Electromeric Effect

Polarisation can easily take place in the electrons of pi-bonds. Hence, the electromeric effect is easily observable in organic reactions. However, such organic reactions need molecules with at least one double or triple bond. The electrons easily attain polarisation when any electrophile or nucleophile approaches such molecules. Therefore, it causes displacement of the electrons in an atom. However, the primary cause of the electromeric effect is either electrostatic repulsion or electrostatic attraction.

Electromeric effect in detail

The electromeric effect is temporary; that is, it is reversible. Hence, it is an organic reaction with a temporary shift of electrons due to specific reagents. Moreover, these reagents are either electrophiles or nucleophiles. 

The electromeric effect is evident in reactions that have organic molecules containing pi-bond. However, the most crucial point of the electromeric effect is that it is not observable in sigma bonds. Hence, a shift happens among the electron when a reagent comes close to such organic molecules. This shift is what causes polarisation.

Since the electromeric effect is temporary, the electrons can shift back to normal positions. However, this happens when the nucleophiles or electrophiles are taken away from their vicinity. Moreover, we can say that the electron becomes polarised once the reagents are removed.

Electromeric effect and its types

The electromeric effect occurs due to the response to two types of reagents. These reagents are electrophiles and nucleophiles. Since there are two different reagents, they cause two different responses. Hence, this causes two different types of electromeric effects. These are described below.

+E effect

+E effect is also known as the positive electromeric effect. So, in the +E effect, the shift of electrons is basically towards the reagent. The cause of this action is the electron-attracting nature of the attacking reagent. Hence, we can say that the attacking reagent is an electrophile. So, the +E effect is due to the electrophiles.

Usually, when there is the addition of acids to an alkene, the +E effect occurs in a reaction.

-E effect

The -E effect is also known as a negative electromeric effect. So, there is repulsion between the attacking reagent and the electrons in the -E effect. This means the attacking reagent is already negatively charged. Hence, the attacking reagent in the -E effect is a nucleophile.

Moreover, the -E effect is observable in reactions containing carbonyl carbon atoms. The molecules containing carbonyl carbon atoms are usually ketones and aldehydes.

Mechanism

Let us keep the explanation of the mechanism of the electromeric effect very simple. This reaction happens in the presence of a double or triple bond. So, when an attacking reagent approaches the pi-electrons, it causes polarisation. Therefore, the pi-electrons move from one atom to another due to polarisation. This is the cause of the electromeric effect.

Examples

The reaction of hydrogen halides

The addition of hydrogen halide is a prominent example of the electromeric effect. Hence, hydrogen halide causes the formation of a carbocation due to pi-electron transfer. Moreover, the nucleophile of the hydrogen halide helps in completing the entire reaction.

The reaction of the benzene ring

These reactions occur when an electrophile attacks benzenoids. Thus, the electrophile causes polarisation and results in the electromeric effect.

Electromeric effect and inductive effect (differences)

To better understand the electromeric effect, let us do a comparative study. This section will compare the electromeric effect with the inductive effect. There are some essential differences between the two effects. These differences are:

For the electromeric effect, the presence of pi bonds is essential. However, for the inductive effect, the presence of a polar covalent bond is more important. Thus, the inductive effect does not need to have a pi bond.

• The visual impact of the electromeric effect is in the pi bond. In contrast, the visual impact of the inductive effect is in the sigma bonds.
• The electromeric effect is temporary. Hence, it is reversible. In comparison, the inductive effect is permanent. Hence, it is irreversible.
• Electromeric effects occur only in the presence of attacking reagents. However, the presence or absence of an attacking reagent does not concern the inductive effect.
• In the electromeric effect, the transfer of electrons is complete. However, there is no transfer of electrons in the inductive effect. There is only a slight displacement of the electrons in the inductive effect.

Structure showing electron shift in electromeric effect

The shift of electrons due to the electromeric effect has been shown in this reaction.

Conclusion

The impact of reagents is very prominent in organic reactions. Hence, the reagent can cause multiple effects on the reaction’s reactants. One such effect is the electromeric effect. There is a complete transfer of electrons from one atom to another in an electromeric effect. Moreover, the electrons participating in the transfer are pi electrons. Besides, the electromeric effect is temporary. Hence, it is also reversible. So, the removal of the attacking reagent causes the return of the electrons. These electrons return to their original position.

Most importantly, there should be at least one double or triple bond for the electromeric effect to occur. Lastly, the electromeric effect occurs in the presence of attacking reagents.