Why is pyridine more basic than pyrrole and Why phenols are more acidic than methanol ?

Why is pyridine more basic than pyrrole ?

The strength of basicity depends on the tendency to donate an electron to the electron deficient element, ion or group. 

In case of pyrrole, the lone of electrons on nitrogen atom takes part in the resonance with the pi electron of pyrrole ring, through which pyrrole attain the Huckel’s rule of aromaticity.

As a result, the availability of electrons for donation  is not easy, which decrease the basicity of pyrrole.

 On the other hand, pyridine is already aromatic compound. The lone of electron on nitrogen atom does not participate in the resonance with the pi electron of pyridine ring . 

Because, pyridine contains three alternating pi bond . So resonance is not possible. In addition to , the p-orbital of carbon atom is mutually perpendicular to the lobe of p-electron orbital of nitrogen atom . 

Therefore , it is impossible to occur resonance between p-orbital of carbon atom and p-orbital of nitrogen atom. 

Hence ,the lone pair on ‘N’-atom is available for donation to the electron deficient element, ion or group.Consequently, pyridine is more basic than pyrrole.

Why is acetic acid more acidic than alcohol ?

If acetic acid is ionized in aqueous solution ,it gives H⁺ cation and acetate anion. On the other hand, on ionization, alcohol gives H⁺ cation and alkoxide anion.

Now, the strength of acid depends on the relative stability of their concerned anion ( that is conjugate base ). Acetate anion have two equivalent resonating structure through which it is stabilized.

But alkoxide anion have no such type of resonating structure. In addition to, +I effect of alkyl group destabilized the alkoxide anion.

Consequently, acetic acid is more acidic than alcohol.

Why phenols are more acidic than methanol ?

Phenols are more acidic than methanol. This can be explained by the resonating structure of phenol. 

The lone pair on oxygen atom takes part  in resonance with the pi electron of benzene ring.

As  a result, the oxygen atom of phenol gets partial positive charge . So, the oxygen atom attract the O – H  bonded electron towards itself.

Hence, O – H bond becomes more polar and also more weak. Due to this, the O – H  bond breaks easily to release H⁺ ion.

But in case of methanol such type of resonance is not possible.Therefore , the O – H bond of methanol is more stronger than phenol and hence the tendency to release proton is very much less.

Consequently, phenol is more acidic than methanol.

Summary

• Why phenols are more acidic than methanol ?
• Why is pyridine more basic than pyrrole ?
• Why is acetic acid more acidic than alcohol ?

C – O bond length in phenol is less than methanol .