1. 9-1 © 2005 John Wiley & Sons, Inc. All rights reserved Chapter 9: Benzene and its Derivatives

2. 9-2 © 2005 John Wiley & Sons, Inc. All rights reserved Benzene and Its Derivatives

3. 9-3 © 2005 John Wiley & Sons, Inc. All rights reserved 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. 9-4 © 2005 John Wiley & Sons, Inc. All rights reserved 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. 9-5 © 2005 John Wiley & Sons, Inc. All rights reserved 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. 9-6 © 2005 John Wiley & Sons, Inc. All rights reserved 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. 9-7 © 2005 John Wiley & Sons, Inc. All rights reserved 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 C 6 H 6. It has alternating three carbon-carbon double and three single bonds. Benzene’s relatively lack of chemical reactivity is due to its structure.

8. 9-8 © 2005 John Wiley & Sons, Inc. All rights reserved 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. 9-9 © 2005 John Wiley & Sons, Inc. All rights reserved 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. 9-10 © 2005 John Wiley & Sons, Inc. All rights reserved 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. 9-11 © 2005 John Wiley & Sons, Inc. All rights reserved 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. 9-12 © 2005 John Wiley & Sons, Inc. All rights reserved 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. 9-13 © 2005 John Wiley & Sons, Inc. All rights reserved 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. 9-14 © 2005 John Wiley & Sons, Inc. All rights reserved Heterocyclic Aromatics Heterocyclic compound: Heterocyclic compound: a compound that contains one or more atoms other than carbon in its ring Heterocyclic aromatic compound: Heterocyclic aromatic compound: a heterocyclic compound whose ring is aromatic pyridine and pyrimidine are heterocyclic analogs of benzene; each is aromatic

15. 9-15 © 2005 John Wiley & Sons, Inc. All rights reserved Pyridine

16. 9-16 © 2005 John Wiley & Sons, Inc. All rights reserved Concept of Aromaticity the five-membered-ring compounds furan, pyrrole, and imidazole are also aromatic

17. 9-17 © 2005 John Wiley & Sons, Inc. All rights reserved Concept of Aromaticity Nature abounds in compounds with a heterocyclic aromatic ring fused to another aromatic ring

18. 9-18 © 2005 John Wiley & Sons, Inc. All rights reserved Nomenclature Monosubstituted alkylbenzenes are named as derivatives of benzene many common names are retained

19. 9-19 © 2005 John Wiley & Sons, Inc. All rights reserved Aromatic Compounds in Nature and Health Many aromatic compounds are common in nature and in medicine.

20. 9-20 © 2005 John Wiley & Sons, Inc. All rights reserved 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. methylbenzenechlorobenzene (toluene)

21. 9-21 © 2005 John Wiley & Sons, Inc. All rights reserved 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. 9-22 © 2005 John Wiley & Sons, Inc. All rights reserved 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. 9-23 © 2005 John Wiley & Sons, Inc. All rights reserved 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

24. 9-24 © 2005 John Wiley & Sons, Inc. All rights reserved

25. 9-25 © 2005 John Wiley & Sons, Inc. All rights reserved Learning Check

26. 9-26 © 2005 John Wiley & Sons, Inc. All rights reserved Learning Check

27. 9-27 © 2005 John Wiley & Sons, Inc. All rights reserved Learning Check Chlorobenzene Meta-dimethylbenzene

28. 9-28 © 2005 John Wiley & Sons, Inc. All rights reserved PAHs Polynuclear aromatic hydrocarbon (PAH) Polynuclear aromatic hydrocarbon (PAH) a hydrocarbon that contain two or more benzene rings, with each pair of rings sharing two adjacent carbon atoms

29. 9-29 © 2005 John Wiley & Sons, Inc. All rights reserved Reactions of Benzene The most characteristic reaction of aromatic compounds is substitution at a ring carbon

30. 9-30 © 2005 John Wiley & Sons, Inc. All rights reserved Reactions of Benzene

31. 9-31 © 2005 John Wiley & Sons, Inc. All rights reserved 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. 9-32 © 2005 John Wiley & Sons, Inc. All rights reserved Phenols Some phenols found in nature

33. 9-33 © 2005 John Wiley & Sons, Inc. All rights reserved Phenols as Antioxidants Vitamin E is a natural antioxidant BHT and BHA are synthetic antioxidants