Draw the structure for 4-ethylpentanedioic acid and explain what is wrong with the name.

"Pentanedioic acid" indicates a 5-carbon chain with a carboxylic acid group ($\text{-COOH}$) at each end.

$$\text{HOOC}-\text{CH}_2-\text{CH}_2-\text{CH}_2-\text{COOH}$$

Numbering from left to right, we place an ethyl group ($\text{-CH}_2\text{CH}_3$) on the fourth carbon.

$$\text{HOOC}-\text{CH}_2-\text{CH}_2-\text{CH}(\text{CH}_2\text{CH}_3)-\text{COOH}$$

This is the structure implied by the name "4-ethylpentanedioic acid".

According to IUPAC rules, the parent chain must be numbered to give the lowest possible locants (numbers) to the substituents. Let's re-number the drawn structure from both ends:

• Option 1 (Numbering from left to right): 1-$\text{COOH}$ 2-$\text{CH}_2$ 3-$\text{CH}_2$ 4-$\text{CH}(\text{CH}_2\text{CH}_3)$ 5-$\text{COOH}$ In this numbering, the ethyl group is at position 4.

• Option 2 (Numbering from right to left): 1-$\text{COOH}$ 2-$\text{CH}(\text{CH}_2\text{CH}_3)$ 3-$\text{CH}_2$ 4-$\text{CH}_2$ 5-$\text{COOH}$ In this numbering, the ethyl group is at position 2.

Since 2 is a lower number than 4, the correct IUPAC numbering for this compound would place the ethyl group at position 2.

The structure is:

$$\text{HOOC}-\text{CH}_2-\text{CH}_2-\text{CH}(\text{CH}_2\text{CH}_3)-\text{COOH}$$

What's wrong with this name?
The name "4-ethylpentanedioic acid" is incorrect because the numbering of the ethyl substituent is not the lowest possible. The correct IUPAC name for this compound should be 2-ethylpentanedioic acid.