Question

Why acetic acid is stronger than phenol whereas formic acid is stronger than acetic acid?

Answer 1

Carboxylic acid are more acidic than phenols because the negative charge in carboxylate anion is more spread out as compared to the phenoxide ion as there are two electronegative O-atoms in carboxylate anion in comparison to one in phenoxide ion. ... So Formic acid is more acidic than acetic acid.

hope it helps you.

Answer 2

YOUR ANSWER IS HERE
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IN DETAIL
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Ethan-1-oic acid ( Acetic acid )

Short but Simple explanation :

Out of all the organic compounds , Carboxylic acids are second only to Sulphonic acids in acidity. Due to which , they have comparatively higher values of Ka but lower values of pKa ,which explains their highly acidic nature than any other organic compound except Sulphonic acids , and Phenol is no exception to it. So, Acetic acid being a Carboxylic acid is more acidic than Phenol.

A little long but more detailed and reasonable explanation:

Reason 1:

Resonance Stabilisation :

In Phenol/Phenoxide ion , negative charge due to lone pair of Oxygen atom disperses between highly electronegative Oxygen atom and low electronegative Carbon atom. Whereas on the other hand, in Carboxylic acid/Carboxylate ion , charge disperses between highly electronegative Oxygen atoms only. Now , negative charge dispersing on highly electronegative atoms provides greater stability than the same being on low electronegative atoms. Therefore , Carboxylate ion is more resonance stabilised than Phenoxide ion. Therefore, Carboxylic acid shows greater polarity and protonation tendency and is associated with higher Ka and lower pKa value and greater acidic character.

Reason 2:

Resonating Structures:

Resonating structures of Phenol/Phenoxide ion are :

Resonating structures of Carboxylate ion ( Acetate ion ) are:

Now , seeing the above two , one may conclude that Phenoxide ion , should be more acidic as it is associated with greater number of resonating structures than Carboxylate ion. However , that is not the case.

Though it is absolutely right to say that greater the number of resonating structures , greater would be the stability. However , there is another factor to be accounted for which explains the high acidity of Carboxylic acids than Phenol. That is, the resonating structures of Carboxylate ion are equally contributing , i.e. both the resonating structures are equal and equally contribute in resonance hybrid for carboxylate ion. However , all the resonating structures in Phenoxide ion are not the same and thus unequally contribute in resonance stabilisation. Thus, comparing the stability of the resonance hybrid of the two structures , we conclude that resonance hybrid of Carboxylate ion proves to be more stable than that of Phenoxide ion. Thus,Carboxylic acids have more protonation tendency in order to obtain greater stability compared to Phenol and are thus, more acidic in nature.

Reason 3:

Weaker Conjugate base:

Phenoxide ion is more stronger conjugate base than carboxylate ion. And stronger the conjugate base , weaker is the acid. Thus , the equilibrium reaction remains more towards in backward direction in dissocation of Phenol to Phenoxide ion and proton than in the case of Carboxylic acids. Thus, Carboxylic acids are more acidic than Phenol.

Well, I suppose that the above three facts are sufficient in explaining the more acidic nature of Carboxylic acids ( or more specifically Acetic acid ) than Phenol.

Hope I am able to explain.
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