1. Chapter 9: Benzene and its Derivatives

2. Benzene and Its Derivatives

3. Benzene - Kekulé
The first structure for benzene was proposed by August Kekulé
this structure, however, did not account for the unusual chemical reactivity of benzene

4. Benzene - Orbital Overlap Model
The concepts of hybridization of atomic orbitals and the theory of resonance, developed in the 1930s, provided the first adequate description of benzene’s structure
the carbon skeleton is a regular hexagon, with all C-C-C and H-C-C bond angles 120°

5. Benzene - Resonance Model
We often represent benzene as a hybrid of two equivalent Kekulé structures
each Kekulé structure makes an equal contribution to the hybrid
the C-C bonds are neither double nor single but something in between

6. Benzene
Alas, benzene is both toxic and carcinogenic. In fact, it might be considered "the mother of all carcinogens," as a large number of carcinogens have structures that include benzene rings. My mentor’s Undergraduate Organic Chemistry professor joked about how he used to "practically bathe in benzene up to the elbows" when he would use it in his research (presumably before it was identified as a carcinogen). He predicted that this would probably lead to his demise. He was right--he died due to leukemia several years ago.

7. Aromatic Compounds and the Structure of Benzene
In the early days the word aromatics was used to described many fragrant molecules isolated from natural sources. Today the term aromatic is used to describe benzene like molecules.
Benzene is a flat, symmetrical molecule with the molecular formula C6H6.
It has alternating three carbon-carbon double and three single bonds.
Benzene’s relatively lack of chemical reactivity is due to its structure.

8. Experimental evidence suggest that all six carbon-carbon bonds in benzene are identical.
The properties, including the above one, of benzene can only be explained by assuming that the actual structure of benzene is an average of the above two possible equivalent structures-known as resonance.
Simple aromatic compounds like benzene are non-polar, insoluble in water, volatile, and flammable.
Unlike alkenes, several aromatic hydrocarbons are toxic. Benzene itself is implicated as a cancer causing chemical.

9. Concept of Aromaticity
The underlying criteria for aromaticity were recognized in the early 1930s by Erich Hückel
To be aromatic, a ring must
have one 2p orbital on each atom of the ring
be planar or nearly planar, so that overlap of all 2p orbitals of the ring is continuous or nearly continuous
have 2, 6, 10, 14, 18, and so forth pi electrons in the cyclic arrangement of 2p orbitals
Benzene meets these criteria
it is cyclic, planar, has one 2p orbital on each atom of the ring, and has 6 pi electrons (the aromatic sextet) in the cyclic arrangement of its 2p orbitals

10. Aromaticity and the 4n + 2 Rule
Huckel’s rule, based on calculations – a planar cyclic molecule with alternating double and single bonds has aromatic stability if it has 4n+ 2  electrons (n is 0,1,2,3,4)
For n=1: 4n+2 = 6; benzene is stable and the electrons are delocalized

11. Aromatic Ions
The 4n + 2 rule applies to ions as well as neutral species
Both the cyclopentadienyl anion and the cycloheptatrienyl cation are aromatic
The key feature of both is that they contain 6  electrons in a ring of continuous p orbitals

12. Compounds With 4n  Electrons Are Not Aromatic (May be Antiaromatic)
Planar, cyclic molecules with 4 n  electrons are much less stable than expected (anti-aromatic)
They will distort out of plane and behave like ordinary alkenes
4- and 8-electron

13. Aromatic Heterocycles: Pyridine and Pyrrole
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 (not homocycles)
Nomenclature is specialized

14. Heterocyclic Aromatics
Heterocyclic compound: a compound that contains one or more atoms other than carbon in its ring
Heterocyclic aromatic compound: a heterocyclic compound whose ring is aromatic
pyridine and pyrimidine are heterocyclic analogs of benzene; each is aromatic

15. Pyridine

16. Concept of Aromaticity
the five-membered-ring compounds furan, pyrrole, and imidazole are also aromatic

17. Concept of Aromaticity
Nature abounds in compounds with a heterocyclic aromatic ring fused to another aromatic ring

18. Nomenclature
Monosubstituted alkylbenzenes are named as derivatives of benzene
many common names are retained

19. Aromatic Compounds in Nature and Health
Many aromatic compounds are common in nature and in medicine.

20. Naming Aromatic Compounds
Aromatic compounds are named with benzene as the parent chain. One side group is named in front of the name benzene.
- No number is needed for mono-substituted benzene since all the ring positions are identical.
methylbenzene chlorobenzene
(toluene)

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

22. Nomenclature Disubstituted benzenes
locate substituents by numbering or
use 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

23. Nomenclature Polysubstituted benzenes
with three or more substituents, number the atoms of the ring
if one group imparts a special name, it becomes the parent name
if no group imparts a special name, number to give the smallest set of numbers, and then list alphabetically

25. Learning Check

26. Learning Check

27. Meta-dimethylbenzene
Learning Check
Chlorobenzene
Meta-dimethylbenzene

28. PAHs Polynuclear aromatic hydrocarbon (PAH)
a hydrocarbon that contain two or more benzene rings, with each pair of rings sharing two adjacent carbon atoms

29. Reactions of Benzene
The most characteristic reaction of aromatic compounds is substitution at a ring carbon

30. Reactions of Benzene

31. Phenols
The functional group of a phenol is a hydroxyl group bonded to a benzene ring
name substituted phenols either as derivatives of phenol or by common names

32. Phenols
Some phenols found in nature

33. Phenols as Antioxidants
Vitamin E is a natural antioxidant
BHT and BHA are synthetic antioxidants