Organic Chemistry

Section 1.20 The Effect of pH on the Structure of an Organic Compound 51

A compound will exist primarily in its

acidic form if the pH of the solution is

less than its pKa.

A compound will exist primarily in its

basic form if the pH of the solution is

greater than its pKa.

We will discuss delocalized electrons in greater detail in Chapter 7. By that time,
you will be thoroughly comfortable with compounds that have only localized elec-
trons, and you can then further explore how delocalized electrons affect the stability
and reactivity of organic compounds.

PROBLEM 41

Which compound would you expect to be a stronger acid? Why?

PROBLEM 42

Draw resonance contributors for the following compounds:

1.20 The Effect of pH on the Structure

of an Organic Compound

Whether a given acid will lose a proton in an aqueous solution depends on the of
the acid and on the pH of the solution. The relationship between the two is given by the
Henderson–Hasselbalch equation. This is an extremely useful equation because it
tells us whether a compound will exist in its acidic form (with its proton retained) or in
its basic form (with its proton removed) at a particular pH.

The Henderson–Hasselbalch equation tells us that when the pH of a solution equals
the of the compound that undergoes dissociation, the concentration of the com-
pound in its acidic form [HA] will equal the concentration of the compound in its
basic form (because ). If the pH of the solution is less than the of
the compound, the compound will exist primarily in its acidic form. If the pH of the
solution is greater than the of the compound, the compound will exist primarily in
its basic form. In other words,compounds exist primarily in their acidic forms in solu-
tions that are more acidic than their values and primarily in their basic forms in
solutions that are more basic than their values.
If we know the pH of the solution and the of the compound, the Henderson–
Hasselbalch equation allows us to calculate precisely how much of the compound will
be in its acidic form, and how much will be in its basic form. For example, when a
compound with a of 5.2 is in a solution of pH 5.2, half the compound will be in
the acidic form and the other half will be in the basic form (Figure 1.20). If the pH is
one unit less than the of the compound there will be 10 times more
compound present in the acidic form than in the basic form (because ). If
the pH is two units less than the of the compound there will be 100
times more compound present in the acidic form than in the basic form (because
). If the pH is 6.2, there will be 10 times more compound in the basic form
than in the acidic form, and at there will be 100 times more compound
present in the basic form than in the acidic form.

pH=7.2

log 100 = 2

pKa (pH=3.2),

log 10 = 1

pKa (pH=4.2),

pKa

pKa

pKa

pKa

pKa

[A-] log 1 = 0 pKa

pKa

pKa = pH + log

[HA]

[A−]

the Henderson–Hasselbalch equation

pKa

• O O–
  C

O

• O O–
  N

O

+

a. b.

CH 3 CO or CH 3 S

O

O

O

H O H