Electrophilic substitution Element effect

The introduction of the halogens onto aromatic rings by electrophilic substitution is an important synthetic procedure. Chlorine and bromine are reactive toward aromatic hydrocarbons, but Lewis acid catalysts are normally needed to achieve desirable rates. Elemental fluorine reacts very exothermically and careful control of conditions is required. Molecular iodine can effect substitution only on very reactive aromatics, but a number of more reactive iodination reagents have been developed. 

Direct fluorination at saturated carbon is possible with either elemental fluorine or CF30F and a radical inhibitor.57-8 The transformation is an electrophilic fluorination, regiospecific by virtue of the highly polar transition state sensitive to the inductive effects of nearby or remote polar substituents. Substitution occurs almost exclusively at tertiary... 

The reaction (a) explains the effect of the above-mentioned catalysts in promoting electrophilic nuclear substitution by elemental halogen ... 

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