Question

Why is phenol a stronger acid than `H-C-=N`?

Answer 1

A stronger acid?

Explanation:

Let us compare `pK_a` values.....

`H-C-=N, 9.21`.

`"phenol,"` `C_6H_5OH`, `9.95`

So on the basis of these data, hydrogen cyanide is a stronger acid than phenol. Phenoxide is resonance stabilized and able to distribute its negative charge over 7 centres. Further nitro substitution on the aryl ring can better distribute the negative charge of the phenoxide; consider`"1,3,5-trinitrophenol"`, whose `pK_a=0.38`.

On the other hand, the `H-C` bond in hydrogen cyanide is formally `"sp-hybridized"`, and should be reasonably strong....as the `pK_a` data illustrate. Both of these species are weak acids.....

Thx to @benintheusa who pointed out an error on my part....