Organic reactions are influenced by several electronic factors like electromeric effect, inductive effect, resonance effect, and hypercoagulation. Compounds having similar formula units and structures can react differently due to the electronic factors.
Examples of electronic effects are:
The inductive effect
The mesomeric effect
What is the Electromeric Effect?
Electrons present in a pi (π) bond can be polarized with little effort. So, when the charged reagents like electrophiles or nucleophiles approach them, the electrons get polarized and displaced towards one of the constituents of the atom. The mechanism involved is electrostatic attraction or repulsion. The atom that acquires the pair of electrons becomes negatively charged while the other atom gets a positive charge.
Electromeric effect is a reversible reaction where there is a complete transfer of a pi-electron pair due to the influence of an electrophile or a nucleophile. The effect disappears upon withdrawal of the attacking reagent. The electromeric effect is also non-specific in terms of its direction which is always that which favours the reaction. The electromeric effect is seen only when there is an electron attacking reagent and is also referred to as the E effect. Curved arrow notation is also used to show the movement of electrons from one site to another.
Direction of the Shift
The direction of the shift of the pair of electrons will be as follows:
If the groups linked to a multiple bond are similar, shift can occur in either direction
If dissimilar groups are linked on the ends of the double bond, shift of the electron pair is decided by the inductive effect
Mechanism of the Electromeric Reaction
The mechanism of an electromeric reaction can be explained in the following manner. When a double or a triple bond is subjected to an attack by an electrophile E+ (a reagent), the 2 pi electrons forming the pi bond are transferred to one atom or the other. The transfer of the shared pi electrons leads to the instantaneous formation of a dipole in the molecule. The electromeric effect can be represented by the following chemical reaction (See figure 1). The curved arrow in this figure depicts the displacement of the electron pair. The atom A loses its share in the electron pair and the atom B has acquired this share. Thus, electronically, A acquires a positive charge while B acquires a negative charge.
Types of Electromeric Effect
There are two distinct types of electromeric effect:
The +E effect, which is also referred to as the positive electromeric effect
The -E effect, which is also referred to as the negative electromeric effect
+E Effect or Positive Electromeric Effect
Electrophiles are electron acceptors and when they attack, the pi-electrons are shifted to the atom with a positive charge. This effect is depicted as +E and can be represented by the chemical reaction shown below (See figure 2). The +E effect is observed in the addition of acids to alkenes.
-E Effect or Negative Electromeric Effect
Nucleophile are electron donors and if they attack, the electrons are shifted away and into the pi system. This effect is represented as -E and can be represented by the following chemical reaction (See figure 3). The -E effect is observed in the reaction of the addition of cyanide ion to carbonyl compounds.
Examples of the electromeric effect:
Reaction of an alkene with Br2 in CCl4
As the reagent bromine approaches alkene, temporary polarization develops with the C2 atom gaining a negative charge and the C1 atom acquiring a positive charge. Alkenes are attacked by the electrophile Br+ to produce a cyclic bromonium ion as an intermediate. The cyclic bromonium ion is then attacked by Br− giving vicinal dibromide as the product
Addition of hydrogen halides
Hydrogen halides provide both an electrophile (proton) and a nucleophile (halide). The electrophile attacks the double bond and takes up a set of pi electrons and attaches it to the resulting molecule (carbocation). The nucleophile (halide) completes the reaction to give rise to a new molecule. Prominent examples of the same have been illustrated in figures 5 and 6.
Nucleophilic addition reaction
When negatively charged nucleophiles come in the vicinity of the carbonyl molecule, the carbonyl group gets polarised and the nucleophile attacks the positive center of the molecule. The reaction can be represented as follows (See figure 7).
Electrophilic addition reaction
Symmetrical alkenes or alkynes involve polarisation of the carbon-carbon double bond due to the action of electrophiles such as H+ (See figure 8).
Electrophilic substitution reactions of benzenoids
These reactions involve polarisation in benzene when attacked by an electrophile (See figure 9).
Differences between electromeric effect and inductive effect:
There are significant differences between the electromeric effect and the inductive effect. While the electromeric effect is reversible the inductive effect is a permanent state of polarization. The differences are shown in Table 1. When both inductive and electromeric effects occur simultaneously, the electromeric effect predominates over the inductive effect.
Table 1: Differences between the electromeric effect and inductive effect