Benzene rings substituents inductive effects

If the reactions of the same substituents on heteroaromatic azoles and on benzene rings are compared, the differences in the reactivities are a measure of the heteroatoms influence. Such influence by the mesomeric effect is smaller when the substituent is /3 to a heteroatom than when it is a or y. The influence by the inductive effect is largest when the substituent is a to a heteroatom. 

In common with a number of heterocyclic iodinations, kinetic effects are found in the iodination of indole and 2-methylindole [68AC(R)1435], When the substituent effects for the reaction are examined it is clear that any resonance effects from the fused benzene ring are only poorly relayed to the reactive 3-position, and the rates appear to be controlled by inductive effects. A 5-methyl group was more activating than 5-methoxy [69AC(R)799]. 

The influence of the inductive effects of the substituents present on both the amine nitrogen and benzene ring on the Co(II)/Co(III) redox potential is quite evident. 

Inductive effect of substituent present on the benzene ring... 

If the electrophile attacks the benzene ring at a position ortho or para to a + / substituent (i.e., to one electron-donating by inductive effect), the activated complex will be similar to 81 or 82, respectively. Resonance structures 81c and 82c are of particularly low energy because in these the positive charge is localized on the carbon that bears the + / group. Attack at the meta position does not allow such a resonance structure to be drawn. 

Aromatic fluorine is replaced by nucleophiles only if the electron density of the aromatic ring is lowered by the presence of electron-withdrawing substituents such as nitro groups. In this case, inductive effect of the four chlorine atoms depletes the electron density of the benzene ring. Inductive effect of fluorine atoms further decreases the electron density at the carbons to which they are attached. The lowest electron density is therefore at carbons 1 and 2. 

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