Substituent effects nucleophilic aromatic substitution

Displacements such as this show all the usual characteristics of electrophilic aromatic substitution (substituent effects, etc., see below), but they are normally of much less preparative significance than the examples we have already considered. In face of all the foregoing discussion of polar intermediates it is pertinent to point out that homolytic aromatic substitution reactions, i.e. by radicals, are also known (p. 331) as too is attack by nucleophiles (p. 167). 

A nitro group behaves the same way m both reactions it attracts electrons Reaction is retarded when electrons flow from the aromatic ring to the attacking species (electrophilic aromatic substitution) Reaction is facilitated when electrons flow from the attacking species to the aromatic ring (nucleophilic aromatic substitution) By being aware of the connection between reactivity and substituent effects you will sharpen your appreciation of how chemical reactions occur... 

Nucleophilic aromatic substitution has been the subject of frequent and extensive reviews1-10. The data on reaction rates, reaction products, substituent effects, salt effects, etc. are all readily available and need not be reassembled here. In spite of this abundance of both data and discussion, some questions of mechanism remain incompletely resolved. 

It might be expected that the activating effect of p-NO2 on nucleophilic aromatic substitution would be related to the a value of the substituent. From various studies of nucleophilic aromatic substitution, Miller and Parker213 obtained a <7 value for /2-NO2 of 1.27, very close to the values based on the ionization of substituted phenols or anilinium ions (Section ELD). 

Due to its strong activating effect in nucleophilic aromatic substitutions and to the possibility of its removal by decarbonylation, the aldehyde function has been used for the preparation of [ F]fluoroarenes not bearing electron-withdrawing substituents. Decarbonylations, possible in the presence of Pd/C [ 161 ], are more efficient in terms of time (15 min vs 1 h) and yields (80%) when using Wilkin-... 

Similarly, those reactions that are strongly assisted by withdrawal of electrons from the reaction site, such as nucleophilic aromatic substitution, give a poor fit to a Hammett plot for the substituents that are capable of withdrawing electrons by delocalization (—N02, —N2 , —C=N, and so on). An example is Reaction 16 in Table 26-7. To correlate reactivity data with structures where strong resonance effects operate, different sets of substituent constants are required.1... 

In nucleophilic as in electrophilic aromatic substitution, then, a substituent group affects reactivity by its ability to attract or release electrons in nucleophilic as in electrophilic aromatic substitution, a substituent group exerts its effect chiefly at the position ortho and para to it. The kind of effect that each group exerts, however, is exactly opposite to the kind of effect it exerts in electrophilic aromatic substitution. In nucleophilic aromatic substitution electron withdrawal causes activation, and electron release causes deactivation. 

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