Electromeric effect: A dipole is formed instantaneously in an organic compound molecule due to the complete transfer of previously shared pi-electron pairs to one of the atoms when an attacking reagent is applied, and this phenomenon is referred to as the Electromeric effect (also known as the electrophilic effect).
This effect can be detected in organic compounds that include at least one multiple bonds, but it is less common in other chemical molecules. When the atoms involved in these multiple bonds come into contact with an attacking reagent, one pi bonding pair of electrons from one of the two atoms is totally transferred to the other atom.
The electromeric effect is a transient effect that lasts only as long as the attacking reagent is present and exposed to the organic component. It is not permanent. It is important to note that after the attacking reagent is withdrawn from the system, the polarised molecule returns to its natural state.
The Different Types of Electromeric Effects
Generally speaking, the electromeric effect can be divided into two types: the positive E effect and the negative E impact. These classes are determined by observing which way the electron pair is moved throughout the transfer process.
When the electron pair of the pi bond is shifted in the direction of the attacking reagent, this phenomenon occurs. The addition of acid to alkenes produces the +E effect, which can be detected. The attacking reagent attaches itself to the atom that received an electron pair as a result of the electron transfer reaction.
The +E effect is most commonly observed when the attacking reagent is an electrophile and the pi electrons are transferred from the negatively charged atom to the negatively charged reagent. The protonation of ethene is an example of the +E effect in action, as depicted in the diagram below.
When the electron pair of the pi bond is shifted away from the attacking reagent, this effect happens, and the pi bond is broken. It attaches itself to the positively charged atom in the compound, i.e. the atom that lost its electron pair as a result of the transfer of electrons.
Most commonly, the -E effect is found in reactions in which the attacking reagent is a nucleophile and where the pi electrons are moved to atoms that the attacking reagent will not form a bond with. An example of the -E effect occurring would be the addition of nucleophiles to carbonyl compounds, as depicted in the illustration below.
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