Organic Chemistry

42 CHAPTER 1 Electronic Structure and Bonding • Acids and Bases

Do not confuse pH and The pH scale is used to describe the acidity of a solution;

the is characteristic of a particular compound, much like a melting point or a boil-

ing point—it indicates the tendency of the compound to give up its proton.

PROBLEM 28

a. Which is a stronger acid, one with a of 5.2 or one with a of 5.8?

b. Which is a stronger acid, one with an acid dissociation constant of or one

with an acid dissociation constant of

PROBLEM 29

An acid has a of in water. What is its

The importance of organic acids and bases will become clear when we discuss how

and why organic compounds react. The most common organic acids are carboxylic

acids—compounds that have a COOH group. Acetic acid and formic acid are exam-

ples of carboxylic acids. Carboxylic acids have values ranging from about 3 to 5.

(They are moderately strong acids.) The values of a wide variety of organic com-

pounds are given in Appendix II.

Alcohols—compounds that have an OH group—are much weaker organic acids,

with values close to 16. Methanol and ethanol are examples of alcohols.

We have seen that water can behave both as an acid and as a base. An alcohol behaves

similarly: It can behave as an acid and donate a proton, or as a base and accept a

proton.

A carboxylic acid can behave as an acid and donate a proton, or as a base and accept a

proton.

+ HO− + H 2 O

+ H 3 O+ + H 2 O

a base

an acid

+OH

CH 3

C

O–

O

CH 3

C

OH

O

CH 3

C

OH

CH 3

C

OH

O

CH 3 OH CH 3 O− + H 2 O

an acid

+ HO−

CH 3 OH CH 3 OH + H 2 O

a base

+ H 3 O+

+

Η

CH 3 OH

methanol

pKa = 15.5

CH 3 CH 2 OH

ethanol

pKa = 15.9

pKa

acetic acid

pKa = 4.76

formic acid

pKa = 3.75

CH 3

C

OH

O

H

C

OH

O

pKa

pKa

Ka 4.53* 10 -^6 Keq? ([H 2 O]=55.5 M)

2.1* 10 -^4?

3.4* 10 -^3

pKa pKa

pKa

pKa :