MCAT Organic Chemistry Review 2018-2019

The acid dissociation constant, or Ka, measures the strength of an acid in solution. In the
dissociation of an acid HA (HA H+ + A–), the equilibrium constant is given by:

Equation    4.1

and the pKa can be calculated as:

pKa =   –log    Ka

Equation    4.2

Thus, more acidic molecules will have a smaller (or even negative) pKa; more basic molecules will
have a larger pKa. Acids with a pKa below –2 are considered strong acids, which almost always
dissociate completely in aqueous solution. Weak organic acids often have pKa values between –2
and 20. pKa values for common functional groups are shown in Table 4.1.