Explain the term electromeric effect. 

It is a temporary phenomenon which operates during some reactions involving molecules containing double or triple bonds. It involves an instantaneous and complete transfer of a shared pair of electrons of the multiple bonds to one of the bonded atoms under the influence of the attacking reagent. It is usually denoted as E.

The effect is indicated with a curved show  which shows the direction of transference of electrons. The atom which receives the electron pair gets negatively charged while the other atoms get positively charged.
The electromeric effect operates as long as the attacking reagent is present. As soon as the attacking reagent is removed, the electromeric effect vanishes i.e.  electron pair comes back to its original position forming multiple bonds again. 
Examples in which electromeric effect operates:
(a) In the case of ethylene.
 
(b) In the case of propylene, the electron transfer is governed by the +effect of the methyl group.

Types of electromeric effect. It is of two types:
(i) + E effect. If the π; electrons are transferred to that atom of the multiple bonds to which the attacking reagent gets finally attached, the effect is called +E effect. For example,
(a) The addition of a proton to ethene.

(b) The addition of a proton to propene.

(ii) -E effect: If the p-electrons are transferred to an atom of the multiple bonds other than to which the attacking reagent gets attached, the effect is called -E effect. For example,