Take the 2-minute tour ×
Chemistry Stack Exchange is a question and answer site for scientists, academics, teachers and students. It's 100% free, no registration required.

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

share|improve this question

3 Answers 3

up vote 18 down vote accepted

Nope. Alcohols consist of an $\ce{OH}$ group bonded to a saturated carbon ($sp^3$ 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

A phenol consists of an $\ce{-OH}$ bonded to an unsaturated $sp^2$ 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 $S_N1$ 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 $S_N1$ 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.

share|improve this answer
    
Actually the question in my book was "Which of these cannot be considered as an alcohol", and phenol was an option. –  Kartik Anand Jul 3 '12 at 14:46
    
@kartik It is not an alcohol by definition. The second part of my answer deals with why we cannot even consider it as one. –  ManishEarth Jul 3 '12 at 14:48
1  
@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. –  bobthejoe Jul 3 '12 at 21:51
    
(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. –  james_ash Jul 9 '12 at 2:15
    
@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. –  ManishEarth Jul 9 '12 at 4:29

Phenols are the aromatic organic compounds that have a hydroxyl group directly connected to a ring and alcohols are the non- aromatic compounds that have a hydroxyl group connected to the main chain. The difference is that one is cyclic and one is non- cyclic.

share|improve this answer

Phenol, the -OH group is attached to benzene, i.e unsaturated compound while alcohol. The -OH group is attached to saturated compound

share|improve this answer
3  
Welcome to Chemistry.SE. This answer is basically a summary of the already accepted answer. Do you have anything new/differnet to add? –  Ben Norris Jan 25 at 21:04

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.