Transcript 15_Lecture - Ventura College

Organic Chemistry
6th Edition
Chapter 15
Paula Yurkanis Bruice
Aromaticity •
Reactions of
Benzene
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Aromatic Compounds Are Unusually Stable
Benzene is an aromatic compound
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Benzene is unusually stable because of electron
delocalization
Compounds with unusually large resonance energies,
such as benzene, are called aromatic compounds
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Criteria for Aromaticity
1. A compound must have an uninterrupted cyclic cloud
of p electrons above and below the plane of the
molecule:
2. The p cloud must contain an odd number of pairs
of p electrons, or 4n + 2 (n = 0, 1, 2 …) total
electrons.
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Hückel’s Rule
For a planar, cyclic compound to be aromatic, its
uninterrupted p cloud must contain (4n + 2) p electrons,
where n is any whole number
Why?
Pattern of bonding orbitals of planar cyclic p systems (e.g.,
benzene):
Needs 4n + 2 (n = 0, 1, 2,
3…) electrons to fill orbitals
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Monocyclic hydrocarbons with alternating single and
double bonds are called annulenes:
Cyclobutadiene and cyclooctatetraene are not aromatic,
because they have an even number of p electron pairs
Cyclooctatetraene is a stable compound because it is
large enough to form a tub shape, thereby removing
degenerate orbitals
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not
aromatic
not
aromatic
aromatic
Cyclopentadiene does not have an uninterrupted ring of
p orbital-bearing atoms
Cyclopentadienyl cation has an even number of p
electron pairs
Cyclopentadienyl anion has an uninterrupted ring of p
orbital-bearing atoms and an odd number of p electron
pairs
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The resonance hybrid shows that all the carbons in the
cyclopentadienyl anion are equivalent
Each carbon has exactly one-fifth of the negative charge
associated with the anion
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These compounds consist of fused benzene rings and
are aromatic:
Any compound consisting of fused benzene rings is
aromatic
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Aromatic Heterocyclic Compounds
Heteroatom donates
one electron
Heteroatom donates two electrons
A heterocyclic compound has ring atoms other than
carbon
The heteroatom donates either one or two electrons to
the p system
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Pyridine Is Aromatic
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Pyrrole Is Aromatic
The lone-pair electrons on the nitrogen atom of pyrrole
are p electrons
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Furan and thiophene are aromatic compounds like
pyrrole
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Examples of Heterocyclic
Aromatic Compounds
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DNA & RNA Bases Are
Aromatic
Purine bases
Pyrimidine bases
These heterocycles are aromatic because of amide
resonance:
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The Effect of Aromaticity on the pKa
Values of Some Compounds
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Why is the pKa of cyclopentadiene so much lower than
that of ethane?
The conjugate base is aromatic:
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Aromaticity influences chemical reactivity:
The cycloheptatrienyl cation is aromatic:
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Antiaromaticity
A compound is antiaromatic if it is a planar, cyclic,
continuous loop of p orbitals with an even number of
pairs of p electrons:
Antiaromatic compounds are highly unstable, but the
nonplanar versions are stable
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Antiaromaticity is important in medicine and biochemistry
The tricyclic antipsychotics interact with the dopamine
receptor because of the butterfly shape:
8-electron
ring
Stabilization of either the flat (aromatic) or the butterfly
form influences a flavoenzyme’s redox potential:
8-electron
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A Molecular Orbital Description of
Aromaticity and Antiaromaticity
Aromatic compounds are stable because they have filled
bonding p molecular orbitals:
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Nomenclature of Monosubstituted Benzenes
Some are named by attaching “benzene” after the name
of the substituent:
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Some have to be memorized:
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A benzene substituent is called phenyl.
A benzene substitutuent with a methylene group is
called benzyl.
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Benzene is either the base name or the substituent
(phenyl):
Aryl group (Ar) is the general term for either an
unsubstituted or a substituted phenyl group
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Aromatic compounds such as benzene undergo
electrophilic aromatic substitution reactions:
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Benzene is a nucleophile that reacts with an electrophile
An electrophilic substitution yields an aromatic product,
which is significantly more stable than the addition
reaction
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Reaction Coordinate Diagrams for the
Two Benzene Reactions
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There are five common electrophilic aromatic substitution
reactions:
1. Halogenation
2. Nitration
3. Sulfonation
4. Friedel–Crafts acylation
5. Friedel–Crafts alkylation
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General Mechanism for Electrophilic
Aromatic Substitution of Benzene
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Halogenation of Benzene
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Lewis acid weakens the Br–Br (or Cl–Cl) bond, which
makes the halogen a better electrophile:
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Mechanism for bromination:
B: Bromide or Benzene
The catalyst is regenerated:
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Nitration of Benzene
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Nitronium ion formation:
Electrophilic aromatic substitution:
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Sulfonation of Benzene
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Sulfonic acid is a strong acid:
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Sulfonation is reversible:
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Reaction coordinate diagram for electrophilic
aromatic substitution:
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Friedel–Crafts Acylation Reactions
The electrophile is an acylium ion:
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Mechanism for Friedel–Crafts acylation:
Must be carried out with more than one equivalent of
AlCl3:
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Friedel–Crafts Alkylation of Benzene
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Mechanism for Friedel–Crafts alkylation:
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The carbocation will rearrange to a more stable species:
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However, 100% of the 2-methyl-2-phenylbutane
product can be obtained if a bulky alkyl halide is used:
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Friedel–Crafts alkylation will not produce a good yield
of an alkylbenzene containing a straight-chain group,
because the carbocation will rearrange:
Acylium ions, however, do not rearrange:
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Methodologies Used for
the Reduction Step
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Using Coupling Reactions to Alkylate Benzene
The Gilman reagent:
The Stille reaction:
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The Suzuki reaction:
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One needs to consider an alternative if another
functional group is present in the compound:
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Chemical Modification of Substituents of Benzene
Reactions of alkyl substituents:
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The resulting halide product can undergo a nucleophilic
substitution reaction:
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Remember that halo-substituted alkyl groups can also
undergo E2 and E1 reactions (Section 9.8)
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Substitutions with double and triple bonds can undergo
catalytic hydrogenation (Sections 4.11 and 6.9)
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Oxidation of an alkyl group bonded to a benzene ring…
Provided that a hydrogen is bonded to the benzylic
carbon,
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The same reagent that oxidizes alkyl substituents will
oxidize benzylic alcohols:
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However, aldehydes or ketones can be generated if a
milder oxidizing agent is used:
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Reducing a Nitro Substituent
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It is possible to selectively reduce just one of the two
nitro groups:
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Summary of Electrophilic Aromatic
Substitution Reactions
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Summary of Friedel–Crafts
Reactions
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