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An in-depth analysis of electrophilic aromatic substitution, focusing on nitration, sulfonation, halogenation, and Friedel-Crafts alkylation and acylation. It explains the mechanisms, limitations, and effects of substituents on the reactivity of aromatic rings.
What you will learn
Typology: Lecture notes
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Nucleophile- Lewis base. Shorthand: Ar-H
E+^ +
H (^) E
Electrophile- Lewis acid: may be or neutral.
Product: neutral if electrophile is ; if electrophile is neutral. Shorthand: Ar-E
Electrophilic substitution is the typical reaction type for aromatic rings.
Generalized electrophilic aromatic substitution:
NO 2
(3) HSO 3 -^ H^ NO^2
Addition of a basic group, eg HSO 4 - , to the ฯ complex would result in formation of a non-aromatic compound, whereas expulsion of H+^ results in an aromatic product.
This intermediate carbocation is sometimes called a ฯ complex.
ฯ bond H NO 2 H NO 2 H NO 2
(2) O=N=O + slow
a nitronium ion, cf. NO 2 +^ BF 4 -
(1) HONO 2 + 2 H 2 SO 4 H^3 O^ + 2^ HSO^4 + NO^2
Mechanism
Since the nitro group can often be reduced to the amine group (tin or iron and HCl are frequently used to effect this reduction), ArNO 2 > Ar-NH 2 , nitration is often used to ultimately make an aryl amine.
Note the following โ
acid, are strong acids and are highly ionized in water.
equilibrium; therefore, the entire reaction is an equilibrium. Thus, Ar-H can be sulfonated using fuming sulfuric acid, H 2 SO 4 CSO 3 , and Ar-SO 3 H can be desulfonated (to Ar-H) by boiling it in a dilute solution of sulfuric acid.
a phenol , Ar-OH. If a sulfonic acid is fused with solid KOH, the -SO 3 H group is replaced by -OH. [Owing to the vigorous reaction conditions, there are limitations with regard to which substituents may be present on the ring.]
polar site capable of hydrogen bonding; this gives rise to water solubility. Dyes are sometimes made water soluble in this way.
, where R is a long-chain alkyl group.
R SO 3 -^ Na+
The ionic "head" is hydrophilic and the long "tail" is hydrophobic. This combination enables this material to disperse oily material in water.
Cl H
(1) Cl 2 + FeCl 3 FeCl 4 -^ Cl+ complex
(2) (^) FeCl 4 -^ Cl+ +^ slow
(3) (^) + FeCl 4 -
Cl
2 Fe + 3 Cl 2 2 FeCl 3
catalyst
electrophile
Cl H
FeCl 4 -
Mechanism
R+
R H
(1) RCl + AlCl 3 AlCl 4 -^ R+ sometimes a complex, sometimes not
(2) +^ slow
(3) (^) + AlCl 4 -
R
catalyst
electrophile
R H
AlCl 4 -
Mechanism
Since a carbocation can be the electrophile in this mechanism, a variety of carbocation precursors could be used: alkenes, for example. Since aryl and vinyl carbocations are unstable, Ar-X and vinyl-X cannot be used as precursors for these species.
Limitations on Friedel-Crafts Alkylation โ
than the halogens ( vide infra ) cannot be present on the ring prior to F-C alkylation, nor can -NH 2 , -NHR, or -NR 2.
electrophilic substitution; therefore, polyalkylation is a problem.
Limitations โ
than the halogens ( vide infra ) cannot be present on the ring prior to F-C acylation, nor can -NH 2 , -NHR, or -NR 2. However, the acylium ion does not rearrange and polyacylation is not a problem because the acyl group deactivates the ring toward further electrophilic substitution.
Major Minor
H 2 SO 4
HNO 3
CH 3
meta
para
ortho (^) CH 3
NO 2
NO 2
CH 3 CH 3
NO 2
Effect of Substituents Already on the Ring
Reactivity: Activating or Deactivating โ
If we allow toluene and benzene to react with mixtures of nitric and sulfuric acids under the same conditions and the toluene reacts 25 times faster than the benzene, we say it is 25 times more reactive. We would also say that the methyl group activates the aromatic ring toward nitration. Since the other electrophilic aromatic substitutions have mechanisms similar to nitration, we might expect the methyl group to activate the aromatic ring toward these reactions; usually, it does.
If we nitrate toluene, we find that the major products are p -nitrotoluene and o -nitrotoluene; only a small amount of m -nitrotoluene is formed. We say the methyl group is an ortho, para director for electrophilic substitutions.
H 2 SO 4
HNO 3
H 2 SO 4
HNO 3
Boxed resonance structures make an especially large contribution if X is electron releasing ; they make an especially small contribution if X is electron withdrawing.
Note: ortho and para attack form carbocations with one boxed structure; the carbocation resulting from meta attack has no boxed structures.
Meta Attack
Para Attack
X
NO 2
H
X
NO 2
H
H 2 SO 4
HNO 3
X
NO 2
X
NO 2 H
X
H
X
Ortho Attack
X
NO 2 H X
NO 2 H X
X
NO 2
H
NO 2
X
H
X
NO 2 H
OK. Electron withdrawing groups on the ring destabilize
the transition state leading to the ฯ-complex, and electron donating ones stabilize the transition state, so reaction occurs faster with electron donating groups. And the effect is greatest at the ortho and para positions so an electron withdrawing group is meta directing because it deactivates the o and p positions.
BUT, two questions.
1) Why are some groups which appear to be electron withdrawing, -NO 2 for example, deactivating and meta directing as expected, while others, -NH 2 for example, are activating and o, p directing?
2) Why are the halogens deactivating and o, p directing?
OK.
COOH
COOH
NH 2
NO 2
OH
O 2 N
HNO 3
H 2 SO 4
OH
O 2 N
NO 2
OH
O 2 N NO 2
CH 3
Cl
HNO 3 H 2 SO 4
CH 3
Cl NO 2
CH 3
Cl
NO 2
CH 3
Cl
O 2 N
32% 59% 9%
Position of Electrophilic Attack for Disubstituted Benzenes
In cases of opposing effects prediction is more difficult and mixtures may result.
Strongly activating groups usually win out over deactivating or weakly activating groups.
The is usually little substitution between two groups which are meta to each other.
