# Why is "phenyl sulfonic acid" a STRONGER acid than "benzoic acid"?

Sep 14, 2017

Because a sulfonic acid is inherently more acidic than a carboxylic acid.....

#### Explanation:

As chemists, as physical scientists we should first examine the data....

$P h C {O}_{2} H + {H}_{2} O r i g h t \le f t h a r p \infty n s P h C {O}_{2}^{-} + {H}_{3} {O}^{+}$

$p {K}_{a} = 4.202$

$P h S {O}_{3} H + {H}_{2} O r i g h t \le f t h a r p \infty n s P h S {O}_{3}^{-} + {H}_{3} {O}^{+}$

$p {K}_{a} = - 2.8$

And thus the sulfonic acid is a million times more acidic......

And we can probably attribute the phenomenon to entropy effects. The sulfonic group is larger, with MORE oxygens atoms, and thus better able to stabilize the negative charge of the conjugate acid. The electronegativities of sulfur versus carbon are to a first approx. equal.