Nucleophilic aromatic substitution the SNAr mechanism

The reaction of a nucleophile with a benzene ring is very rare. This can only occur when strongly electron-withdrawing substituents (e.g. N02) are at the ortho- and/or para- positions of aryl halides. The reaction 

The order of reactivity of aryl halides is Ar T Ar CI Ar-Br Ar I. which is the reverse order for SN2/SN1 reactions of alkyl halides (see Section 5.3.1). As fluorine is the most electronegative halogen atom, it strongly polarises the C—F bond. This makes the carbon 8+, and hence it is susceptible to nucleophilic attack. The fluorine atom is also small, and hence the incoming nucleophile can easily approach the adjacent carbon atom (as there is little steric hindrance). 

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