Mesomeric Effect in Phenol

The concept of mesomeric effect, mesomerism and Mesomers was laid down by scientist Ingold in the year 1938. 

 Apparently, Mesomer is a connotation of the word resonance whose idea was given by the scientist Pauling. It is the property that is possessed by the functional groups in a chemical compound. 

The mesomeric effect is the phenomenon in which a molecule can exist in more than one structure, but none of the structures are fundamental structures. 

What is a mesomeric effect?

The polarity developed between atoms of a conjugated system by the electron transfer or pi–bond electron transfer is known as the Mesomeric effect.  

In other or simpler words, it can be described as the phenomenon which occurs when pi-electrons either move away or towards a substituent group in a conjugated system of orbits. The mesomeric effect creates resonance in the molecules. It is also known as the electromeric effect as it provides stability by shifting a pair of electrons from one place to another.

There are two types of mesomeric effects – +M and -M.

Electron is present only in ortho or para position as meta is always silent towards resonance or mesomeric effect. 

Note – alkyl groups to activate the benzene ring uses alpha-H

         A double-bonded system is directly attached to sp3 carbon. 

 – M effect aka Negative Mesomeric effect 

It is the condition when the pi electrons are taken from a conjugate system and is attached to a particular group. 

Therefore it decreases the electron density of the conjugated system

The essential atom will have a positive charge 

They are also known as deactivators of the ring

CF3, CCl4 are examples of -M groups

+M effect aka Positive mesomeric effect

It is the condition when pi electrons are taken from a particular group and is attached to a conjugate system. 

Therefore, it increases the electron density in the conjugated system.

The +M group circulated the negative charge throughout the benzene ring.

The mesomeric effect in phenol 

The mesomeric effect is used to describe characteristics of phenol like acidity, basicity etc.

Phenol shows a positive mesomeric effect which helps in the ionisation of phenol from hydrogen ions and therefore shows acidity. It also affects the bond length of the compound. 

The electrons will not be present in the Meta position, showing a positive mesomeric effect. 

Phenols are more acidic than aliphatic alcohols as the benzene ring stabilises the phenoxide ion by resonance delocalisation of the negative charge.

Subsequent effects of acidity of phenols

Electron-donating substituents make a phenol less acidic by destabilising the phenoxide ion due to mesomeric or resonance effect

Electron-withdrawing substituents make phenol more acidic by stabilising the phenoxide ion through delocalisation of the negative charge and through inductive effects as it counteracts mesomeric effects.

Stability of the phenyl carbocation 

The phenyl carbocation is unstable because it has high bond breaking energy of the 

C-H benzene bond

Explanation 

C-H bonds of the benzene rings are sp2 hybridised 

This signifies that there is a high s character in the ring due to which the electrons are closer to the nucleus, which makes the bonds tougher to break. 

Apparently, the sp2 has a vacant orbital that lies at the planar area of the given benzene ring, which cannot be stabilised by the mesomeric effect as the pi bonds can not overlap it.

Conclusion 

 The mesomeric effect lays the basics of acidity and basicity in the phenols. The positive and negative mesomeric effects are wholly and solely dependent on the groups attached and the conjugated system. Resonance alters the bond length of the benzene ring, making it more stable. 

 Only ortho and para positions are involved in the mesomeric effect. The mesomeric effect, especially the positive mesomeric effect, always overpowers the negative inductive effect. The meta position always stays silent in the mesomeric effect. The phenoxide ion is more stable than phenol as the mesomeric effect works on the stability of the carbocation. 

Still, the phenyl carbocation is pretty unstable as the resonance does not work there as the gap is present in the planar sp2, and pi bonds cannot overlap it. Higher the resonance higher the stability