Lewis adds electrophilic aromatic substitution

Electrophilic aromatic substitution (Section 14.4) The classic substitution reaction of aromatic compounds with Lewis adds. 

An obvious consequence of this deiocaiization is the decreased basicity of the nitrogen atom and the increased acidity of the NH group. In fact, the pK of pyrrole acting as a base is about -4, and protonation occurs at carbon below pH -4. By contrast, the NH proton pK 16.5) can be removed by much weaker bases than those that can remove protons on normal secondary amines. The nucieophiiic nature of the ring means that pyrrole is attacked readily by electrophiles. Reaction with bromine requires no Lewis add and leads to substitution (confirming the aromaticity of pyrrole) at all four free positions. Contrast pyridine s reactivity with bromine (p. 731) it reacts just once, at nitrogen. 

Alkylation is the paramount electrophilic substitution reaction in industrial aromatic chemistry, for example, in the production of ethylbenzene, cumene, diisopropylbenzenes and diisopropylnaphthalenes. A carbonium ion generally acts as the electrophilic agent and is produced by reaction of a Lewis add with an olefin. The most stable of the possible carbonium ions normally predominates in the reaction nevertheless, attention must also be paid to the formation of isomers. 

The electrophile 4 adds to the aromatic ring to give a cationic intermediate 5. Loss of a proton from 5 and concomitant rearomatization completes the substitution step. Subsequent hydrolysis of the iminium species 2 yields the formylated aromatic product 3. Instead of the highly toxic hydrogen cyanide, zinc cyanide can be used. The hydrogen cyanide is then generated in situ upon reaction with the hydrogen chloride. The zinc chloride, which is thereby formed, then acts as Lewis acid catalyst. 

---