Chapter 15, Lecture 3, Organic 2
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Transcript of Chapter 15, Lecture 3, Organic 2
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Chapter 15 2
Effects of Substituents on Reactivity and Orientation
The nature of groups already on an aromatic ring affect both thereactivity and orientation of future substitution. We say, therefore,that substituent groups affect both reactivity and orientation inEAS
Electron-donating substituents on a benzene ring make thering react more rapidly than benzene, and are therefore calledactivating groups.
Furthermore, most activating groups cause the location ofelectrophilic aromatic substitution to be ortho and/or para tothe activating group. Therefore, activating groups are ortho-para directing groups.
Electron-withdrawing substituents on a benzene ring makethe ring react less rapidly than benzene, and are therefore called
deactivating groups.
-- Most deactivating groups cause the location of electrophilicaromatic substitution to be meta to the deactivating group.Therefore, most deactivating groups are meta directors.
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Activating Groups: Ortho-Para Directors
All activating groups are also ortho-para directors
The halides are also ortho-para directors but are mildlydeactivating
The methyl group of toluene is an ortho-para director
Toluene reacts more readily than benzene, we observe the greater
reactivity of toluene in several ways:
1) With toluene, milder conditions (lower temp. and lower conc.of the electrophile ) can be used in EAS than with benzene.
2) Under the same conditions toluene reacts faster than benzene.
EX: In nitration, toluene reacts 25 times as fast as benzene.
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Chapter 15 4
The methyl group of toluene is an ortho-para director.
Example: When we nitrate toluene with nitric and sulfuric
acid, we get mononitrotoluenes in the following relativeproportions
The same behaviour is observed in halogenation, sulfonation
and so forth.
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Alkyl groups are not the only activating groups. Others include
groups containing oxygen or nitrogen atoms directly bonded to the
ring, such as hydroxyl, alkoxyl, amino and amide or ester groups with
the nitrogen or oxygen bonded to the benzene ring.
Example: The hydroxyl group and the amino group are very powreful
activating groups and are also ortho-para directors.
Phenol and aniline react with bromine in water to produce products in
which both of the ortho positions and the para positions are
substituted.
Alkyl groups and atoms with one or more unshared electron pairs directly bonded to
the aromatic ring will be ortho-para directors .
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Chapter 15 6
Deactivating Groups: Meta Directors
The nitro group is a very strong deactivating group.
Nitrobenzene undergoes nitration at a rate only 10-4
times that ofbenzene. The nitro group is a meta director.
When nitrobenzene is nitrated with nitric acid and sulfuric acids, 93%
of the substitution occurs at the meta position.
The carboxyl group (-CO2H), the sulfonic acid group (-SO3H), andthe trifluoromethyl group (-CF3) are also deactivating groups; they are
also meta directing groups.
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Halo Substitutents: Deactivating Ortho-Para DirectorsThe chloro and bromo groups are ortho-para directors . However ,
even though they contain unshared electron pairs, they are deactivating
toward electrophilic aromatic substitution because of the electronegativeeffect of the halogens.
Chlorobenzene and bromobenzene, undergo nitration at a rate
approximately 30 times slower than benzene.
The relative percentages of monosubstituted products that are
obtained when chlorobenzene is chlorinated, brominated, nitrated, or
sulfonated are shown in the following table.
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Chapter 15 8
Classification of Substituents
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The following five rules summarize the aforementioned
table:
1) Activating substituents are ortho, para directors.
2) Ortho, para directors, except for alkyl, aryl, and vinyl
groups, have nonbonding electrons on the atom attached
to the aromatic ring.
3) Deactivating substituents are meta directors.
4) Meta directing groups have at least a partial positive
charge on the atom that bonds to the ring carbons.
5) Halogens are an exception to the above rules. They are
deactivating, but are ortho, para directing groups, andthey have nonbonding electrons.
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An activating group activates all positions to the
benzene ring even the positions meta to it. It
directs ortho and para simply because it activates
the ortho and para positions much more than it
does to the meta.
A deactivating group deactivates all positions inthe ring even the positions meta to it. It directs
meta simply because it deactivates the ortho and
para positions even more than it does to the
meta. Then the effect of any group whether activating
or deactivating- is strongest at the ortho and para
positions.
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Chapter 15 11
Theory of Substituent Effects on Electrophilic Substitution
Reactivity: The Effect of Electron-Releasing and Electron-
Withdrawing Groups
Any factor that increases the energy of the transition staterelative to that of the reactants decreases the relative rate of the
reaction.
It does this because it increases the energy of activation of the
reaction.
In the same way, any factor that decreases the energy of thetransition state relative to that of the reactants lowers the
energy of activation and increases the relative rate of the
reaction.
The rate-determining step in EAS of substituted benzenes is
the step that results in the formation of the arenium ion.
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IfQ is an electron-releasing group, the reaction occurs faster than the
corresponding reaction of benzene.
IfQ is an electron-withdrawing group, the reaction is slower than that
of benzene.
It appears, then, that the substituent (Q) must affect the stability of the
transition state relative to that of the reactants.
Electron-releasing groups apparently make the transition state more
stable. Electron-withdrawing groups apparently make the transition state less
stable.
Since the arenium ion is positively charged, we would expect the
following remarks:
An electron-releasing group stabilizes the arenium ion and thetransition state leading to it.
An electron-withdrawing group make the arenium ion less stable, and
in a corresponding way it should make the transition state leading to
the arenium ion less stable.