Aromatic Electrophilic Substitution (Ar-Se) Reactions
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
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