chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a physical science within the natural sciences that studies the chemical elements that make up matter and chemical compound, compounds made of atoms, molecules a ...

, the electromeric effect is a molecular

polarization Polarization or polarisation may refer to: Mathematics *Polarization of an Abelian variety, in the mathematics of complex manifolds *Polarization of an algebraic form, a technique for expressing a homogeneous polynomial in a simpler fashion by ...

occurring by an intramolecular

electron The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...

displacement, characterized by the substitution of one

electron pair In chemistry, an electron pair or Lewis pair consists of two electrons that occupy the same molecular orbital but have opposite spins. Gilbert N. Lewis introduced the concepts of both the electron pair and the covalent bond in a landmark paper ...

for another within the same atomic octet of electrons. It is sometimes called the ''conjugative mechanism'', and previously, the '' tautomeric mechanism''. The electromeric effect is often considered along with the

inductive effect In Organic chemistry, the inductive effect in a molecule is a local change in the electron density due to electron-withdrawing or electron-donating groups elsewhere in the molecule, resulting in a permanent dipole in a bond. It is present in a ...

as types of electron displacement. Although some people refer to it as an effect produced by the presence of a

reagent In chemistry, a reagent ( ) or analytical reagent is a substance or compound added to a system to cause a chemical reaction, or test if one occurs. The terms ''reactant'' and ''reagent'' are often used interchangeably, but reactant specifies a ...

like an

electrophile In chemistry, an electrophile is a chemical species that forms bonds with nucleophiles by accepting an electron pair. Because electrophiles accept electrons, they are Lewis acids. Most electrophiles are positively Electric charge, charged, have an ...

or a

nucleophile In chemistry, a nucleophile is a chemical species that forms bonds by donating an electron pair. All molecules and ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they are ...

IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...

does not define it as such. The term ''electromeric effect'' is no longer used in standard texts and is considered as obsolete. The concepts implied by the terms electromeric effect and

mesomeric effect In chemistry, the mesomeric effect (or resonance effect) is a property of substituents or functional groups in a chemical compound. It is defined as the polarity produced in the molecule by the interaction of two pi bonds or between a pi bond an ...

are absorbed in the term

resonance effect In chemistry, resonance, also called mesomerism, is a way of describing Chemical bond, bonding in certain molecules or polyatomic ions by the combination of several contributing structures (or ''forms'', also variously known as ''resonance struc ...

. This effect can be represented using curved arrows, which symbolize the electron shift, as in the diagram below:

Types of electromeric effects

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.

References

{{Reflist Physical organic chemistry Chemical bonding