chain.

37. Vitamin K
Both natural and synthetic vitamin K (menadione) are 1,4-naphthoquinones.

38. Benzylic Oxidation
Benzene is unaffected by strong oxidizing agents such as H2CrO4 and KMnO4
Halogen and nitro substituents are also unaffected by these reagents.
An alkyl group with at least one hydrogen on its benzylic carbon is oxidized to a carboxyl group.

39. Benzylic Oxidation
If there is more than one alkyl group on the benzene ring, each is oxidized to a -COOH group.

40. Benzylic Chlorination
Chlorination and bromination occur by a radical chain mechanism.

41. Mechanism: Benzylic Reactions
Benzylic radicals (and cations also) are easily formed because of the resonance stabilization of these intermediates.
The benzyl radical is a hybrid of five contributing structures.

42. Benzylic Halogenation
Benzylic bromination is highly regioselective.
Benzylic chlorination is less regioselective.

43. Hydrogenolysis Hydrogenolysis: Cleavage of a single bond by H2
Benzylic ethers are unique in that they are cleaved under conditions of catalytic hydrogenation.

44. Synthesis, Protecting Group: Benzyl Ethers
The value of benzyl ethers is as protecting groups for the OH groups of alcohols and phenols.
To carry out hydroboration/oxidation of this alkene, the phenolic -OH must first be protected; it is acidic enough to react with BH3 and destroy the reagent.