Aromatic Electrophilic Substitution (Ar-Se) Reactions

Only available on StudyMode
  • Download(s) : 490
  • Published : January 16, 2012
Open Document
Text Preview
Engineering Chemistry III

Prof. K. M. Muraleedharan

Aromatic electrophilic substitution (Ar-SE) Reactions

The special reactivity of aromatic systems towards electrophiles arises mainly from two factors: the presence of π electron density above and below the plane of the ring - making it nucleophilic, and the drive to regain the aromatic character by opting for substitution as opposed to a simple addition reaction. Preference towards addition reactions in the case of alkenes and substitution in the case of aromatic compounds becomes evident if we analyze the energy profiles of these reactions (Figures 1 and 2).

Figure 1.

Indian Institute of Technology Madras

Engineering Chemistry III

Prof. K. M. Muraleedharan

Figure 2. Note: consider all the resonance structures of the wheland complex

The mechanism of electrophilic aromatic substitution involves an initial rate determining interaction of the π system with the electrophile to give a benzenonium ion intermediate (σcomplex or wheland complex), which undergoes a rapid de-protonation by the base in the second step to restore aromaticity (Figure 3). E H

E H

+ E+

E H

fast

E

+ HB+

B

Figure 3. Some common electrophilic aromatic substitution reactions are: halogenation, nitration, sulfonation, Friedel-Crafts Acylation and Friedel-Crafts alkylation. These differ only in the

Indian Institute of Technology Madras

Engineering Chemistry III

Prof. K. M. Muraleedharan

nature and mode of generation of electrophiles, but in general follow the same two-step mechanism described above. Reagent combinations that lead to the generation of electrophiles in these reactions are shown in Figure 4.

Indian Institute of Technology Madras

Engineering Chemistry III

Prof. K. M. Muraleedharan

Reaction

Electrophile

Generation of electrophiles
Cl2 + FeCl3 Cl3Fe Cl Cl First step

Chlorination

Cl+

bromination

Br+

Br2 + FeBr3

Br3Fe Br

Br

Iodination

I+

I2 + 2Cu2+

2 I+ + 2Cu+

Nitration

O=N=O

HO NO2 +

H OSO3H

H2O NO2 + HSO4

O=N=O + H2O

Sulfonation

O

S O O

O HO S OH + O

O H O S OH O

O HO S OH2 O

+

O O S OH O

O O S O Cl Alkylation eg. 1

+ H3O

O HO S O

+ H2O

+

AlCl3

+ AlCl4

eg. 2

H3C

H C

C H

CH3+ HF

H3C Δ

H C

C H2

CH3 +

F

OH eg. 3 + H2SO4

O + O S OH + H2O O

O Acylation H3C H3C O Cl + AlCl3 H3C AlCl3 O R O R O H3C acylium ion O R R O O + AlCl3 R O O + AlCl3(RCO2) O + AlCl4

Figure 4

Indian Institute of Technology Madras

Engineering Chemistry III

Prof. K. M. Muraleedharan

Note: since the product of acylation is a ketone which can complex with AlCl3, two equivalents of the Lewis acid is necessary to bring about the conversion efficiently. The complex can later be hydrolyzed using water

Among the reactions mentioned above, Friedel crafts alkylation suffers from two main draw backs: a) the possibility of multiple substitutions due to ring activation on mono-alkylation and b) the formation of products arising from rearranged electrophiles (carbocations) to more stable ones (Figure 5). Multiple substitutions can be avoided by using a large excess of the substrate (aromatic system) to ensure the collision of the electrophile with an un-substituted substrate and not a mono-substituted one. H3C AlCl3 + CH3CH2CH2CH2Cl 0oc 65% 35% H3C CH3 CH3 CH3 H3C CH3 100% 0% CH2 H3C H3C 1,2-methyl shift CH2 CH3 CH3 CH2 H2 C CH3 H2 C H CH2 H2 C CH3

CH

H2C

C H2

H3C

H3C

1,2-hydride shift

C H2

H2C CH3 H3C CH2Cl CH3 AlCl3

CH3

C

H3C C CH3

H2C

+

Figure 5. Since acylium ions do not undergo rearrangement, an acylation-reduction strategy can be conveniently used to introduce alkyl groups which are prone to rearrangement as demonstrated by the example below (Figure 6)

Indian Institute of Technology Madras

Engineering Chemistry III

Prof. K. M. Muraleedharan

O +...
tracking img