Chapter 15, Lecture 3, Organic 2

8/2/2019 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.

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Chapter 15, Lecture 3, Organic 2