Why acetic acid is more acidic than phenol and Why phenol are more acidic than aliphatic alcohol ?

Why acetic acid is more acidic than phenol?

Both acetic acid and phenol are ionized in aqueous solution and give acetate and phenoxide anion respectively. Now ,the strength of acid depends on relative stability of their concerned anion ( conjugate base ).

Acetate anion have two equivalent resonating structure. The negative charge on oxygen atom equally spread out between two oxygen atom.

On the other hand,in case of phenoxide anion , there are five resonating structure. But the negative charge carry the less electronegative carbon atom.

Consequently, the relative stability of acetate anion is higher than phenoxide anion. Therefore , acetic acid is more acidic than phenol.

Why is acetic acid more acidic than ethanol?

If acetic acid is ionized in aqueous solution ,it gives H⁺ cation and acetate anion. On the other hand, on ionization, ethanol gives H⁺cation and ethoxide anion.

Now, the strength of acid depends on the relative stability of their concerned anion ( that is conjugate base ).

Acetate anion have two equivalent resonating  structure through which it is stabilized.

But ethoxide anion have no such type of resonating structure. In addition to, +I effect of ethyl group destabilized the ethoxide anion.

            Consequently, acetic acid more acidic than ethanol.

Why phenol are more acidic than aliphatic alcohol?

Phenol are more acidic than aliphatic alcohol. This can be explained by the resonating structure of phenol.The lone pair on oxygen atom takes part  in resonance with the pi electron of benzene ring.

As  a result, the oxygen atom of phenol gets partial positive charge. So, the oxygen atom attract the O – H bonded electron towards itself.

Hence, O – H bond becomes more polar and also more weak. Due to this, the O – H  bond breaks easily to release H⁺ ion.

But in case of aliphatic alcohol ( R–CH2-OH ) such type of resonance is not possible.

Therefore , the O – H bond of aliphatic alcohol is more stronger than phenol and hence the tendency to release proton is very much less.

Consequently, phenol is more acidic than aliphatic alcohol.

Summary



Why acetic acid is more acidic than phenol ?
Why is acetic acid more acidic than ethanol ?
Why phenol are more acidic than aliphatic alcohol?
Why acetic acid is more acidic than benzenol?

Ionic bond  is non directional but covalent bond is directional.