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.