It involves the complete transfer of electrons of a multiple bond to one of the bonded atom in presence of an electron attacking reagent. It is called the E effect. This effect is temporary and takes place only in the presence of a reagent. This effect is also of two types i.e. +E effect and –E effect.
if the electrons of the Π – bond are transferred to that atom to which the reagent gets finally attached is called +E effect.
if the electrons of the Π – bond transferred to that atom other than the one to which the reagent gets finally attached is called -E effect.
Example for +E effect is addition of acid to alkene
Example for -E effect is addition of cyanide ion to the carbonyl group.