Chapter 12 : 
Reactions of Arenes. Electrophilic Aromatic Substitution		 Ch 12 contents

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 of alkenes generally react via an electrophilic addition pathway:

electrophilic addition to C=C

So why don't arene C=C react in a similar fashion to alkenes and give overall addition ?
To answer this, we need to compare the two possibilities.
 


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.

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