Inchemistry, theelectromeric effect is a molecularpolarization occurring by an intramolecularelectron displacement, characterized by the substitution of oneelectron pair for another within the sameatomic octet of electrons. It is sometimes called theconjugative mechanism, and previously, thetautomeric mechanism. The electromeric effect is often considered along with theinductive effect as types of electron displacement. Although some people refer to it as an effect produced by the presence of areagent like anelectrophile or anucleophile,IUPAC does not define it as such. The termelectromeric effect is no longer used in standard texts and is considered as obsolete.[1] The concepts implied by the terms electromeric effect andmesomeric effect are absorbed in the termresonance effect.[2] This effect can be represented using curved arrows, which symbolize the electron shift, as in the diagram below:
The effect can be classified into two types, namely the +E effect and the −E effect. This classification is based on the direction of electron pair transfer. When the attacking reagent is electrophile, the +E effect is generally observed and π-electrons are transferred to the positively charged atom. When the attacking reagent is a nucleophile, there is generally an −E effect, where π electrons are transferred to atoms to which the attacking reagent will not bind.[citation needed]