Why is phenol more acidic than an alkyl alcohol?

2 Answers
Jan 4, 2016

The phenoxide anion is stabilized somewhat by resonance.


As physical scientists, we should look up measures of acidity, say phenol versus ethanol.

#pK_a#, phenol #=# #10#. #pK_a#, ethanol #=# #16#.

So there is a a difference in acidity by 6 orders of magnitude. The negative charge of the base is distributed over 7 centres in phenoxide versus 1 centre in ethoxide. Phenoxide has several resonance structures available.

Jan 5, 2016

Because phenol's conjugate base is more weakly acidic than that of an alkyl alcohol, making the regular phenol more strongly acidic than an alkyl alcohol.

To examine the acidities, let us look at the resonance structures of ethoxide vs. phenoxide:

As you can see, ethoxide would violate the octet rule, which is supposed to hold for carbon, and that would be unfavorable. Hence, it has no reasonable resonance structures.

Phenoxide on the other hand, has resonance stabilization all throughout the molecule. It has good #\mathbf(pi)# electron delocalization, distributing the energy more evenly, thus making it overall more stable and less acidic.

Since phenoxide is less acidic than ethoxide, if you protonate it, it has a greater tendency to donate the proton away than ethoxide does, making phenol more acidic than ethanol.

We can see this in their pKas:

Phenol: #~~ 9.9#
Ethanol: #~~15.9#

where a lower pKa corresponds to a stronger acid.