t is observed that most of the attacking reagents always possess either a positive or a negative charge, therefore for a reaction to take place on the covalent bond the latter must possess oppositely charged centres. This is made possible by displacement (partial or complete) of the bonding electrons. The electronic displacement in turn may be due to certain effects, some of which are permanent and others are temporary. The former effects are permanently operating in the molecule and are known as polarization effects, while the latter are brought into play by the attacking reagent and as soon as the attacking reagent is removed, the electronic displacement disappears; such effects are known as the polarisability effects.

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Various effects have been distinguished in molecules in which electrons are displaced away from or towards a specified atom or group. The symbols used are as follows:

E – electromeric effect;

I – inductive effect (sometimes Is);

Id – inductomeric effect;

M – mesomeric effect;

R – resonance effect;

T – tautomeric effect, use mesomeric effect.

In addition, a plus sign before the symbol indicates that electron displacement is away from the atom or group (e.g. +I); a minus sign indicates displacement towards an atom or group (e.g. −I)   !!!!!