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Mesomeric Effect and Its Applications for JEE

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Last updated date: 17th Apr 2024
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What is the Mesomeric Effect?

In 1938, scientist Ingold developed the concept of the mesomeric effect. Interestingly, mesomerism is synonymous with resonance, introduced by scientist Pauling. Until 1950, the word mesomerism was widely used in French and German. However,  the word "resonance" is very popular in English and is widely used today. Broadly speaking, they refer to the same concept.


The polarization developed between atoms in a conjugated system by electron transfer or electron transfer from pi bonds is known as the mesomeric effect. In short, we can describe the mesomeric effect that occurs when pi electrons move away from or near a substituent in a conjugated orbital system.


Types of Mesomeric Effect

Mesomeric effects can be divided into two categories:

  • +M effect

  • -M effect


What is the +M effect?

When pi electrons or electrons are transferred from a particular group to a conjugated system, thereby increasing the electron density of the conjugated system, such a phenomenon is known as the (+M) or positive mesomeric effect.

For the +M effect, the group must have a lone pair of electrons or a negative charge.

The +M effect gives a negative charge to the conjugated system, or it can be said that the increased electron density in the conjugated system is due to it. These conjugated systems show higher reactivity for electrophiles and less reactivity for nucleophiles.


Positive Mesomeric Effect Examples:

$-N{{H}_{2}},-OH,$ and $-SH$

Positive Mesomeric Effect Order:

If the substituent is from the electron releasing group, then the effect is positive.

$-{{O}^{-}}>-N{{H}_{2}}>-NHR>-OR>-NHCOR>-OCOR>-Ph>-F>-Cl>-Br>-I$


What is the -M effect?

When the electrons of the pi bond are transferred from the conjugated system to a particular group, the electron density of the conjugated system will therefore decrease, a phenomenon known then as the negative mesomeric effect (– M).

For the -M effect, the group must have a positive charge or a free orbital. The

-M effect makes the compound more reactive with the nucleophile because it reduces the electron density in the conjugated system. At the same time, it is less reactive towards the electrophile for the same reasons.


Negative Mesomeric Effect Examples:

$-N{{O}_{2}},-COH,$ and $-CN$

Negative Mesomeric Effect Order:

If the substituent is from the electron-withdrawing group, then the effect is negative.

$-N{{O}_{2}}>-CN>-S\left(=O\right)2-OH>-CHO>-C=O>-COOCOR>-COOR>-COOH>-CON{{H}_{2}}>-CO{{O}^{-}}$


What is the Significance of the Mesomeric Effect?

  • It describes the charge distribution in a compound, which helps to determine when electrophiles or nucleophiles strike.

  • Useful for describing physical properties such as dipole moment and bond length.


Resonance Effect

If two or more different structures can be drawn for a molecule or ion with the same arrangement of the atomic nuclei but different distributions of electrons, then these structures are said to be in resonance with each other. The different structures are called resonance structures. Not all properties of molecules or ions are represented by a single resonance structure, but the actual structure is a resonance hybrid of all resonance structures. Resonance structures are hypothetical structures of conjugated compounds used to explain the motion of electrons. The actual structure of the conjugated compound is the result of the hybridization of all resonance structures. This phenomenon is called delocalization, resonance fascination.


The difference between the calculated energy (heat of hydrogenation) and the experimental energy that contributes to the stability of a conjugated compound is called the resonance or delocalization energy. If there is a lot of resonance energy, it is better to stabilize the resonance.


Electron Displacement Effect

Organic reactions cannot occur until a certain charge or polarity is formed on the reactants, and they attach to each other. This only happens when there is a shift of electrons due to the polarization that develops in the reactant molecules. Such effects involving the displacement of electrons in the substrate (reactant) molecule are called "electron displacement effects".


There are four basic electron displacement effects.

  • Inductive effect

  • Electromeric effects

  • Resonance

  • Hyperconjugation


Applications of the Mesomeric Effect

  • Carbocation Stability: The stability of the carbocation is enhanced by resonance. All aromatic compounds are always more stable than non-aromatic compounds due to the resonance effect.

  • Carbanion Stabilization: Resonance increases therefore stability increases.

  • Free Radical Stability: Resonance increases the stability of free radicals.

  • Strength of Acids:

  • The strength of acids is directly proportional to the -M effect.

  • Acid strength is proportional to –I effect.

  • Acid strength is inversely proportional to +M effect.

  • Acid strength is inversely proportional to +I effect.


  • Order of Base Forces:

  • Base strength is proportional to + M effect.

  • Base Strength is proportional to +I effects.

  • Base Strength is inversely proportional to -M effects.

  • Base Strength is inversely proportional to the -I effect.


Conclusion

The mesomeric effect in chemistry is a phenomenon induced in a chemical compound due to substituents or functional groups on a chemical compound. It is defined as the polarization induced in a molecule by the interaction of two pi bonds or between a pi bond and a lone pair of electrons present on an adjacent atom. This effect is used qualitatively and describes the electron-withdrawing or releasing properties of the substituents depending on the resonance structure involved and is denoted by the letter M.


The mesomeric effect is negative (–M) when the substituent is an electron-withdrawing the substituent group, and the effect is positive (+M) when the substituent is the electron donor group.

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FAQs on Mesomeric Effect and Its Applications for JEE

1. Why is the mesomeric effect stronger than the inductive effect?

As resonance increases, the stability of the carbocation increases. The charge movement and number of resonance structures are the main reasons for the decreased electron shortage and increased stability. Furthermore, resonance is always more efficient and dominant than inductive effects.

2. Why do higher halogens not prefer to show mesomeric effects?

The higher halogens are weaker resonant groups because their p orbital does not overlap well with the 2p atomic orbital of the cyclic carbon hence the longer carbon-halogen bond length and the size of the 3p, 4p or 5p atomic orbital is big. Therefore, the higher halogens do not prefer to show mesomeric effects.