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I know phenols are more acidic as compared to alcohols, but are they considered different from alcohols?

Sure, you can study something as a subset, but are phenols considered a subset of alcohols, or are they considered as completely different from alcohols? My confusion is due to the fact that both of them contain a hydroxyl group.

Phenol
Alcohol

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  • $\begingroup$ Then what are the difference between above mentioned ones and carboxylic acid? $\endgroup$
    – On the way to success
    Jun 19, 2015 at 14:39

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Nope. Alcohols consist of an -$\ce{OH}$ group bonded to a saturated carbon ($\mathrm{sp^3}$ hybridized, no multiple bonds).

IUPAC says:

alcohols

Compounds in which a hydroxy group, -$\ce{OH}$, is attached to a saturated carbon atom $\ce{R3COH}$. The term 'hydroxyl' refers to the radical species, $\ce{HO^.}$.

and

phenols

Compounds having one or more hydroxy groups attached to a benzene or other arene ring, e.g., 2-naphthol:

2-naphthol structure
(source: iupac.org)

A phenol consists of an -$\ce{OH}$ bonded to an unsaturated $\mathrm{sp^2}$ carbon. Thus, it does not qualify as an alcohol. One can classify it as an enol, though.

Really, to me, the classification doesn't matter. Classifications are artificial, what is important is, how well the properties fit in the classification.

Many of the alcohol properties depend upon:

  • Its unsaturated nature: oxidation to ketone/aldehyde/acid
  • The weaker $\ce{R-O}$ bond and its ability to easily break and form an $\ce{R+}$ cation (this makes it a good participant in $\mathrm{S_N1}$ reactions)

Phenol can obviously not be oxidised at the $\ce{OH}$ to a ketone/acid (though one can do stuff to make it into a quinone). Phenylic carbocations are unstable, thus we don't get any $\mathrm{S_N1}$ reactions, and the $\ce{Ph-O}$ bond stays put.

On the other hand, most of the reactions of phenol depend upon its

  • Aromatic phenyl ring: All the EAS reactions
  • Weaker $\ce{O-H}$ bond (i.e., acidic nature): Reimer-Tiemann reaction, etc.

Thus phenols and alcohols don't have too many reactions in common. So, in this case, they have been classified in a sensible manner--if phenols were classified as alcohols, we would basically be clubbing two radically different classes of compounds under one umbrella.

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    $\begingroup$ Actually the question in my book was "Which of these cannot be considered as an alcohol", and phenol was an option. $\endgroup$
    – Kartik Anand
    Jul 3, 2012 at 14:46
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    $\begingroup$ @kartik It is not an alcohol by definition. The second part of my answer deals with why we cannot even consider it as one. $\endgroup$
    – ManishEarth
    Jul 3, 2012 at 14:48
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    $\begingroup$ @KartikAnand, one must remember that what is taught in a course is not always necessarily agreed on in the field. The "Central Dogma" comes to mind. $\endgroup$
    – bobthejoe
    Jul 3, 2012 at 21:51
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    $\begingroup$ (Re:last paragraph) I agree that classifications are somewhat artificial. But I disagree that phenols and alcohols have nearly no reactions in common. Both alcohols and phenols are capable of acting as weakly acidic species; when deprotonated, their conjugate bases are both strong bases and good nucleophiles; and they can undergo many reactions in common at oxygen, such as alkylation, silylation, acylation, and many others. This is a significant facet of the chemistry of alcohols and should not be overlooked. $\endgroup$
    – james_ash
    Jul 9, 2012 at 2:15
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    $\begingroup$ @james_ash: (I converted your post to a comment, once you have 50 rep you can comment yourself). Hmm, I sort of disagree with your comments on their strengths as acids/bases/nucleophiles, but that is only because their relative strengths are quite different. I'll edit my post, though--you're absolutely right about the other reactions. Basically I want to say that their nature is different enough for them to be classified differently. $\endgroup$
    – ManishEarth
    Jul 9, 2012 at 4:29

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