Types of Electromeric Effects

Reactions are influenced by various factors like inductive effects, resonance effects, hyperconjugation, and electromeric effects. Compounds having similar formulas and structures can react differently to these effects. Among all these effects, the electromeric effect is the most significant one. The electromeric effect is the shift of pi electrons when a reagent attacks a pi bonded compound. And this total shift produces polarity. In this article, we will talk about types of electromeric effects. 

What is the electromeric effect?

The electromeric effect can be defined as a complete transfer of pi electrons due to the influence of an electrophile and a neutrophile. It is a reversible reaction. It is a fleeting reaction that occurs when a reagent attacks a multi-bonded molecule, causing a complete shift of pi electrons to one of the bonds between two atoms. When the attacking reagent is removed, the effect goes away.

This effect also doesn’t have a specific direction, favouring the reaction at all times. The electromeric effect is only seen when an electron attacking reagent is there. This effect is also popularly known as the +E effect. A curved arrow notation is also used to present electron movement from one location to another.

Why Does The Shift Of The Reaction Occur?

The direction of shift of electrons according to types of electromeric effects is given below:

• If the groups connected by multiple bonds are similar, a transition in either direction is possible. 
• The inductive effect determines the shift of the electron pair when dissimilar groups are linked on the ends of the double bond.

Mechanism Of Electromeric Effect:

When a compound with a double bond and triple bond is subjected to attack by an electrophile E+, the two pi electrons are shifted to one atom or other. This shift of shared pi electrons leads to the generation of dipoles in the molecule. 

The spontaneous initiation of a dipole in the molecule is aided by the transfer of the shared pi electrons. The Electromeric effect is depicted by an arrow that abides by the electron pair’s displacement.

Characteristics Of Electromeric Effect:

• It is a transient effect involving the complete shift of pi-electrons to one of the bonded atoms.

• It works with organic compounds that have at least one multiple bond.

• This reaction takes place when an attacking reagent is present. 

• In the absence of an attacking reagent, the effect is lost.

Types Of Electromeric Effect:

There are two types of electromeric effect: the +E effect and the -E effect. The types of electromeric effects have been explained below. 

+E electromeric effect:

The +E effect occurs when the place attacking the reagent moves towards the pi bond’s electron pair. It usually occurs when an acid is added to alkenes. This reagent attaches to the atom that gained an electron pair when the transfer occurred. 

This effect is seen when the attacking reagent is an electrophile, and the pi electrons are transferred to an atom with a positive charge. Another example of the +E effect at work is the protonation of ethene. 

-E electromeric effects:

The -E effect occurs when the attacking reagent moves away from the electron pair of the pi bond. The attacking reagent binds to an atom charged positively in the molecule, i.e. the atom that lost the electron pair during the transmission.

If the attacking reagent is a nucleophile, then the -E effect occurs, and the pi electrons are transferred to the atom with which the attacking reagent will not bond. The addition of nucleophiles to carbonyl compounds is an example of the -E effect, as shown below.

Examples Of Electromeric Effect:

• The reaction of an alkene with Br2 in CCl4 solution. Temporary polarisation occurs as the reagent bromine approaches alkene, with the C2 atom obtaining a negative charge and the C1 atom gaining a positive charge. The electrophile Br+ attacks alkenes, producing a cyclic bromonium ion as an intermediate. The cyclic bromonium ion is then attacked by Br, yielding vicinal dibromide.

• Another example of the electromeric effect is the addition of hydrogen halides. Hydrogen halide has both electrophile and nucleophile hydrogen. The electrophile attacks the double bond, absorbing a set of pi electrons and attaching them to the resulting molecule (carbocation). The reaction is completed by the nucleophile (halide), which results in the formation of a new molecule.

• One more example of the electromeric effect is the nucleophilic addition reaction. In this, a negatively charged nucleophile approaches a carbonyl molecule, the carbonyl group becomes polarised, and the nucleophile attacks the positive centre of the molecule. 

• An electrophilic addition reaction is another example. When electrophiles like H+ cause polarisation of the carbon-carbon double bond in alkenes or alkynes that are symmetrical. 

Conclusion:

This article explains the electromeric effect in detail. The shift of pi electrons when an attacking reagent attacks the multiple bonded atom is known as an electromeric effect. The electromeric effect is not permanent; it is a temporary effect. There are two types of electromeric effect: +E effect and -E effect. It is considered along with inductive effects as a type of electron displacement.