chapter 4 benzene and its derivatives. aromatic compounds aromatic compound: aromatic compound: a...
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Chapter 4Chapter 4
Benzene and Its Derivatives
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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).
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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
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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.
12 1
2
Resonanance contributor Resonanance contributor
Resonance hybridThe real structure
electron delocation
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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.
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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.
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NomenclatureNomenclature◦ The common names for these monosubstituted
benzenes are also retained
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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
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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).
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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".
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ExamplesExamplesName the following compounds
CH3
Cl
OHI
COOH
NO2
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ExamplesExamplesDraw the structure of the
following compoundsa. 2-phenyl-4-hexyneb.m-ethylphenol
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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.
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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:
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Reactions of BenzeneReactions of BenzeneNitration:
◦ A value of nitroarenes is that the nitro group can be reduced to a primary amino group.
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Reactions of BenzeneReactions of BenzeneSulfonation:
◦ An application of sulfonation is in the preparation of synthetic detergents.
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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
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ExampleExampleWhat product(s) would result
from the following compoundsCl
H2SO4
heat+ HNO3
chlobenzene
COOH
+ Cl2
FeCl3
Benzoic acid
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ExamplesExamplesPredict the products of the
following reaction
+ H2SO4
heat
H2SO4
heat+ HNO3
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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.
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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.
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PhenolsPhenolsSome phenols found in nature.
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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).
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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.
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Phenols as AntioxidantsPhenols as AntioxidantsChain propagation:Chain propagation: Reaction of a radical to form a
new radical.
Propagation step 1:
◦ Propagation step 2:
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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.