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Chapter 15: Aromaticity & Reactions of Benzene
Aromatic compounds such as benzene are unusually
stable because of electron delocalization (resonance).
The p orbitals overlap around the ring (b) to form a bonding
molecular orbital with electron density above and below the plane
of the ring (c).
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resonance
energy
Resonance energy of benzene
{151 KJ/mole (36 Kcal/mole)}
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The Two Criteria for Aromaticity
(Huckels Rule: The 4n+2p Electron Rule)
1. A compound must have an uninterrupted cyclic cloud
ofT electrons above and below the plane of the
molecule: {cyclic, planar, conjugated (a continuous
system ofp orbitals )}
2. The Tcloud must contain an odd number of pairs
ofTelectrons, or 4n + 2 (n = 0, 1, 2 ) total electrons.Benzene is aromatic: it is planar, cyclic, has a p
orbital at every carbon, and 6 p electrons (n=1). There
is a polygon-and-circle methodfor deriving the
relative energies of orbitals of a system with a cyclic
continuous array ofp orbitals: Aromatic compounds are
stable because they have filled bonding p molecular
orbitals
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Monocyclic hydrocarbons with alternating single and
double bonds are called annulenes:
not aromatic aromatic Even # ofp odd # ofp Even # ofp
electron pairs electron pairs electron pairs
Not stable highly stable stable: tub shape
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not aromatic not aromatic aromatic
The resonance hybrid shows that all the carbons in
the cyclopentadienyl anion are equivalent. Eachcarbon has exactly one-fifth of the negative charge
associated with the anion.
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Aromatic Heterocyclic Compounds
Heteroatom donates two electronsHeteroatom
donates one
electron
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The Effect of Aromaticity on the pKaValues of Some Compounds
Why is the pKa of cyclopentadiene so much lower than that of ethane?
The conjugate base is aromatic:
The cycloheptatrienyl cation is aromatic:
Aromaticity influences chemical reactivity:
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A compound is antiaromatic if it is a planar, cyclic with continuous
loop ofp orbitals and an even number of pairs ofTelectrons:
Antiaromatic compounds are highly unstable, but the nonplanarversions are stable.
Antiaromaticity
benzene cyclopentadienyl cation
cyclobutadienecyclopentadienyl cation
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Nomenclature of Monosubstituted Benzenes
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Reactions of benzene:
Aromatic compounds such as benzene undergo
electrophilic aromatic substitution reactions:
Benzene is a nucleophile that reacts with an
electrophile
Mechanism for Electrophilic Aromatic Substitution of Benzene
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Halogenation of Benzene
The Mechanism:
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Nitration of Benzene
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Sulfonation of Benzene
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FriedelCrafts Acylation Reactions
The electrophile is an acylium ion:
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FriedelCrafts Alkylation of Benzene
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Methodologies Used for the Reduction Step
One needs to consider an alternative if another
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One needs to consider an alternative if another
functional group is present in the compound:
Ch i l M difi ti f S b tit t f B
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Chemical Modification of Substituents of Benzene
Reactions of alkyl substituents:
Remember that halo-substituted alkyl groups can also
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Remember that halo substituted alkyl groups can also
undergo E2 and E1 reactions (Section 9.8)
Substitutions with double and triple bonds can undergo
catalytic hydrogenation (Sections 4.11 and 6.9)
Oxidation of an alkyl group bonded to a benzene ring
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Oxidation of an alkyl group bonded to a benzene ring
Provided that a hydrogen is bonded to the benzylic carbon,
The same reagent that oxidizes alkyl substituents will
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The same reagent that oxidizes alkyl substituents will
oxidize benzylic alcohols:
However, aldehydes or ketones can be generated if a
milder oxidizing agent is used:
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