Nucleophilic Substitution Reactions of Aryldiazonium Salts

Nucleophilic substitution reactions via an SN2 reaction are not possible in aromatic compounds because the configuration at the reacting C atom cannot be inverted. 

the reaction Ar-N =N + H3P02 — Ar-H represents an important procedure for the defunctionalization of aromatic compounds. It is frequently employed to remove N112 groups from aromatic compounds if the NH2 group was previously only introduced to accomplish a certain Ar-SE reaction of a particular regioselectivity—where the same reaction with the same regioselectivity would not have been possible without this NH2 group. 

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Fig. 5.53. Mechanistic aspects I of nucleophilic aromatic substitution reactions of aryldiazonium salts via radicals introduction of Nu=Cl, Br, CN or N02 according to Figure 5.52. Following step 2 there are two alternatives either the copper(II) salt is bound to the aryl radical (step 3) and the compound Ar-Cu(III)NuX decomposes to Cu(I)X and the substitution product Ar-Nu (step 4), or the aryl radical reacts with the cop-per(II) salt in a one-step radical substitution reaction yielding Cu(I)X and the substitution product Ar-Nu.

Fig. 5.55. Mechanistic aspects III of nucleophilic aromatic substitution reactions of aryldiazonium salts via radicals introduction of Nu = I through reaction of aryldiazonium salts with KI. In this (chain) reaction the radical I2 —apart from its role as chain-carrying radical—plays the important role of initiating radical. The scheme shows how this radical is regenerated the initial reaction by which it presumably forms remains to be provided, namely (1) Ar-N =N + I -> Ar- N=N + h ...

Fig. 5.56. Nucleophilic substitution reactions on a masked aryldiazonium salt. The introduction of fluorine takes place by the SN1 mechanism in Figure 5.50, the introduction of iodine occurs by the radical mechanism of Figure 5.56.

Although nitriles lack an acyl group, they are considered acid derivatives because they hydrolyze to carboxylic acids. Nitriles are frequently made from carboxylic acids (with the same number of carbons) by conversion to primary amides followed by dehydration. They are also made from primary alkyl halides and tosylates (adding one carbon) by nucleophilic substitution with cyanide ion. Aryl cyanides can be made by the Sandmeyer reaction of an aryldiazonium salt with cuprous cyanide. a-Hydroxynitriles (cyanohydrins) are made by the reaction of ketones and aldehydes with HCN. 

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