Organic Chemistry

The Henderson–Hasselbalch equation is also useful when one is working with
buffer solutions. A buffer solutionis a solution that maintains a nearly constant pH
when small amounts of acid or base are added to it. Buffer solutions are discussed in
detail in the Study Guide and Solutions Manual (see Special Topic I).

PROBLEM 43

As long as the pH is greater than __________, more than 50% of a protonated amine with

a of 10.4 will be in its neutral, nonprotonated form.

PROBLEM 44 SOLVED

a. At what pH will 99% of a compound with a of 8.4 be in its basic form?

b. At what pH will 91% of a compound with a of 3.7 be in its acidic form?

c. At what pH will 9% of a compound with a of 5.9 be in its basic form?

d. At what pH will 50% of a compound with a of 7.3 be in its basic form?

e. At what pH will 1% of a compound with a of 7.3 be in its acidic form?

SOLUTION TO 44a If 99% is in the basic form and 1% is in the acidic form, the

Henderson–Hasselbalch equation becomes

There is a faster way to get the answer: If about 100 times more compound is present in the

basic form than in the acidic form, the pH will be two units more basic than the Thus,

SOLUTION TO 44b If 91% is in the acidic form and 9% is in the basic form, there is

about 10 times more compound present in the acidic form. Therefore, the pH is one unit

more acidic than the pKa.Thus, pH=3.7-1.0=2.7.

pH=8.4+2.0=10.4.

pKa.

pH=10.4

8.4=pH-2.0

8.4=pH+log .01

pKa=pH+ log

1

99

pKa

pKa

pKa

pKa

pKa

pKa

Section 1.20 The Effect of pH on the Structure of an Organic Compound 53

BLOOD: A BUFFERED SOLUTION

Blood is the fluid that transports oxygen to all the

cells of the human body. The normal pH of human

blood is 7.35 to 7.45. Death will result if this pH decreases to a

value less than or increases to a value greater than

for even a few seconds. Oxygen is carried to cells by a protein

in the blood called hemoglobin. When hemoglobin binds

hemoglobin loses a proton, which would make the blood more

acidic if it did not contain a buffer to maintain its pH.

A carbonic acid/bicarbonate buffer is used to

control the pH of blood. An important feature of this buffer is

that carbonic acid decomposes to CO 2 and H 2 O:

(H 2 CO 3 >HCO 3 - )

HbH+ ++O 2 HbO 2 H+

O 2 ,

'6.8 '8.0

Cells need a constant supply of with very high levels re-

quired during periods of strenuous exercise. When is con-

sumed by cells, the hemoglobin equilibrium shifts to the left to

release more so the concentration of decreases. At the

same time, increased metabolism during exercise produces

large amounts of This shifts the carbonic acid/bicarbonate

equilibrium to the right, which increases the concentration of

Significant amounts of lactic acid are also produced during

exercise, and this further increases the concentration of

Receptors in the brain respond to the increased concentra-

tion of and trigger a reflex that increases the rate of breath-

ing. This increases the release of oxygen to the cells and the

elimination of by exhalation. Both processes decrease the

concentration of H+in the blood.

CO 2

H+

H+.

H+.

CO 2.

O 2 , H+

O 2

O 2 ,

H 2 CO 3 HCO 3 −

carbonic acid bicarbonate

CO 2 + H 2 O + H+

Tutorial:

Effect of pH on structure