Friedel—Crafts reaction disguised chemical selectivity

In a previous section (Section 6.1.3), we discussed the problem of disguised chemical selectivity for extremely fast competitive consecutive reactions. This problem could be solved using micromixers, in which the mixing takes place in a very short period by virtue of a small diffusion path caused by the microstructure. Friedel-Crafts alkylation using N-acyliminium ion pools provides a nice example of the effectiveness of micromixing. 

Both the reaction of an initiator (In) with a monomer (M) and the reactions of propagating polymer (Pn) with a monomer (M) are very fast (Scheme 9.8). Therefore, we have to consider the possibility of disguised chemical selectivity, which is observed for Friedel-Crafts reactions of reactive aromatic compounds and a cation pool. If mixing is slow, the consecutive propagation reactions take place before all of the initiators react with monomers, even if the consecutive propagation reactions are slower than the initiation reaction. This is also an example of disguised... 

Like Friedel-Crafts reactions, halogenation such as bromination and iodination of aromatic compounds are classified as electrophilic aromatic substitution reactions. Bromine and iodine substituents are weakly electron-withdrawing groups, and introduction of such substituents causes a decrease in reactivity, and therefore, the first halogenation is faster than the second halogenation. However, the reactions in batch macroreactors suffer from the problem of disguised chemical selectivity, i.e, the formation of dihalogenated products. 

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