The Foundations of Chemistry

EXAMPLE 27-12 Isomeric Carboxylic Acids

Write the structural formula of each of the four carboxylic acids having the molecular formula

of C 5 H 10 O 2. Give the IUPAC name of each.

Plan

We first write the structural formula of the carboxylic acid that does not have branching. Then

we remove one carbon from the straight-chain structure and reposition it to form branched

isomers. We check to see whether branching is possible at more than one location. We find

that branching is possible at two different locations. Remove a second carbon from the

unbranched isomer and, if possible, from additional branching. The two removed carbons can

be added as methyl branches. Each isomer is named by naming the longest chain that contains

the carboxylic acid group. The final -eof the name is changed to -oic acid.Names for branches

appear at the beginning of the name.

Solution

Isomer 1.

Isomer 2.

Isomer 3.

Isomer 4.

You should now work Exercise 72a, b.

O

CH 3 C C OH

CH 3

CH 3

dimethylpropanoic acid

CH 3 CHCH 2 C

O

OH

CH 3

3-methylbutanoic acid

CH 3 CH 2 CH

O

C OH

CH 3

2-methylbutanoic acid

OH

O

CH 3 CH 2 CH 2 CH 2 C pentanoic acid

C

O

OH

CH

Br

CH 3

2-bromopropanoic acid

( -bromopropionic acid)

C

O

OH OH

CH 2 CH 2 CH 2

4-hydroxybutanoic acid

( - hydroxybutyric acid)

Derivatives of -hydroxy carboxylic

acids are currently used in some

cosmetic preparations.

H

C

H

H

H

(a) acetic acid
(an aliphatic acid)

(b) benzoic acid
(an aromatic acid)

C

O

O

H

C

O

O

Figure 27-17 (a) Models of acetic

acid. (b) Models of benzoic acid.