25.1 Carboxylic Acids – Structure and Naming

Learning Objectives

By the end of this section, you will be able to:

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

Structure of Carboxylic Acids

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 25.1a.). Carboxylic acids are considered weak acids that can neutralize bases and tend to have a sour or tart taste to them. Not surprisingly, many of them are best known by common names based on Latin and Greek words that describe their source.

The image shows a variety of carboxylic acids in the home. It includes vinegar, aspirin, lemons, vitamin C and spinach.
Figure 25.1a. 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. (credit a: Photo by Joe Shlabotnik, CC BY-NC-SA 2.0; b: Photo by U.S. Food and Drug Administration, CC BY-SA 2.0; c: Photo by Pete, PD; d: Photo by eggbank, Unsplash; e: Photo by Willis Lam, CC BY-SA 2.0)

A carboxylic acid is an organic compound in which a carbon is double bonded to an oxygen atom (referred to as a carbonyl group) while also being single bonded to a hydroxyl group (-OH). This combination of carbonyl group with a hydroxyl group creates what is known as a carboxyl group, the functional group found in carboxylic acids.

Indigenous Perspectives: Inuit Love Soy Sauce

a white cup with soy sauce within it
Figure 25.1b. Cup of soy sauce (credit: Image by Tim Reckmann, CC BY 2.0).

Soy sauce was first produced in China about 2,200 years ago. It is now commonly used in Inuit culture as the condiment of choice for Arctic Char also known as iKaluk in Inuttut. Spiced soy sauces can contain different carboxylic acids which can add to the flavour profile. A healthy ingredient within soy sauce is Niacin which is an aromatic carboxylic acid (Figure 25.1c.) (Anderson & Rayner-Canham, 2022).

Nitrogen within a benzene ring with a carboxylic acid substituent.
Figure 25.1c. Molecular structure of Niacin (credit: Image by Mysid, PDM)

More information about the use of soy sauce in Inuit culture can be found Soy sauce: An essential Inuit condiment in Chem 13 News Magazine.

Carboxylic acids, RCO2H, occupy a central place among carbonyl compounds. Not only are they valuable in themselves, they also serve as starting materials for preparing numerous carboxylic acid derivatives such as acid chlorides, esters, amides, and thioesters (Figure 25.1d.). In addition, carboxylic acids are present in the majority of biological pathways.

Image displays structure of a carboxylic acid at the top of a tree diagram. The branches of the tree diagram point to the 4 acid derivatives that can be produced in reactions of carboxylic acids; acid chlorides, esters, amides and thioesters.
Figure 25.1d. Carboxylic acids, the starting point for formation of several acid derivatives; acid chlorides, esters, amides and thioesters. (credit: Organic Chemistry (OpenStax), CC BY-NC-SA 4.0.)

Common Names of Carboxylic Acids

Many carboxylic acids are named using their common names which use the prefixes: form-, acet-, proprion-, and butyr-. The simplest carboxylic acid, formic acid (HCOOH), was first obtained by the distillation of ants (Latin formica, meaning “ant”) (Figure 25.1e.). The bites of some ants inject formic acid, and the stings of wasps and bees contain formic acid (as well as other poisonous materials).

Structural digram for formic acid highlighting the carbonyl group and hydroxyl group.
Figure 25.1e. Functional group for carboxylic acids representing formic acid (credit: Intro Chem: GOB (v. 1.0), CC BY-NC-SA 3.0).

The next higher homolog is acetic acid (Figure 25.1f.), 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 flavour. Acetic acid is probably the most familiar weak acid used in educational and industrial chemistry laboratories.

Structural digram for acetic acid highlighting the carbonyl group and hydroxyl group. Two carbons in a chain with a double bonded oxygen and a hydroxyl group.
Figure 25.1f. Functional group for carboxylic acids representing acetic acid (credit: Intro Chem: GOB (v. 1.0), CC BY-NC-SA 3.0).

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. Vinegar comes in a variety of forms which we commonly use in everyday cooking practices. Infographic 25.1a. showcases the changes to chemical structure that provides us with the differing flavours we experience from some of the vinegar varieties.

Spotlight on Everyday Chemistry: Vinegar

Below we can see the chemical structures that make up the varieties of vinegars we use in everyday cooking practice.

Infographic 25.1a. Vinegar or acetic acid is a common carboxylic acid. It comes in a variety of form based on changes to the chemical structure. Read more about “The sour science of vinegar varieties” by Andy Brunning / Compound Interest, CC BY-NC-ND, or access a text-based summary of infographic 25.1a [New tab].

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 odour. By recognizing extremely small amounts of this and other chemicals, bloodhounds are able to track fugitives.

Below (Figure 25.1g.) is an example of a carboxylic acid with a substituent group. This acid is named 2-bromo-propanoic acid. 2-bromo-propanoic acid is used in the production of herbicides and the synthesis of pharmaceutical intermediates.

The image displays the condensed structural formula for 2-bromo-propanoic acid. CH3CH2CH(Br)COOH
Figure 25.1g. The structural diagram here represents 2-bromo-propanoic acid (credit: Intro Chem: GOB (v. 1.0), CC BY-NC-SA 3.0).

Watch Carboxylic Acids, Typical Acids and Esters – Organic Chemistry – YouTube (4 min)

Video source: Fuse School – Global Education. (2014, August 10). Carboxylic acids, typical acids and esters – Organic Chemistry – Chemistry – Fuse School [Video]. YouTube.

The most simplistic aromatic carboxylic acid in which the carboxyl carbon is attached directly to carbon 1 is called benzoic acid (C6H5COOH) (Figure 25.1h.). When substituent groups are added to benzoic acid, they are numbered in the direction that gives the smallest numbers possible.

The image displays a ring structure (benzene) containing the functional groups for a carboxylic acid (a carbonyl and hydroxyl group) highlighted in red.
Figure 25.1h. Image represents a ring structure containing the functional groups for carboxylic acids (credit: Intro Chem: GOB (v. 1.0), CC BY-NC-SA 3.0).

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 (Figure 25.1i.). These letters refer to the position of the carbon atom in relation to the carboxyl carbon atom.

The image shows two examples of Greek lettering to outline positioning of substituent groups on the parent chain. The two examples provided are alpha-methylpropionic acid and beta-hydroxybutyric acid.
Figure 25.1i. Use of Greek letters to represent the position of substituent groups (credit: Intro Chem: GOB (v. 1.0), CC BY-NC-SA 3.0).

IUPAC Naming of Carboxylic Acids

Here are some basic rules for naming carboxylic acids from the International Union of Pure and Applied Chemistry (IUPAC):

  1. Select the longest carbon chain containing the carboxyl group, this is the parent chain. The -e ending of the parent alkane name is replaced by the suffix -oic acid.
  2. The carboxyl carbon is always numbered “1” but the number is NOT included in the name.
  3. Name the substituents attached to the chain in the usual way (providing you with the lowest numbering possible for these groups).
  4. Aromatic carboxylic acids (ie. with a COOH) directly connected to a benzene ring) are named after the parents compound benzoic acid.

In the IUPAC nomenclature system, 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, if there are substituents the carboxyl carbon atom is counted first; numbers are used to indicate the substituted carbon atoms in the parent chain.

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

Example 25.1a

Give the common and IUPAC names for each compound.

  2. Image displays the structural diagram for 2-bromobutanoic acid.
    (Credit: Intro Chem: GOB (v. 1.0), CC BY-NC-SA 3.0).


  1. The LCC contains four carbon atoms; the compound is therefore named as a substituted butyric (or butanoic) acid.
    Image shows only the carbon chain from example 1 and numbers the carbons in the parent chain.
    (Credit: Intro Chem: GOB (v. 1.0), CC BY-NC-SA 3.0).

    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.
    Image shows the parent chain from example 2 and numbers the carbons within the parent chain.
    (Credit: Intro Chem: GOB (v. 1.0), CC BY-NC-SA 3.0).


    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 25.1a

Write the condensed structural formula for β-chloropropionic acid.

Check Your Answer[1]

Exercise 25.1b

Give the IUPAC name for each compound.


Check Your Answers:[2]

Exercise 25.1c

Give the IUPAC name for each compound.

  1. Line diagram with 4 carbons in the parent chain plus a CH3 group attached to carbon 2 and a double bonded O and hydroxyl group.
  2. Condensed structural formula with a parent chain of 5 carbons and a double bonded oxygen as well as a hydroxyl group. Also 2 chlorine substituents on carbon's 2 and 4.

Check Your Answer[3]

Image source: Adapted from course materials by Caryn Fahey and JR van Haarlem.

Dicarboxylic Acids

A dicarboxylic acids is an organic compound containing two carboxyl groups (-COOH) often referred to as a diacid. The general molecular formula for dicarboxylic acids can be written as HO−R−COOH. Dicarboxylic acids are used in the preparation of copolymers such as polyamides and polyesters. The most commonly used diacid in industry is adipic acid, which is a precursor to nylon production. Other examples of diacids include aspartic acid and glutamic acid, both of which are amino acids in the human body.

Example 25.1b

Name the following dicarboxylic acids.

2 separate line diagrams of dicarboxylic acids to be named.
(credit: Organic Chemistry (Vollhardt & Schore), CC BY-NC-SA 4.0


  1. butanedioic acid
  2. propanedioic acid

Example 25.1b source: Organic Chemistry (Vollhardt & Schore), CC BY-NC-SA 4.0

Attribution & References

Except where otherwise noted, this page is adapted by Caryn Fahey from:

References cited in-text

Anderson, C. C., & Rayner-Canham, G. (2022, Fall). Soy sauce: An essential Inuit condiment. Chem 13 News Magazine.

  1. 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.

  2. a. 5-chloropentanoic acid, b. 1-bromo-5-methylpentanoic acid

  3. 1) 2-methylbutanoic acid 2) 2,4-dichloropentanoic acid



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Organic and Biochemistry Supplement to Enhanced Introductory College Chemistry Copyright © 2024 by Gregory Anderson; Caryn Fahey; Adrienne Richards; Samantha Sullivan Sauer; David Wegman; and Jen Booth is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

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