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General, Organic, and General, Organic, and Biochemistry, 7eBiochemistry, 7e

Bettelheim,Bettelheim,

Brown, and MarchBrown, and March

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Chapter 13Chapter 13

Benzene and its Benzene and its DerivativesDerivatives

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Aromatic CompoundsAromatic 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)

C

CC

C

CC

H

H

H

H

H

H

A Kekulé structureshowing all atoms

A Kekulé structureas a line-angle formula

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BenzeneBenzene• 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 hybridresonance hybrid of these structures

• each individual Lewis structure is called a contributing contributing structurestructure

• 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

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BenzeneBenzene• 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

CC

CCC

C CCC

CCC

H

HH

H

HH H

H

HH

H

H

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NomenclatureNomenclature• 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;

CH2CH3 CH3 CH=CH2

TolueneEthylbenzene Styrene

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NomenclatureNomenclature• the common names for these monosubstituted

benzenes are also retained

• phenyl group (Cphenyl group (C66HH55- or Ph-):- or Ph-): the substituent group derived by loss of an H from benzene

OCH3 C-OHO

NH2OH C-HO

Phenol Aniline Benzoic acidAnisole Benzaldehyde

C6H5

1-Phenylcyclohexene 4-Phenyl-1-butenePhenyl group

124

3

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NomenclatureNomenclature• 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)

COOHBr

CH3

CH3Cl

CH2CH3

1

2-Bromobenzoic acid(o-Bromobenzoic acid)

1,3-Dimethylbenzene(m-Xylene)

1-Chloro-4-ethylbenzene(p-Chloroethylbenzene)

12

2

23

3

4

1

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NomenclatureNomenclature• 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"

CH3NO2

Cl

OH

Br

BrBrNO2

CH2CH3

Br

43

2

1

5

6

43

21

43

1 2

4-Chloro-2-nitrotoluene

2,4,6-Tribromophenol 2-Bromo-1-ethyl-4-nitrobenzene

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PAHsPAHs• 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

PhenanthreneAnthraceneNaphthalene Benzo[a]pyrene

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Reactions of BenzeneReactions of Benzene• By far the most characteristic reaction of

aromatic compounds is substitution at a ring carbon• this reaction is called aromatic substitutionaromatic 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

H Cl2FeCl3 Cl HCl+ +

ChlorobenzeneBenzene

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Reactions of BenzeneReactions of Benzene• Nitration

• a value of nitroarenes is that the nitro group can be reduced to a primary amino group

H HNO3H2SO4

NO2 H2O++

Nitrobenzene

COOHO2N 3H2

NiCOOHH2N 2H2O++

3 atm

4-Nitrobenzoic acid(p-Nitrobenzoic acid)

4-Aminobenzoic acid(p-Aminobenzoic acid, PABA)

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Reactions of BenzeneReactions of Benzene• Sulfonation

• an application of sulfonation is in the preparation of synthetic detergents

H H2SO4 SO3H H2O+

Benzenesulfonic acid

+

CH3(CH2)10CH2

1. H2SO4

2. NaOH

CH3(CH2)10CH2 SO3- Na+

Dodecylbenzene

Sodium 4-dodecylbenzenesulfonate, SDS(an anionic detergent)

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PhenolsPhenols• 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

1,2-Benzenediol(Catechol)

1,3-Benzenediol(Resorcinol)

1,4-Benzenediol(Hydroquinone)

3-Methylphenol(m-Cresol)

Phenol

OH OH OH OH OHOH

OHOH

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PhenolsPhenols• Some phenols found in nature

2-Isopropyl-5-methylphenol

(Thymol)

4-Hydroxy-3-methoxy-benzaldehyde

(Vanillin)Urushiol

(Poison ivy)

OH

HO

H3CO CHO

CH2(CH2)13CH3

OH

OH

NCO

H

CH3O

HO Capsaicin(from various types of peppers)

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Phenols as AntioxidantsPhenols as Antioxidants• Autoxidation:Autoxidation: a reaction that converts an R-H

group to an R-O-O-H (hydroperoxide)

CH2CH=CH-CHH

O2 CH2CH=CH-CH-CH2

O-O-Hlight

or heatSection of a fatty

acid hydrocarbon chain

+Oxygen A hydroperoxide

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Phenols as AntioxidantsPhenols as Antioxidants• Autoxidation is a radical chain reaction

• chain initiation:chain initiation: formation of a radical from a nonradical compound

CH2CH=CH-CHH

CH2CH=CH-CHSection of a fatty

acid hydrocarbon chainA carbon radical

lightor heat

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Phenols as AntioxidantsPhenols as Antioxidants• chain propagation:chain propagation: reaction of a radical to form a new

radical• propagation step 1

• propagation step 2

CH2CH=CH-CH O-O CH2CH=CH-CHO-O

A hydroperoxy radical

+Oxygen is adiradical

CH2CH=CH-CHO-O

CH2CH=CH-CHH

CH2CH=CH-CH-CH2

O-O-HCH2CH=CH-CH

A hydroperoxide

+

+ A new carbon radical

Section of a new fattyacid hydrocarbon chain

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Phenols as AntioxidantsPhenols 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

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Phenols as AntioxidantsPhenols 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

HO

OH

OH OH

OCH3

3

Vitamin E Butylated hydroxy-toluene (BHT)

Butylated hydroxy-anisole (BHA)

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End End Chapter 13Chapter 13

Benzene and its DerivativesBenzene and its Derivatives