Substitution, SNAr, activating mechanism

Substrates 4 have a fluorine and a hydrogen atom located in similarly activated positions of the same nitroarene molecule. The reaction of substrates 4 with nucleophiles would result in the substitution of either the hydrogen or the fluorine atom, leading to a mixture of SNAr (6) and VNS (7) products (Scheme 36.5). In consequence, compounds 4 are ideal to evaluate the effect of base concentrations on the VNS mechanism. As the SwAr mechanism is not influenced by base concentration (a reasonable assumption in most of the cases), the study of the VNS/ SNAr product ratio gives information about the effect of the base on the VNS reaction rate. 

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