# What functional groups can act as acids?

Feb 19, 2014

The sulfonic, phosphoric, and carboxylic acid groups are the strongest acids. Many functional groups behave as weaker acids.

A functional group is an acid if it can donate a proton to a base.

Here is a list of functional groups that can behave as acids, along with their approximate $p {K}_{a}$ values.

R-SO₃H — sulfonic acids — $p {K}_{a}$ = -1 (important in industry)

R-PO₄H₂ — phosphoric acids — $p {K}_{a}$ = 2 (important in biochemistry)

R-COOH — carboxylic acids — $p {K}_{a}$ = 4

R-SH — Thiols — $p {K}_{a}$ = 13

R-OH — Alcohols — $p {K}_{a}$ = 17

R-NH₂ — Amines — $p {K}_{a}$ = 35

C-H bonds can also behave as acids. Their $p {K}_{a}$ values range from 13 to 50, depending on the groups that are next to the C atom.