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25.12: Carboxylic Acids

Last updated

Mar 21, 2025
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- 25.11: Aldehydes and Ketones
- 25.13: Esters

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Vinegar (dilute acetic acid) is available in every grocery store in the country. The large one-gallon jugs can be found in households everywhere. One magazine published an article listing 150 ways to use vinegar, from getting dirt off of your computer to cleaning window blinds. A mixture of vinegar and baking soda is an excellent way to clean sink drains. You can also clean your carpets and your piano keys. Not bad for something that may have struck you as merely an ingredient in salad dressing!

Carboxylic Acids

Organic acids such as acetic acid all contain a functional group called a carboxyl group.

Chemical structure of a carboxyl group, showing a carbon atom double-bonded to an oxygen atom and single-bonded to a hydroxyl group (OH). Labeled Carboxyl Group.

The carboxyl group contains the $\ce{C=O}$ of the carbonyl group, with the carbon atom also being bonded to a hydroxyl $\left( \ce{-OH} \right)$ group. A carboxylic acid is an organic compound that contains the carboxyl functional group. The general formula for a carboxylic acid can be abbreviated as $\ce{R-COOH}$ . The carbon atom of the carboxyl group may be attached to a hydrogen atom or to a carbon chain. The naming of a carboxylic acid is as follows:

Name the parent compound by finding the longest continuous chain that contains the carboxyl group.
Change the -e at the end of the name of the alkane to -oic acid.

Structural formulas of methanoic acid and butanoic acid are shown. Methanoic acid has a single carbon, while butanoic acid has four carbons in its chain.

Properties of Carboxylic Acids

Carboxylic acids are all weak acids. In aqueous solution, the $\ce{O-H}$ bond of the hydroxyl group can break, yielding a negative carboxylate ion and the hydrogen ion.

Chemical equilibrium showing a carboxylic acid dissociating into a carboxylate ion and a hydrogen ion, with labels underneath each component. — Figure $\PageIndex{1}$ : Equilibrium between carboxyl group and carboxylate ion.

The smaller members of the aliphatic carboxylic acid series are colorless, volatile liquids with strong odors. Ethanoic acid is commonly known as acetic acid; common household vinegar is a $5\%$ solution of acetic acid. Larger carboxylic acids are solids with low melting points. There are numerous aromatic carboxylic acids, which are all crystalline solids. Carboxylic acids can form intermolecular hydrogen bonds, and thus have relatively high melting and boiling points compared to other organic compounds that cannot hydrogen bond. Carboxylic acids with shorter carbon chains are very soluble in water, while those with longer carbon chains are less soluble.

Many carboxylic acids occur naturally in plants and animals. Citrus fruits such as oranges and lemons contain citric acid.

Graphical representation of ascorbic acid with its molecular structure and 3D model on the left, and a stack of whole and halved oranges on the right. — Figure $\PageIndex{2}$ : Citric acid is a large carboxylic acid with three ionizable hydrogen atoms. It is found in citrus fruits and gives them their sour or tart flavor.

Ethanoic and citric acids are frequently added to foods to give them a tart flavor. Benzoic, propanoic, and sorbic acids are used as food preservatives because of their ability to kill microorganisms that can lead to spoilage. Methanoic and ethanoic acids are widely used in industry as starting points for the manufacture of paints, adhesives, and coatings.

Summary

Organic acids, such as acetic acid, all contain a functional group called a carboxyl group.
The carboxyl group contains the $\ce{C=O}$ of the carbonyl group, with the carbon atom also being bonded to a hydroxyl $\left( \ce{-OH} \right)$ group.
A carboxylic acid is an organic compound that contains the carboxyl functional group.

Carboxylic Acids

Properties of Carboxylic Acids

Summary

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