Why pyridine does not exhibits resonance and Why is phenol more acidic than cyclo hexanol ?

Why pyridine does not exhibits resonance?

Pyridine is a hetero cyclic aromatic compound. It has three alternating double bond.

But,the lone of electron on nitrogen atom does not participate in the resonance with the pi electron of pyridine ring.

Because, the p-orbital of carbon atom is mutually perpendicular to the lobe of p-electron orbital of nitrogen atom, that is they are present in different plane.

Consequently, pyridine does not exhibits resonance with the pi electron of pyridine ring.

Why phenol is more acidic than cyclo hexanol ?

In case of phenol, the lone pair of electron on oxygen atom takes part in the resonance with the pi electron of benzene ring.

As a result, the oxygen atom of phenol gets partially positive charge. Hence, more electronegative oxygen atom attract the O — H bonded electron towards itself.

Thus the O — H bond becomes more polar and more weak. So, it break easily to release H+ ion.

On the other hand, in case of cyclo hexanol, such type of resonance does not occur due to absence of conjugation.

Hence O –H bond of cyclo hexanol is less polar with compare to phenolic O –H bond. So, cyclo hexanol does not release proton easily. Consequently, phenol is more acidic than cyclo hexanol.

Why is m-nitro phenol or 3-nitro phenol more acidic than phenol ?

All the mono nitro phenol are more acidic than phenol. Because — NO2 is a strong electron withdrawing group.

Due to — I effect and — R effect, — NO2 group attract O — H bonded electron. Hence O –H bond becomes more polar and more weak. So O –H bond break easily to release H+ ion.

Although, in case of m-nitro phenol or 3-nitro phenol, conjugation between –NO2 group and lone pair of oxygen atom of — OH group does not occur due to absence of conjugation between them.

But, due to –I effect of –NO2 group the density of electron on oxygen atom decrease. Hence oxygen atom gets partially positive charge. So more electronegative oxygen atom attract the O — H bonded electrons towards itself.

As a result, O –H bond becomes more polar and more weak. So O –H bond break easily to release H+ ion. Consequently, m-nitro phenol or 3-nitro phenol is more acidic than phenol.

Summary