Question

(i) Why phenol has a higher boiling point than toluene?
(ii) Why are alcohols easily protonated but phenol is not protonated?

Answer 1

Hint: Phenols have intermolecular H-bonding present in the, but there is no such bonding in the toluene and the lone pair of electrons on oxygen atom in case of phenols are in resonance with the benzene ring but in alcohols, there is no resonance and the lone pair of electrons are delocalized on the oxygen atom itself. Now answer the given statements.

Complete step by step solution:
First of all, let’s discuss what the boiling point is. By the boiling point we mean the temperature at which the substance begins to boil and changes its phase from liquid to vapor.
(i) In phenols there is intermolecular hydrogen bonding(i.e. the bonding between the different molecules, between the hydrogen atom of one molecule and the electronegative atom of another molecule) and therefore, they exist as associated molecules and therefore ,have higher boiling point. On the other hand in toluene, there is no such bonding and thus, have lower boiling point.
Thus, due to intermolecular hydrogen bonding and polar nature of -OH group in phenol, they have higher boiling point points than toluene.
(ii) In alcohols (R-OH) due to the presence of the electron donating alkyl group having positive inductive effect and high electron density of oxygen atom due to the presence of lone pairs of electrons on it, it easily undergoes protonation ( i.e. there is addition of proton). On the other hand , in case of phenols , the benzene is electron withdrawing and the lone pair of electrons present on the oxygen atom are in resonance with the benzene ring and thus, are not easily available for protonation. So, thus, they are not easily protonated.
That’s why alcohols are easily protonated but phenol is not easily protonated.

Note: Boiling point increases with increase in the surface area, atomic mass of the compound and the intramolecular hydrogen bonding but decreases with branching of the compound. This is so because due to branching, the surface area of the compound decreases and hence, the boiling point also decreases.