1. 13 13-1 © 2003 Thomson Learning, Inc. All rights reserved General, Organic, and Biochemistry, 7e Bettelheim, Brown, and March

2. 13 13-2 © 2003 Thomson Learning, Inc. All rights reserved Chapter 13 Benzene and its Derivatives Benzene and its Derivatives

3. 13 13-3 © 2003 Thomson Learning, Inc. All rights reserved Aromatic Compounds Aromatic compound: Aromatic compound: a hydrocarbon that contains one or more benzene-like rings arene:arene: a term used to describe aromatic compounds Ar-:Ar-: a symbol for an aromatic group derived by removing an -H from an arene Kekulé structure for benzene (1872)

4. 13 13-4 © 2003 Thomson Learning, Inc. All rights reserved Benzene Resonance structure for benzene (1930s) the theory of resonance, developed by Linus Pauling, provided the first adequate description of the structure of benzene resonance hybridaccording to the theory of resonance, certain molecules and ions are best described by writing two or more Lewis structures; the real molecule or ion is a resonance hybrid of these structures contributing structureeach individual Lewis structure is called a contributing structure we show that the real molecule is a resonance hybrid of the two or more Lewis structures by using a double- headed arrow between them

5. 13 13-5 © 2003 Thomson Learning, Inc. All rights reserved Benzene here are two contributing structures for benzene the resonance hybrid has some of the characteristics of each Lewis contributing structure the length of a carbon-carbon bond in benzene, for example, is midway between that of a carbon-carbon single bond and a double bond

6. 13 13-6 © 2003 Thomson Learning, Inc. All rights reserved Nomenclature monosubstituted alkylbenzenes are named as derivatives of benzene; for example, ethylbenzene the IUPAC system retains certain common names for several of the simpler monosubstituted alkylbenzenes;

7. 13 13-7 © 2003 Thomson Learning, Inc. All rights reserved Nomenclature the common names for these monosubstituted benzenes are also retained phenyl group (C 6 H 5 - or Ph-):phenyl group (C 6 H 5 - or Ph-): the substituent group derived by loss of an H from benzene

8. 13 13-8 © 2003 Thomson Learning, Inc. All rights reserved Nomenclature When two substituents occur on a benzene ring, three isomers are possible; they may be located by numbering the atoms of the ring or using the locators ortho (o), meta (m), and para (p)

9. 13 13-9 © 2003 Thomson Learning, Inc. All rights reserved Nomenclature For three or more substituents: if one of the substituents imparts a special name, name the molecule as a derivative of that parent if none of the substituents imparts a special name, number the substituents to give the smallest set of numbers, and list them in alphabetical order before the ending "benzene"

10. 13 13-10 © 2003 Thomson Learning, 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

11. 13 13-11 © 2003 Thomson Learning, Inc. All rights reserved Reactions of Benzene By far the most characteristic reaction of aromatic compounds is substitution at a ring carbon aromatic substitutionthis reaction is called aromatic substitution some groups that can be introduced directly on the ring are the halogens, the nitro (-NO 2 ) group, and the sulfonic acid (-SO 3 H) group Halogenation

12. 13 13-12 © 2003 Thomson Learning, Inc. All rights reserved Reactions of Benzene Nitration a value of nitroarenes is that the nitro group can be reduced to a primary amino group

13. 13 13-13 © 2003 Thomson Learning, Inc. All rights reserved Reactions of Benzene Sulfonation an application of sulfonation is in the preparation of synthetic detergents

14. 13 13-14 © 2003 Thomson Learning, 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

15. 13 13-15 © 2003 Thomson Learning, Inc. All rights reserved Phenols Some phenols found in nature

16. 13 13-16 © 2003 Thomson Learning, Inc. All rights reserved Phenols as Antioxidants Autoxidation: Autoxidation: a reaction that converts an R-H group to an R-O-O-H (hydroperoxide)

17. 13 13-17 © 2003 Thomson Learning, Inc. All rights reserved Phenols as Antioxidants Autoxidation is a radical chain reaction chain initiation:chain initiation: formation of a radical from a nonradical compound

18. 13 13-18 © 2003 Thomson Learning, Inc. All rights reserved Phenols as Antioxidants chain propagation:chain propagation: reaction of a radical to form a new radical propagation step 1 propagation step 2

19. 13 13-19 © 2003 Thomson Learning, Inc. All rights reserved Phenols as Antioxidants Hydroperoxides are unstable under biological conditions, they degrade to short- chain aldehydes and carboxylic acids with unpleasant "rancid" smells similar formation of hydroperoxides in the low-density lipoproteins deposited on the walls of arteries leads to cardiovascular disease in humans in addition, many effects of aging are thought to be the result of hydroperoxide formation and their subsequent degradation

20. 13 13-20 © 2003 Thomson Learning, Inc. All rights reserved Phenols as Antioxidants vitamin E is a natural antioxidant BHT and BHA are synthetic antioxidants these compounds are radical scavengers they form stable radicals and thus break the cycle of chain propagation steps; they prevent further formation of destructive hydroperoxides

21. 13 13-21 © 2003 Thomson Learning, Inc. All rights reserved End Chapter 13 Benzene and its Derivatives