REACTIONS OF ARENES ELECTROPHILIC AROMATIC SUBSTITUTION

Characteristically, the reagents that react with the aromatic ring of benzene and its derivatives are electrophiles. We aheady have some experience with electrophilic reagents, particularly with respect to how they react with alkenes. Electrophilic reagents add to alkenes. 

A different reaction takes place when electrophiles react with arenes. Substitution is observed instead of addition. If we represent an arene by the general formula ArH, where Ar stands for an aryl group, the electrophilic portion of the reagent replaces one of the hydrogens on the ring  

CHAPTER TWELVE Reactions of Arenes Electrophilic Aromatic Substitution 

We call this reaction electrophilic aromatic substitution it is one of the fundamental processes of organic chemistry. 

1 REPRESENTATIVE ELECTROPHILIC AROMATIC SUBSTITUTION REACTIONS OF BENZENE 

The three most stable resonance structures for cyclohexadienyl cation are 

The positive charge is shared equally by the three carbons indicated. Thus the two carbons ortho to the. s/Z-hybridizcd carbon and the one para to it each bear one third of a positive charge (+0.33). None of the other carbons is charged. The resonance picture and the simple MO treatment agree with respect to the distribution of charge in cyclohexadienyl cation. 

Electrophilic aromatic substitution leads to replacement of one of the hydrogens directly attached to the ring by the electrophile. All four of the ring hydrogens of p-xylene are equivalent so it does not matter which one is replaced by the nitro group. 

The aromatic ring of 1,2,4,5-tetramethylbenzene has two equivalent hydrogen substituents. Sul-fonation of the ring leads to replacement of one of them by —S03H. 

Aluminum chloride coordinates with 1-chloropropane to give a Lewis acid/Lewis base complex, which can be attacked by benzene to yield propylbenzene or can undergo an intramolecular hydride shift to produce isopropyl cation. Isopropylbenzene arises by reaction of isopropyl cation with 

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