The Foundations of Chemistry

Sources of some naturally occurring
carboxylic acids.

1116 CHAPTER 28: Organic Chemistry II: Shapes, Selected Reactions, and Biopolymers

Acetic acid is 1.3% ionized in 0.10 Msolution. Regardless of the lengths of the chains,

the acid strengths of the monocarboxylic acids are approximately the same, with Kavalues

(in water) in the range 10^5 to 10^4. Their acid strengths increase dramatically when elec-

tronegative substituents are present on the
-carbon atom (Kavalues in water range from

10 ^3 to 10^1 ). Compare acetic acid and the three substituted acetic acids in Table 28-1.

There are two main reasons for this increase: (1) the electronegative substituents pull elec-

tron density from the carboxylic acid group, and (2) the more electronegative substituents

help to stabilize the resulting carboxylate anion by spreading the negative charge over

more atoms.

The alcohols are so very weakly acidicthat they do not react with strong bases. They

have about the same acid strength as water (see Table 28-1), and some of their reactions

are analogous to those of water.

The reactive metals react with alcohols to form alkoxideswith the liberation of

hydrogen.

2CH 3 CH 2 XOH2Na88nH 2 2[NaCH 3 CH 2 O]

ethanol sodium ethoxide

(an alkoxide)

Alkoxides are strong bases that react with water (hydrolyze) to form the parent alcohol

and a metal hydroxide.

[NaCH 3 CH 2 O]HXOH88nCH 3 CH 2 OH[NaOH]

sodium ethoxide ethanol

Phenols react with metallic sodium to produce phenoxides;the reactions are analo-

gous to those of alcohols. Because phenols are more acidic than alcohols, their reactions

are more vigorous.

2Na H 2 Na

OH O

2 2

phenol sodium phenoxide

In Section 18-3 we defined

pKalog Ka

When Kagoes up by a factor of 10,
pKagoes down by one unit. We see
that the stronger an acid, the lower its
pKavalue.

This is similar to the reaction of water
with active metals.

2HXOH2Na88n
H 2 2[NaOH]

TABLE 28-1 Kaand pKaValues of Some Carboxylic Acids

Name Formula Ka pKa

formic acid HCOOH 1.8 10 ^40 3.74

acetic acid CH 3 COOH 1.8 10 ^50 4.74

propanoic acid CH 3 CH 2 COOH 1.4 10 ^50 4.85

monochloroacetic acid ClCH 2 COOH 1.5 10 ^30 2.82

dichloroacetic acid Cl 2 CHCOOH 5.0 10 ^20 1.30

trichloroacetic acid Cl 3 CCOOH 2.0 10 ^10 0.70

benzoic acid C 6 H 5 COOH 6.3 10 ^50 4.20

phenol* C 6 H 5 OH 1.3 10 ^10 9.89

ethanol* CH 3 CH 2 OH 
 10 ^18 
 18.

*Phenol and ethanol are not carboxylic acids. Phenol is weakly acidic compared with carboxylic acids, whereas ethanol

is even weaker than water.