Chapter 4Chapter 4

Benzene and Its Derivatives

Aromatic CompoundsAromatic CompoundsAromatic 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).

BenzeneBenzene General properties: Display aromaticity. The carbon-hydrogen ratio is high. They burn with a sooty yellow flame because of the

high carbon-hydrogen ratio. They undergo electrophilic subsitution and

nucleophilic substitution Give sweet smell

BenzeneBenzeneResonance 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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Resonanance contributor Resonanance contributor

Resonance hybridThe real structure

electron delocation

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.

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.

NomenclatureNomenclature◦ The common names for these monosubstituted

benzenes are also retained

NomenclatureNomenclature Phenyl group (CPhenyl group (C66HH55- or Ph-):- or Ph-): The substituent group

derived by removal of an H from benzene.

Ph

3-methyl-2-phenyl-2-pentene

NomenclatureNomenclatureWhen 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).

NomenclatureNomenclatureFor 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".

ExamplesExamplesName the following compounds

CH3

Cl

OHI

COOH

NO2

ExamplesExamplesDraw the structure of the

following compoundsa. 2-phenyl-4-hexyneb.m-ethylphenol

PAHsPAHs

Polynuclear aromatic hydrocarbon (PAH)Polynuclear aromatic hydrocarbon (PAH)◦A hydrocarbon that contains two or more

benzene rings, with each pair of rings sharing two adjacent carbon atoms.

Reactions of BenzeneReactions of BenzeneBy 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:

Reactions of BenzeneReactions of BenzeneNitration:

◦ A value of nitroarenes is that the nitro group can be reduced to a primary amino group.

Reactions of BenzeneReactions of BenzeneSulfonation:

◦ An application of sulfonation is in the preparation of synthetic detergents.

The effects of subsituents on The effects of subsituents on Reactivity of a Benzene ringReactivity of a Benzene ring

Make benzene more reactive toward substitution

Make benzene less reactive toward substitution

ExampleExampleWhat product(s) would result

from the following compoundsCl

H2SO4

heat+ HNO3

chlobenzene

COOH

+ Cl2

FeCl3

Benzoic acid

ExamplesExamplesPredict the products of the

following reaction

+ H2SO4

heat

H2SO4

heat+ HNO3

PhenolsPhenolsThe functional group of a phenol is a hydroxyl ( -OH) group bonded to a benzene ring.◦ Name substituted phenols either as derivatives of

phenol or by common names.

PhenolsPhenols Most phenols are weak acids, with pKa values

approximately 10. They are insoluble in water but react with strong

bases, such as NaOH and KOH to form water-soluble salts.

PhenolsPhenolsSome phenols found in nature.

Phenols as AntioxidantsPhenols as Antioxidants

Autoxidation:Autoxidation: A reaction that converts an R-H group to an R-O-O-H (hydroperoxide).

Phenols as AntioxidantsPhenols as Antioxidants

Autoxidation is a radical chain reaction:◦Radical: An atom of molecule with an

unpaired electron.◦ Chain initiation:Chain initiation: Formation of a radical

from a nonradical compound.

Phenols as AntioxidantsPhenols as AntioxidantsChain propagation:Chain propagation: Reaction of a radical to form a

new radical.

Propagation step 1:

◦ Propagation step 2:

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.

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.