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In phenol, pulling the $\mathrm{p}_z$ electrons from the oxygen atom into the ring causes the hydrogen atom to be more partially positive than it is in aliphatic alcohols. This means it is much more easily lost from phenol than it is from aliphatic alcohols, so phenol has a stronger acidic property than ethanol.

Could someone please explain the link between the $\mathrm{p}_z$ electrons of oxygen overlapping with the cloud of delocalised electrons of the phenol ring and the increase in positivity of the bonded hydrogen … which in turns allows phenol to lose a proton more easily?

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    $\begingroup$ If there's one thing to be learned from the nice question and the answers, it is: Draw, draw, draw :) Keep a pen ready and draw resonance structures on paper, beer mats, etc. $\endgroup$
    – Klaus-Dieter Warzecha
    Commented Mar 10, 2014 at 8:37

4 Answers 4

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In fact I find a more simple reasoning with resonance structure. When phenol loses the $\ce{H+}$ the phenolate ion is stabilized due to the resonance effect, as shown below:

Phenoxide resonance forms

The energy of the dissociated form is lower and so phenol has more chance to be in the solution dissociated with the phenolate ion. Aliphatic alcohols are not stabilized by resonance so they are not very prone to dissociation.

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    $\begingroup$ While this is by far the most common explanation for the acidity of phenol, it may not be the largest factor. Comparing a simple saturated alcohol with phenol, there is also an inductive effect operating due to the $sp^2$ carbon in the latter case, as opposed to a $sp^3$ carbon in an alcohol, and it must be taken into account. Of course, the importance of the mesomeric effect can be increased by substitution, and picric acid is a clear example of this. $\endgroup$
    – Nicolau Saker Neto
    Commented Jul 21, 2015 at 17:50
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    $\begingroup$ You are assuming that the phenol and alcohol are in water. In a less polar solvent I suspect no or very little dissociation of phenol occurs, thus the polar nature of the solvent has a real role to play in stabilising the ions and driving the reaction what ever the nature of the 'resonance'. BTW is there any experimental evidence for these resonance structures even though they are commonly drawn? $\endgroup$
    – porphyrin
    Commented Jul 8, 2016 at 8:29
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The effect is indeed amazing, if you compare the $\mathrm{p}K_\mathrm{a}$ of tert-butanol (17.0) with that of phenol (9.95).

Deprotonation is facilitated when the reaction goes downhill (energywise). In order to obtain stabilization of the anion, the negative charge needs to be distributed over a larger number of centres. This distribution (only) is possible if the participating orbitals overlap in space.

So, which orbitals may play a role here? On the one hand, there is the $\pi$ system of the benzene, formed from the $\mathrm{p}_z$ orbitals on the carbon atoms. On the other hand, there's the $\mathrm{p}_z$ on the oxygen with the same spatial orientation.

Now, can we get comparable molecules, more acidic than phenol itself?

The answer is yes, if we allow further substituents that (a) pull the negative off the oxygen by inductive effects or (b) even allow for a further distribution of the negative charge by additional resonance.

resonance of picric acid

An impressive example for the latter is 2,4,6-trinitrophenol, also known as picric acid with a $\mathrm{p}K_\mathrm{a}$ of 0.29.

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Here's a simple explanation The acidity of any molecule is decided by the stability of it's ANION. We know that one of the lone pairs of oxygen in phenol gets conjugated with pie e- of the ring, distributing the charge, making phenoxide ANION very stable when H+ leaves in aq.

In alcohols, alkyl groups rather pushes the e- on already negative oxygen, concentrating the charge on ANION rather making less stable hence less acidic.

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    $\begingroup$ This doesn't add anything that hasn't already been covered by the previous answers. $\endgroup$
    – bon
    Commented Nov 13, 2015 at 14:58
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Phenol is more acidic than ethanol because in phenol lone pair of electrons are utilized in resonance stabilization so bond length between O and H atom increases and $\ce{H+}$ ion is easily released. While in case of alcohol no resonance stabilization takes place so no release of $\ce{H+}$ ion.

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