20.1 Naming Carboxylic Acids and Nitriles

Carboxylic Acids and Nitriles

IUPAC naming of carboxylic acids

Carboxylic acids contain the carboxyl group ($-COOH$), which combines a carbonyl ($C=O$) and a hydroxyl ($-OH$) on the same carbon. The naming system builds directly off alkane names, with a specific suffix swap.

To name a carboxylic acid using IUPAC rules:

1. Find the longest carbon chain that includes the carboxyl carbon. This is your parent chain.
2. Replace the -e ending of the parent alkane with -oic acid (e.g., pentane → pentanoic acid).
3. Number the chain starting from the carboxyl carbon, which is always carbon 1. You don't need to specify "1-" in the name because the $-COOH$ is always at the end.
4. Name and number any substituents as prefixes. For example, a methyl group on carbon 2 of a five-carbon acid gives 2-methylpentanoic acid.

For dicarboxylic acids ($-COOH$ at both ends of the chain), add -dioic acid to the parent name. Both carboxyl carbons count as part of the parent chain. Ethanedioic acid (oxalic acid) is the simplest example.

Common names for simple carboxylic acids show up constantly in organic chemistry, so you should memorize these:

• Formic acid ($HCOOH$), the simplest carboxylic acid, one carbon total
• Acetic acid ($CH_3COOH$), two carbons, the acid in vinegar
• Propionic acid ($CH_3CH_2COOH$), three carbons
• Butyric acid ($CH_3CH_2CH_2COOH$), four carbons, responsible for the smell of rancid butter

Acyl groups ($RCO-$) come from removing the $-OH$ from a carboxylic acid. They're named by swapping the -ic acid ending for -yl:

• Acetic acid → acetyl ($CH_3CO-$)
• Propionic acid → propionyl ($CH_3CH_2CO-$)
• Butyric acid → butyryl ($CH_3CH_2CH_2CO-$)

You'll see acyl groups when naming acid derivatives like acyl halides (e.g., acetyl chloride, propionyl bromide).

Nomenclature of nitriles

Nitriles contain the $-C \equiv N$ functional group. There are two ways to name them: the suffix method and the prefix method.

Suffix method (IUPAC standard):

1. Find the longest carbon chain that includes the nitrile carbon (the $C$ of $-CN$).
2. Replace the -e of the parent alkane with -nitrile (e.g., butane → butanenitrile).
3. The nitrile carbon is always carbon 1, so you don't specify its position.
4. Number and name substituents as usual. A methyl group on carbon 2 of a four-carbon nitrile gives 2-methylbutanenitrile.

Prefix method (cyano-):

This approach treats the $-CN$ group as a substituent rather than the principal functional group. You use it when a higher-priority group (like $-COOH$) is present, or sometimes for complex structures.

1. Name the parent compound based on the longest chain (ignoring the $-CN$ carbon).
2. Use the prefix cyano- and a number to show where the $-CN$ is attached. For example, 2-cyanopentane has a $-CN$ group on carbon 2 of a five-carbon chain.

Notice the carbon count differs between the two methods. In butanenitrile, the nitrile carbon is part of the four-carbon parent chain. In 2-cyanopentane, the $-CN$ carbon is not counted in the five-carbon parent chain.

Structures from acid and nitrile names

Being able to go from a name back to a structure is just as important as naming. Here's how to approach each type.

Drawing carboxylic acids:

1. Pull out the parent alkane from the name (pentanoic acid → pentane → five carbons).
2. Draw the carbon chain and place a $-COOH$ group at carbon 1.
3. Attach substituents at their numbered positions. For 2-methylpentanoic acid, put a methyl group on carbon 2.

Drawing nitriles (suffix name):

1. Identify the parent alkane (butanenitrile → butane → four carbons).
2. Draw the chain and place a $-CN$ group at carbon 1.
3. Add substituents where indicated. For 2-methylbutanenitrile, a methyl group goes on carbon 2.

Drawing nitriles (prefix name):

1. Draw the parent chain first (2-cyanopentane → pentane → five carbons).
2. Attach a $-CN$ group at the numbered position. For 2-cyanopentane, the $-CN$ goes on carbon 2.

IUPAC Nomenclature and Common Names

IUPAC nomenclature gives every organic compound a unique, systematic name. Common names, though, persist for many simple carboxylic acids (formic, acetic, propionic, butyric) and you're expected to know both.

When multiple functional groups are present, priority rules determine which group gets the suffix. Carboxylic acids rank higher than nitriles in IUPAC priority, so if both are present, the compound is named as an acid and the $-CN$ group is indicated with the cyano- prefix.

Substituents are always named as prefixes with position numbers. The parent chain is chosen to include the highest-priority functional group and to be as long as possible.