The Foundations of Chemistry

Genes, the units of chromosomes that carry hereditary characteristics, are long stretches

of double helical deoxyribonucleic acid, or DNA. DNA is composed of four nucleotide

bases:adenine, guanine, cytosine, and thymine. The first two are modified purines, and

the latter two are modified pyrimidines. The sequence of these building blocks in DNA

acts as a code for the order of amino acids in the proteins of an organism. The DNA in

each cell of an organism contains the instructions for making the complete organism (see

Section 28-10).

CARBOXYLIC ACIDS

Compounds that contain the carboxyl group, , are acidic. They are called

carboxylic acids.Their general formula is RXCOOH. Most are weak acids.They are

much stronger acids than most phenols, however. Carboxylic acids are named systemati-

cally by dropping the terminal -efrom the name of the parent hydrocarbon and adding

• oic acid(Table 27-11). Many older names survive, however, and organic acids are often
  called by common names. In aromatic acids the carboxyl group is attached to an aromatic
  ring (Figure 27-17).
  Organic acids occur widely in natural products, and many have been known since
  ancient times. Their common (trivial) names are often derived from a Greek or Latin
  word that indicates the original source (see Table 27-11).
  The names of modified carboxylic acids are often derived from the trivial names of the
  acids. Positions of substituents are sometimes indicated by lowercase Greek letters, begin-
  ning with the carbon adjacentto the carboxyl carbon, rather than by numbering the carbon
  chain.

C OH

O

27-13

N

N

N

N

H

NH 2

N

N

N

N

H

O

adenine

N

N

H

NH 2

thymine

H

H 2 N

N

H

O N

H

O

CH 3

O

guanine cytosine

Aliphatic carboxylic acids are
sometimes referred to as fatty acids
because many have been obtained from
animal fats.

1076 CHAPTER 27: Organic Chemistry I: Formulas, Names, and Properties

TABLE 27-11 Some Aliphatic Carboxylic Acids

Formula Common Name IUPAC Name

HCOOH formic acid methanoic acid

CH 3 COOH acetic acid ethanoic acid

CH 3 CH 2 COOH propionic acid propanoic acid

CH 3 CH 2 CH 2 COOH butyric acid butanoic acid

CH 3 CH 2 CH 2 CH 2 CH 2 COOH caproic acid hexanoic acid

CH 3 (CH 2 ) 10 COOH lauric acid dodecanoic acid

CH 3 (CH 2 ) 14 COOH palmitic acid hexadecanoic acid

CH 3 (CH 2 ) 16 COOH stearic acid octadecanoic acid

In the general formulas for alcohols
and ethers, R cannot be H. In
carboxylic acids, R can be H.

Formic acid was obtained by
distillation of ants (L. formica,“ant”);
acetic acid occurs in vinegar (L.
acetum,“vinegar”); butyric acid in
rancid butter (L. butyrum,“butter”);
and stearic acid in animal fats (Gr.
stear,“beef suet”). Caproic acid is one
of the so-called “goat acids.” Its odor
is responsible for the name.

A space-filling model of a portion of
the DNA double helical structure.