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In chemistry, the mesomeric effect, also known as the resonance effect, is a feature of substituents or functional groups in a molecule. The effect is symbolized by the letter ‘M’ and is used to describe the electron-withdrawing or releasing properties of substituents depending on the relevant resonance structure. The mesomeric effect is negative when a substituent is an electron-withdrawing group, and the effect is positive when a substituent is an electron-releasing group. Acetyl-nitrile-nitro is an example of a -M substituent. Alcohol, amine, and benzene are examples of +M substituents. The inductive action also determines the net electron flow from or to the substituent. 

Mesomeric Effect types 

  • +M effect mesomeric effect  
  • -M effect mesomeric effect 

+M Mesomeric Effect 

In a positive mesomeric effect, the electron density of the molecule or conjugated system increases when an electron moves away from a specific group and towards the rest of the molecules. This is demonstrated by electron-releasing substituents. OH, SH, -OR, -OCOR, NH2, -NR2, –NHCOR, Ph, –CH3, –F, –Cl, –Br, –I are some of the +M substituents.    

-M Mesomeric Effect 

In a negative mesomeric effect, the electron density of the molecule or conjugated system falls when electrons are transported from the rest of the molecules to a specific group.  This is demonstrated by electron-withdrawing substituents. NO2, –CN, –SO3H, –COOH, –COOR, –CONH2, –COO, –CHO, –COR are some of the -M substituents.  

Significance of Mesomeric Effect 

  1. The effect depicts the charge distribution in the molecule. This provides an effective way of predicting the point of attack of electrophiles and nucleophiles. 
  2. Useful in explaining physical characteristics i.e. bond length, dipole moment etc. 

Applications of Mesomeric Effect 

  1. The negative resonance effects of the carbonyl group delocalize electrons, by removing electrons and lowering electron density, especially on the third carbon.  
  2. The cyanide group in acrylonitrile has a negative mesomeric impact (-R or -M). The electron density on the third carbon decreases due to electron delocalization towards the cyanide group. 
  3. The nitro group, -NO2, in nitrobenzene exhibits the -M effect due to the delocalization of conjugated electrons. It is worth noting that the electron density on the benzene ring is lower in the ortho and para positions. It is the reason why the nitro group deactivates the benzene ring against an electrophilic substitution reaction. 
  4. The -OH group in phenol exhibits a +M effect due to the delocalization of the lone pair on the oxygen atom towards the ring. As a result, the electron density on benzene rings increases, especially in the ortho and para positions. As a result, phenol is more prone to electrophilic substitution reactions. The substitution is preferred at ortho and para positions. 
  5. In aniline, the -NH2 group likewise has a +R impact. Through delocalization, it releases electrons towards the benzene ring. The electron density on the benzene ring increases and as a result, aniline activates the ring, allowing it to undergo electrophilic substitution. It is also worth noting that delocalization leads to a decrease in electron density on nitrogen in aniline which is why it has lower basic strength than ammonia and alkylamines. 

Mesomeric Effect and Resonance Effect 

The mesomeric effect is the dipole created in a molecule or a conjugated system by the movement of electrons towards or away from a substituent group. It is represented by M.  Various canonical structures show how delocalization of pi electrons causes the withdrawal or release of electrons associated with a specific substituent.  The +R (electron releasing group) is equivalent to the + M effect, while the -R (electron attracting) group is equivalent to the -M effect. 

Mesomeric Effect vs Inductive Effect 

Mesomeric Effect Inductive Effect 
The mesomeric effect is the  polarity created in a molecule or conjugated system by electron movement towards or away from a substituent group An inductive effect occurs when the polarization of one bond is caused by the polarization of another bond
It is a long term effect in which the substituent or functional groups in a chemical compound plays a crucial role The difference in the electronegativity in the two atoms in the bond has a direct impact on the inductive action
The mesomeric impact is negative (-M) when a substituent is an electron-withdrawing group A negative inductive effect describes the electron-withdrawing characteristic of groups of atoms. It is denoted by the letter -I

Conclusion 

In a conjugated system, the mesomeric effect can be transmitted with any number of carbon atoms. Charge delocalization results in the resonance stabilization of the molecule. The discrepancy in the negative values of atoms in these functional groups causes the occurrence. Higher negativity, for example, indicates a greater capacity to donate electrons. The electron-donating or releasing impact of these substituents is described by the negative mesomeric effect. The electron-withdrawing effect of these substituents is described by the positive mesomeric effect