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Chapter 4

Chapter 4. Benzene and its Derivatives. Aromatic Compounds. Aromatic compound: a hydrocarbon that contains one or more benzene-like rings arene: a term used to describe aromatic compounds Ar-: a symbol for an aromatic group derived by removing an -H from an arene

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Chapter 4

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  1. Chapter 4 Benzene and its Derivatives

  2. Aromatic Compounds • Aromatic compound: a hydrocarbon that contains one or more benzene-like rings • arene: a term used to describe aromatic compounds • Ar-: a symbol for an aromatic group derived by removing an -H from an arene • Kekulé structure for benzene (1872)

  3. Aromatic Compounds • Experiments show that the Kekulé structure • is not correct. • All C-C bonds are identical and • benzene does not undergo addition reactions • typical of double bonds. • A correct description is given by • resonance theory or by orbital models – valence • bond or molecular orbital.

  4. Benzene • Resonance structure for benzene (1930s) • the theory of resonance, developed by Linus Pauling, provided the first adequate description of the structure of benzene • according 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 • each 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. 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. Delocalized electrons are not confined between two adjacent bonding atoms, but actually extend over three or more atoms. 10.8

  7. Molecules for which you can write resonance structures have an greater stability due to the electron delocalization. Aromatic compounds are particularly stable because of extensive delocalization.

  8. 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;

  9. Nomenclature • the common names for these monosubstituted benzenes are also retained • phenyl group (C6H5- or Ph-): the substituent group derived by loss of an H from benzene

  10. 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)

  11. 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"

  12. Reactions of Benzene • The most characteristic reaction of aromatic compounds is substitution at a ring carbon • this reaction is called aromatic substitution • some groups that can be introduced directly on the ring are the halogens, the nitro (-NO2) group, and the sulfonic acid (-SO3H) group • Halogenation

  13. Reactions of Benzene • Nitration • a value of nitroarenes is that the nitro group can be reduced to a primary amino group

  14. Reactions of Benzene • Sulfonation • an application of sulfonation is in the preparation of synthetic detergents

  15. 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

  16. Phenols • Some phenols found in nature

  17. Acidity of Phenols • Phenols are weak acids but much stronger acids • than alcohol. • They will react with NaOH to produce sodium • salts. • Phenols are not water soluble, but their sodium • salts are.

  18. Phenols as Antioxidants • Autoxidation is a radical chain reaction • chain initiation: formation of a radical from a nonradical compound

  19. Phenols as Antioxidants • chain propagation: reaction of a radical to form a new radical • propagation step 1 • propagation step 2

  20. 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

  21. 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

  22. Benzene and its Derivatives End Chapter 4

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