Electromeric Effect

 

The temporary or time variable effect involving the complete transfer of a shared pair of π electrons to one of the atoms joined by multiple bond double or triple at the requirement of an attacking reagent is known as electromeric effect. As soon as the attacking reagent is removed, the transferred π electron pair again form the bond and the molecule reverts to its ground state electronic condition. For this reason, the electromeric effect may be called polarisability of multiple bond. There are permanent polarizations of sigma bond in the multiple bonds, but this inductive effect is negligible with respect to the electromeric effect.

To understand this time variable effect letters now consider a Proton which comes close to A carbon-carbon double bond. To provide electron to the proton, π bond will break and electron pair will be transferred completely to one of the carbon atoms as shown below

The electromeric effect has no specific direction. Its direction is always that which favours the reaction. However, if one of the atoms joined to the multiple bond is more electronegative than the other, the transfer of electron pair usually occurs on the more electronegative atom; example the electromeric effect on -C=O bond as follows

 

This is because of the greater electronegativity of O then that of C.

Inductive effect does also play an important role in determining the direction of the electromeric effect. If a group with +I or -I effect is attached to a multiple bonded carbon atom of the system -C=C-, the direction of the electromeric effect is the same as that of the inductive effect does in propane the transfer of electron pair will occur on the first carbon atom.

Similarly, in allyl alcohol, the -I effect of -OH reverses the electromeric effect as shown here. Here to the direction of the electromatic effect is the same as that of the inductive effect.

The electromeric effect being temporary, it is not reflected in the physical properties of the concerned molecule. The electromeric effect is denoted by the symbol E. It is said to be -E or +E as the transfer of π electron pair is away from or towards the carbon chain respectively.

+E Effect (Positive Electromeric Effect):

1) In the +E effect, electrons shift towards an atom or group connected to a double or triple bond, heightening electron density at that site.

2) This shift is typically induced by an electron-donating group (EDG) adjacent to the double or triple bond.

3) Consequently, electron density concentrates towards the atom or group possessing greater electronegativity, resulting in the development of a positive charge on the adjacent carbon atom.

4) The +E effect serves to stabilize carbocations by favoring the formation of a positively charged carbon atom.

-E Effect (Negative Electromeric Effect):

1) In the +E effect, electrons are displaced towards an atom or a group attached to a double or triple bond, thereby increasing the electron density at that position.

2) This diversion typically occurs due to the presence of an electron-withdrawing group (EWG) neighboring the double or triple bond.

3) As a result, electron density disperses away from the atom or group with higher electronegativity, inducing a partial positive charge on the adjacent carbon atom.

4) The -E effect destabilizes carbocations by discouraging the formation of a positively charged carbon atom.

Now, let us consider an example of vinyl cyanide undergoing nucleophilic attack by chloride ion.

It is the –E effect of cyano group which is directing the orientation of attack of chloride ion shown. Other such groups which causes –E effect are –NO₂, -NO, -CHO, -COOH, etc.

The electromeric effect is polarisability effect and operates in the exited state. The combined mesomeric and electromeric effect of the atom or group is known as the conjugative effect or tautomeric effect. All permanent electron displacements in the molecule in ground state are described by resonance. Electromeric effects may be supported or opposed by other permanent effects in various conditions as follows:

Case 1 In the following two example

(I) is the condition in which E-effect is supported by +I effect while (II) is the condition in which E-effect is opposed by +I effect; hence (I) is easily possible in the comparison of (II).

Case 2 In the case of vinyl bromide,

(I) is the condition of E effect supported by +M effect and opposed by –I effect therefore it is easier than (II) because, in the condition of (II) E-effect is supported by –I effect and opposed by +M effect.

Case 3 If the multiple bond is present between two different atoms, then electromeric shift will take place in the direction of the more electronegative atom.

Therefore it can be said that electromeric effect influences several part of organic chemistry:

1) Reactivity

 The reactivity of organic molecules is profoundly impacted by the presence of electron-donating or electron-withdrawing groups, dictating the likelihood of nucleophilic or electrophilic substitution reactions, respectively.

2) Stability of Intermediates

Intermediates such as carbocations, carbanions, and free radicals play crucial roles in organic reactions. The electromeric effect can modulate their stability, thereby shaping reaction pathways and product distributions.

3) Resonance

In resonance phenomena, electrons undergo delocalization via conjugated π bonds. The electromeric effect is instrumental in determining the extent of electron delocalization in resonance structures.

4) Acidity and Basicity

 Electron-donating or electron-withdrawing groups can influence the acidity or basicity of organic compounds. For instance, electron-withdrawing groups enhance acidity by stabilizing the conjugate base through the -E effect.

Therefore it can be concluded that electromeric effect can help chemists predict and rationalize structures, reaction, mechanisms etc. in organic chemistry. It furnishes valuable insights into organic chemistry empowering chemists to engineer new compounds. 

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Reference

1) Jonathan Clayden, Nick Greeves, Stuart Warren, organic chemistry book second edition.