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

Why Substitution not Addition ?

As we just saw, 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 the question is why don't arene C=C react in a similar fashion to alkene C=C and also give overall addition ?

To answer this, we need to compare the two possibilities.  


The first step is the same in each of the pathways, 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 - substitution retains the aromatic unit whereas addition removes the C=C.

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organic chemistry © Dr. Ian Hunt, Department of Chemistry University of Calgary