Overview

Chapter 12 : Reactions of Arenes. Electrophilic Aromatic Substitution

Electrophilic Aromatic Substitution
Overall an electrophilic aromatic susbtitution can be represented as follows:

Electrophilic aromatic substitution
There are three fundamental components to an electrophilic substitution reaction:
  1. formation of the new s bond from a C=C in the arene nucleophile
  2. removal of the proton by breaking the C-H s bond
  3. reform the C=C and restore aromaticity
curly arrows for electrophilic aromatic substitution

The mechanism is represented by the following series of events:

cyclohexadienyl cation = arenium ion

The reaction of the electrophile E+ with the arene is the slow step since it results in the loss of aromaticity even though the resulting cation is still resonance stablised.

 

Why Substitution not Addition ?

Overall an electrophilic aromatic substitution can be represented as follows:

electrophilic aromatic substitution
But we have previously seen the C=C generally react via an electrophilic addition pathway:
electrophilic addition to C=C
So why don't arenes react in a similar fashion to alkenes and give overall addition ?
 
The first step is common to both, E+ adds a C=C to give a carbocation intermediate. 
 

For an addition pathway, the nucleophile then adds to the electrophilic carbocation giving a cyclohexadiene derivative.
 

electrophile reacting with the arene, to give the carbocation intermediate
 

susbtitution pathway and alternate addition pathway

For a substitution pathway, the "nucleophile" functions as a base and removes a proton from the sp3 C to recreate the C=C and restore the aromaticity.

Recall that the resonance energy of benzene is about 152 kJ / mol (36 kcal / mol) and a conjugated diene is 16 kJ / mol (4 kcal/mol). This extra stability of the aromatic system is responsible for favouring the substitution reaction.

Reactions and Reagents:

The following table contains a summary of the key reactions of aromatic systems covered by this chapter.  More detailed information on each reaction can be accessed by following the link from the reaction column. Note that the reagent combination and reaction conditions for reactions of benzenes are usually more severe than reactions of alkenes.... this tends to make them more readily recognizable.

The following pointers may aid your understanding of these reactions:

Reaction
Reagents
Electrophile
Product
Comments
Nitration
HNO3 / H2SO4
NO2+
E+ formed by loss of water from nitric acid
Sulfonation
H2SO4 or SO3 / H2SO4
SO3
Reversible
Halogenation
Cl2 / Fe or FeCl3
Cl+
E+ formed by Lewis acid removing Cl

Br2 / Fe or FeBr3
Br+
E+ formed by Lewis acid removing Br-
Alkylation
R-Cl / AlCl3
R+
E+ formed by Lewis acid removing Cl-

R-OH / H+
R+
E+ formed by loss of water from alcohol

C=C / H+
R+
E+ formed by protonation of alkene
Acylation
RCOCl / AlCl3
  RCO+
E+ formed by Lewis acid removing Cl-

RCO2COR / AlCl3
 RCO+
E+ formed by Lewis acid removing RCO2-