Question

Why are alcohols not considered acids?

More simply, why can alcohols not donate protons from their hydroxyl group under s.t.p. ?

Answer 1

You know that not ALL hydroxides or hydrogen halides are strong acids....

Explanation:

For the hydrogen halide series...

`HX(aq) + H_2O(l) rightleftharpoonsH_3O^+ + X^-`

For `X=Cl, Br, I` the equilibrium lies to the right as we face the page. But for `X=F`, the smaller fluorine atom competes for the proton, and the fluoride conjugate base is entropically disfavoured.

Now SOME hydroxides are also strong acids, for instance sulfuric acid:

`(HO)_2S(=O)_2+2H_2O rightleftharpoons 2H_3O^+ + SO_4^(2-)`

And here the negative charge of the dianion is distributed about the 5 centres of the sulfate anion....the which enhances the acidity of the acid.

Nitric acid is another example...

`(O=)stackrel(+)N(O^(-))OH+H_2OrarrH_3O^+ + (O=)stackrel(+)N(O^(-))O^-`

Again, the formal charges are delocalized over FOUR centres....

But now consider the action of an alcohol as an acid...

`ROH(aq) + H_2O(l) rightleftharpoons H_3O^+ + RO^(-)`

Here the alkoxide is strongly charge-localized...with the negative charge confined to the oxygen, and not distributed over several centres...the given equilibrium lies strongly to the left as written..

The alcohol could react with sodium metal...

`ROH(l)+ Na(s) rightleftharpoons RO^(-)Na^+ + 1/2H_2(g)uarr`

...but without such strong reagents, the acidity of the alcohol does not manifest itself.