Electromeric Effect
Electromeric Effect
This is a
temporary effect involving the complete transfer of a shared pair of electrons to
one or other atom joined by a multiple bond, i.e., a double or triple bond. The
electromeric effect is brought into play only at the requirements of the
attacking reagent, and because of this, the direction of the electromeric
effect is always that which facilitates reaction.
The electromeric
effect is represented as follows :
The curved
arrow shows the displacement of the shared electron pair, beginning at the position
where the pair was originally, and ending where the pair has migrated. Since A has lost its share in the electron pair
and B has gained this share, A acquires a positive charge and B a negative
charge.
The electromeric
effect is represented by the symbol E, and is said to be +E when the
displacement is away from the atom or group, and –E when towards the atom or group.
N.B.
– The displacement of the electron pair forming a covalent bond when a u it charge
is brought up is a measure of the polarisability of that bond. It is not
a permanent polarization since, when the charge is removed, the electron displacement
disappears. Thus, the electromeric effect is a polarisability effect and
operates in the excited state.
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