Halogen Substituents Deactivating, but Ortho, Para-Directing

17-8 Halogen Substituents Deactivating, but Ortho, Para-Directing 

The halobenzenes are exceptions to the general rules. Halogens are deactivating groups, yet they are ortho, para-directors. We can explain this unusual combination of properties by considering that 

the halogens are strongly electronegative, withdrawing electron density from a carbon atom through the sigma bond (inductive withdrawal), and 

the halogens have nonbonding electrons that can donate electron density through pi bonding (resonance donation). 

These inductive and resonance effects oppose each other. The carbon-halogen bond (shown at right) is strongly polarized, with the carbon atom at the positive end of the dipole. This polarization draws electron density away from the benzene ring, making it less reactive toward electrophilic substitution. 

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