Chemistry - A Molecular Science

Chapter 13 Organic Chemistry

306

CARBOXYLIC ACIDS Carboxyl groups

consist of a hydroxyl group attached to a carbonyl group (Figure 13.19)

and have the general formula RCOOH. The

carbonyl oxygen removes electron density

from the O-H bond, which makes carboxyl groups acidic (Figure 13.20).

Carboxylic acids are named by identifying the longest continuous chain that contains

the carboxyl group and replacing the ‘e’ in the name of the corresponding alkane with -

oic

and adding the word

acid

. Thus, the four-carbon acid C

H 3

COOH is butanoic acid and the 7

six-carbon acid C

H 5

COOH is hexanoic acid if there is no branching. The most common 11

exception is CH

COOH, which is referred to as acetic acid rather than ethanoic acid. 3

When R = H, the acid is called formic acid,

HCOOH. The anions of acids are named by

replacing the

-ic

ending of the acid with

-ate.

Thus, C

H 5

COO 11

1- is the hexanoate ion,

HCOO

1- is the formate ion, and CH

COO 3

1- is the acetate ion.

Carboxylic acids with long chains are called

fatty acids

. The long chains of animal

fatty acids are saturated, while many of the vegetable fatty acids are unsaturated. As discussed at the end of Section 13.1, saturated chains contain no carbon-carbon double bonds, while unsaturated chains contain carbon-carbon double bonds.

Soaps

are the salts

of fatty acids. CH

(CH 3

) 216

COOH is stearic acid, a saturated fatty acid found in animal

tissue, while CH

(CH 3

) 216

COONa is sodium stearate, a soap. Soaps contain a hydrophobic

tail (CH

-(CH 3

) 2 n

-) and a polar head (COO

1-), so they are also detergents (Section 10.4).

AMINES AND AMIDES Organic derivatives of ammonia that are not bound to a carbonyl group are called

amines

.^

Two examples of simple amines are methyl amine, CH

NH 3

, and dimethyl amine, 2

(CH

) 32

NH. When the nitrogen is bound to a carbonyl group, it is called an

amide

. The

groups around the N atom in amides are coplanar, so the N is sp

2 hybridized. The planarity

of the amide group is accounted for with the resonance structure in Figure 13.21. Amine groups are present in many naturally occurring

materials, four of which are shown in

Figure 13.22. Note that caffeine also contains two amide nitrogens.

Compounds containing an amine and a carboxylic acid are called

amino acids

.

However, like ammonia, amines are good Brønsted and Lewis bases, so they can react with carboxylic acids. As shown in Figure 13.23, amino acids can exist in a charge-separated form, in which the carboxylic acid has given up its proton, and the amine has gained a proton. In the human body, most of the amino acids are found in the charge-separated form. In all biologically important

amines, the amine group is connected to the

O C

ROH

carbonylhydroxyl

+

carboxyl

Figure 13.19 A carboxylic acid A carboxylic acid contains a carboxyl group = a carbonyl + a hydroxyl group. R is used to

represent an unspecified group.

O

HC^3

C

OH

H

H

H^3

C

C

O

O H

HH

pK

~ 5a

O

O

Figure 13.20 Reaction of acetic acid and water Acetic acid is ~10

13 times stronger as an acid than is ethanol.

R^1

N R^2

O C

R^3

R^1

N R^2

O C

R^3

Figure 13.21 Resonance structures of amides

All of the groups in the figure lie in a plane, so the N atom must be sp

2 hybridized as shown in the structure on the right.

N

HC^3

O

O O O

CH

3

N

N

N N

HC^3 O

O CH

3

CH

3

Cocaine

Caffeine

NH

N

NH

2

Histamine

NH

CH

3

HO

HO

OH

Epinephrine (adrenaline)

Figure 13.22 Some naturally occurring amines

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