Aromatic Comp. Lec.3

Dr.M_T The 3rd Vision Academy 01156281369

I

Aromatic Compound

INTERPRETATION OF RATE EFFECTS OF Rx.

The influence of a substituent on both the rate and distribution of

products in electrophilic aromatic substitution reactions can be

understood with one model based on the ability of the substituents to

either donate or withdraw electrons from the aromatic ring.

*Activating gp.

groups which make the aromatic ring reactive relative to benzene.

*Deactivating gp.

groups which make the aromatic ring less reactive than benzene.

STRUCTURAL EFFECTS IN ELECTROPHILIC AROMATIC

SUBSTITUTION;

By examination the effect that a substituent already bonded to the

aromatic ring has on the introduction of a second substituent. The

substituent could affect the rate of the reaction compared to benzene.

Also, any of three possible products—the ortho, meta, and para

isomer—could be produced.


II

Let's study the above table….

1. Orientation Effects of Ring Substituents;

for ex. Nitration of Benzene

The ortho and para isomers predominate, and very little of the meta

isomer forms. The methyl group is said to be an ortho, para director.

That is, the methyl group directs or orients the incoming substituent

into positions ortho and para to itself. It turns out that all activating

groups are ortho, para directors. Halogens, which are weakly

deactivating, are also ortho, para directors.

To imagine the case,,,

I. Inductive Effect

II. Mesomeric "Resonance" Effect


III

I. Inductive Effects of Ring Substituents

- more easily visualized than resonance effects ,,,

because they are related to the concept of electronegativity.

- "attraction of e' from chemical bond"

+ I : repel e' from itself.

- I : attract e' to itself.

- Regioselectivity;

* It depends on stability of the formed intermediate because Rx. Goes to

the most stable intermediate so the reagent will select the positions that

gives more stabilized intermediate.

for ex.

alkyl groups are electron donating relative to hydrogen, and tend to

stabilize double bonds; they also stabilize carbocations.

Alkyl groups transfer electron density through the s bonds to sp2-

hybridized carbon atoms.

Thus, they also donate electron density to the benzene ring by an

inductive effect.

The trifluoromethyl group, whose fluorine atoms pull electron density

away from the carbon atom to which they are bonded, withdraws

electron density from the ring by an inductive effect.

- Any functional group that has a formal positive charge on the atom

bonded to the aromatic ring, such as the nitro group, withdraws

electron density from the ring.


IV

- Groups such as the carbonyl group or the nitrile group have a partial

positive charge on the atom bonded to the aromatic ring, so they also

withdraw electron density from the ring.

"All such groups deactivate the aromatic"

II. Resonance Effects of Ring Substituents

- Describe how electron density is shifted into or out of a benzene ring

by resonance effects.

- "complete migration of e' "

+ M : e' releasing effect. ---------- gp. releases its e' to the ring.

- M : e' withdrawing effect. ------------- gp. attract the e' cloud to itself.


V

* Resonance (+M) for O, 𝝆 directors

- gp. such as OH = Z; has +M effect.

- carry unshared pairs of e'

- Net result; increase the density of e' cloud at O, 𝝆 positions.

* Resonance (-M) for m director

-gp. such as N𝑶𝟐 = Z; (-M), decrease e' density at O, 𝝆 positions & Z=Y

- terminal double bond is conjugated with 𝝅 system of the ring.

- m-director


VI

1. Orientation of Alkyl group or Aryl group

* +I effect ,,, increase arenium ion stability at O, 𝝆 position

2. Orientation of N𝑯𝟐


VII

3. Orientation of RC𝑶− group

4. Orientation of S𝑶𝟑−− group


VIII

5. Orientation of N𝑶𝟐 group

6. Orientation of O𝑯−group


IX

7. Orientation of Trihalomethyl group


X

8. Orientation of Halogen group


XI

2. REACTIVITY;

*Activating group;

- reacts faster than benzene.

- G = e' releasing gp. stabilize 𝝈 complex.

*Deactivating group;

- reacts slower than benzene.

- G = e' withdrawing gp. destabilize 𝝈 complex.


XII

1. Strong Activators

* +M > - I

*Net result is e' releasing effect (+M) & they are strong activators.

ex; Aniline ---- (N𝑯𝟐)

***Give Reason!

- The +M effect of 𝑵𝑯𝟐 & O𝑯− groups exceed their -I effect!

Answer;

1. The unshared pairs of N& O atoms occupy 2p orbital; "the p-orbital

of the 𝟐𝒏𝒅 shell". Also, the 𝝅 e' of the C atoms of the aromatic ring are

present in 2p-orbital of the 𝟐𝒏𝒅 shell.

So, Overlapping between electrons in the same energy level result in

powerful resonance so, +M will be very strong than -I.

2. 𝝈 complex


XIII

2. Moderate Activators

ex; Acetamide gp. "N with lone pair attached to carbonyl gp.; strong

withdrawing gp."

*lone pair isn't available all time for benzene, it's divided between

benzene& carbonyl group so the migration of e' is consumed in the side

chain resonance so, it's moderate activator.

*Due to steric effect;

Only para position is attacked!

3. Weak Activators

*weak +I effect so, it's weak activator.

ex; Alkyl gp.


XIV

4. Weak Deactivators

*Cl, Br, I; Resonance is NOT effective as it occurred between p-orbitals

of different energy level therefore, deactivator.

*exception!

- F; Resonance occurred between p-orbitals of same energy level but F

has the highest electronegativity (-I effect) so it -slightly- exceeds

resonance (+M effect) therefore, deactivator.

as mentioned into slides!


XV

5. Strong& Moderate Deactivators

*Strong;

*Moderate;

,,, ,,, ,,,

ex; N𝑶𝟐 grp. have e' withdrawing effect through resonance so, it

decreases e' cloud density from the whole structure.

therefore, hard for N to attract the more E.N atom (O) so, hard to

attract 𝑬+ so, deactivator.

*Nitro grp.s were found to deactivate rate of Rx. by 104 times!

***Give Reason!

Q; Aniline reacts with electrophiles hardly in strong acidic medium at

m-position.

- As at acidic medium there is release of 𝑯+atoms.

- The 𝑵+ attracts e' to itself so, hard to attract 𝑬+

- Decrease the rate of Rx. so, deactivator "m-director"


XVI

Q; Fluorobenzene, Phenol& Benzaldehyde

1. Compare between them during the Rx.

2. What are the products of mono-nitration for these compounds? G.R!

Ans.

(1)

- The most reactive of them is Phenol.

- Reasons;

+M > -I because 2p of O & 2p of C are in the same energy

level so, strong resonance.

Therefore, Strong Activator.

--------------------------------

(2)

- Less reactive than Phenol.

- Reasons;

+M < -I as it's the highest E.N atom (F).

Therefore, Deactivator.

--------------------------------

(3)

- less reactive than both above.

Reasons;

double bond of carbonyl grp. is conjugated with the ring.

Therefore, deactivator.

--------------------------------

Nitration in;

1. Flourobenzene gives ortho, para product.

2. Phenol gives ortho, para product.

3. Benzaldehyde gives meta product.

Reason; -Draw 𝝈 complex for each one-


XVII

ORIENTATION IN DI-SUBSTITUTED BENZENE

1. Reinforce each other

a) When two or more substituents are on the ring the reactivity and

orientation of further is determined by the cumulative effects of the

substituents.

b) The effects of substituents can be reinforcing:

c) When two groups oppose each other in directing, the more activating

substituent controls regioselectivity:

2. Activator& Another Activator (The more Powerful)


XVIII

3. Activator& Deactivator (Activator control& determine the

orientation)

4. Deactivator& Another Deactivator (The more powerful)

*N𝑶𝟐; Strong deactivator. ------ "control& determine"

*COOH; Moderate deactivator.

5. Deactivator& Halogen (weak deactivator but o, p position director)

***Halogen determine the orientation.


XIX

Science

Aromatic Comp. Lec.3