15.2: Carboxylic Acids - Structures and Names

Learning Objectives

• Name carboxylic acids with common names.
• Name carboxylic acids according to IUPAC nomenclature.

Carboxylic acids occur widely in nature, often combined with alcohols or other functional groups, as in fats, oils, and waxes. They are components of many foods, medicines, and household products (Figure \(\PageIndex{1}\)). Not surprisingly, many of them are best known by common names based on Latin and Greek words that describe their source.

Figure \(\PageIndex{1}\): Carboxylic Acids in the Home. Carboxylic acids occur in many common household items. (a) Vinegar contains acetic acid, (b) aspirin is acetylsalicylic acid, (c) vitamin C is ascorbic acid, (d) lemons contain citric acid, and (e) spinach contains oxalic acid. © Thinkstock

The simplest carboxylic acid, formic acid (HCOOH), was first obtained by the distillation of ants (Latin formica, meaning “ant”). The bites of some ants inject formic acid, and the stings of wasps and bees contain formic acid (as well as other poisonous materials).

The next higher homolog is acetic acid, which is made by fermenting cider and honey in the presence of oxygen. This fermentation produces vinegar, a solution containing 4%–10% acetic acid, plus a number of other compounds that add to its flavor. Acetic acid is probably the most familiar weak acid used in educational and industrial chemistry laboratories.

Pure acetic acid solidifies at 16.6°C, only slightly below normal room temperature. In the poorly heated laboratories of the late 19th and early 20th centuries in northern North America and Europe, acetic acid often “froze” on the storage shelf. For that reason, pure acetic acid (sometimes called concentrated acetic acid) came to be known as glacial acetic acid, a name that survives to this day.

The third homolog, propionic acid (CH₃CH₂COOH), is seldom encountered in everyday life. The fourth homolog, butyric acid (CH₃CH₂CH₂COOH), is one of the most foul-smelling substances imaginable. It is found in rancid butter and is one of the ingredients of body odor. By recognizing extremely small amounts of this and other chemicals, bloodhounds are able to track fugitives. Models of the first four carboxylic acids are shown in Figure \(\PageIndex{2}\).

Figure \(\PageIndex{2}\): Ball-and-Stick Models of Carboxylic Acids. Carboxylic acids feature a carbon atom doubly bonded to an oxygen atom and also joined to an OH group. The four acids illustrated here are formic acid (a), acetic acid (b), propionic acid (c), and butyric acid (d).

The acid with the carboxyl group attached directly to a benzene ring is called benzoic acid (C₆H₅COOH).

The common names of carboxylic acids use Greek letters (α, β, γ, δ, and so forth), not numbers, to designate the position of substituent groups in acids. These letters refer to the position of the carbon atom in relation to the carboxyl carbon atom.

In the nomenclature system of the International Union of Pure and Applied Chemistry (IUPAC), the parent hydrocarbon is the one that corresponds to the longest continuous chain (LCC) containing the carboxyl group. The -e ending of the parent alkane is replaced by the suffix -oic and the word acid. For example, the carboxylic acid derived from pentane is pentanoic acid (CH₃CH₂CH₂CH₂COOH). As with aldehydes, the carboxyl carbon atom is counted first; numbers are used to indicate any substituted carbon atoms in the parent chain.

Greek letters are used with common names; numbers are used with IUPAC names.

 

Table of the First 10 Carboxylic Acids

IUPAC Name	Common Name	Structural Formula
Methanoic Acid	Formic Acid	HCOOH
Ethanoic Acid	Acetic Acid	CH₃COOH
Propanoic Acid	Propionic Acid	CH₃CH₂COOH
Butanoic Acid	Butyric Acid	CH₃(CH₂)₂COOH
Pentanoic Acid	Valeric Acid	CH₃(CH₂)₃COOH
Hexanoic Acid	Caproic Acid	CH₃(CH₂)₄COOH
Heptanoic Acid	Enanthic Acid	CH₃(CH₂)₅COOH
Octanoic Acid	Caprylic Acid	CH₃(CH₂)₆COOH
Nonanoic Acid	Pelargonic Acid	CH₃(CH₂)₇COOH
Decanoic Acid	Capric Acid	CH₃(CH₂)₈COOH

Example \(\PageIndex{1}\)

Give the common and IUPAC names for each compound.

1. ClCH₂CH₂CH₂COOH
2. 

Solution

1. The LCC contains four carbon atoms; the compound is therefore named as a substituted butyric (or butanoic) acid.

   

   The chlorine atom is attached to the γ-carbon in the common system or C4 in the IUPAC system. The compound is γ-chlorobutyric acid or 4-chlorobutanoic acid.

2. The LCC contains four carbon atoms; the compound is therefore named as a substituted butyric (or butanoic) acid.

The bromine (Br) atom is at the α-carbon in the common system or C2 in the IUPAC system. The compound is α-bromobutyric acid or 2-bromobutanoic acid.

Exercise \(\PageIndex{1}\)

Give the IUPAC name for each compound.

1. ClCH₂CH₂CH₂CH₂COOH
2. (CH₃)₂CHCH₂CHBrCOOH

Example \(\PageIndex{2}\)

Write the condensed structural formula for β-chloropropionic acid.

Solution

Propionic acid has three carbon atoms: C–C–COOH. Attach a chlorine (Cl) atom to the parent chain at the beta carbon atom, the second one from the carboxyl group: Cl–C–C–COOH. Then add enough hydrogen atoms to give each carbon atom four bonds: ClCH₂CH₂COOH.

Exercise \(\PageIndex{2}\)

Write the condensed structural formula for 4-bromo-5-methylhexanoic acid.

Key Takeaways

• Simple carboxylic acids are best known by common names based on Latin and Greek words that describe their source (e.g., formic acid, Latin formica, meaning “ant”).
• Greek letters, not numbers, designate the position of substituted acids in the common naming convention.
• IUPAC names are derived from the LCC of the parent hydrocarbon with the -e ending of the parent alkane replaced by the suffix -oic and the word acid.

IUPAC and Common Nomenclature for Carboxylic Acids

IUPAC Naming Rules

1. Identify the Longest Carbon Chain: The chain containing the carboxyl group (-COOH) is identified.
2. Replace the Alkane Suffix: The suffix “-e” of the parent alkane is replaced with “-oic acid.”
3. Number the Chain: The carboxyl carbon is always numbered as 1.
4. Name Substituents: Identify and name any substituents attached to the main chain.

Examples:

• Methanoic Acid: HCOOH (Formic Acid)
• Ethanoic Acid: CH₃COOH (Acetic Acid)
• Propanoic Acid: CH₃CH₂COOH (Propionic Acid)

Carboxylic Acids with Alkyl Group Substituents

1. Identify and Number the Longest Chain: The chain containing the carboxyl group is numbered starting from the carboxyl carbon.
2. Name the Alkyl Substituents: Name and number the alkyl groups attached to the main chain.
3. Combine the Names: Combine the names of the substituents with the base name of the carboxylic acid.

Examples:

• 2-Methylpropanoic Acid: (CH₃)₂CHCOOH (Isobutyric Acid)
• 3-Methylbutanoic Acid: CH₃CH₂CH(CH₃)COOH (Isovaleric Acid)
• 2,2-Dimethylpropanoic Acid: (CH₃)₃CCOOH (Pivalic Acid)

Common Naming Rules

1. Historical Names: Many carboxylic acids have common names based on their historical sources or properties.
2. Prefixes and Suffixes: Common names often use prefixes like “form-” for one carbon, “acet-” for two carbons, etc.

Examples:

• Formic Acid: HCOOH (Methanoic Acid)
• Acetic Acid: CH₃COOH (Ethanoic Acid)
• Propionic Acid: CH₃CH₂COOH (Propanoic Acid)

These guidelines help in systematically naming carboxylic acids, ensuring clarity and consistency in chemical communication.