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

1 Answer

Answer:

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

Explanation:

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

#PhCO_2H +H_2O rightleftharpoonsPhCO_2^(-) + H_3O^+#

#pK_a=4.202#

#PhSO_3H +H_2O rightleftharpoonsPhSO_3^(-) + H_3O^+#

#pK_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.