# How do you determine acidity of aromatic compounds?

Jun 22, 2017

#### Answer:

How else but by measurement.........?

#### Explanation:

The $p {K}_{a}$ of $\text{benzoic acid}$, ${C}_{6} {H}_{5} C {O}_{2} H$, is known and widely reported, $p {K}_{a} = 4.202$. This means that is a stronger acid than that of $\text{acetic acid}$, $p {K}_{a} = 4.76$, and this is probably an entropy effect in that $\text{benzoate}$ anion is less charge dense than $\text{acetate}$.

On the other hand, the $p {K}_{a}$ of $\text{phenol}$, ${C}_{6} {H}_{5} O H$ is $9.95$.

There are various arguments that we could use to justify the apparent order of acidities, but BEFORE we make them, i.e. before the BS, we first make the measurement, or at least consult the data in the literature.