Carboxylic Acid Functional Group

carboxylic acid structure

The carboxylic acid is a common functional group in many organic molecules and biomolecules. This article describes the basics of the carboxylic acid functional group, as well as some of its reactions and chemical effect.

What is the carboxylic acid functional group?

A carboxylic acid can be thought of as a combination between two functional groups: an alcohol group, involving a hydrogen bound to an oxygen, which attaches to a carbonyl group, involving a carbon double bound to an oxygen. Often, chemists abbreviate carboxylic acid to COOH group in chemical literature.

Carboxylic acid group (blue) with carbonyl (magenta) and alcohol (red) groups indicated
Carboxylic acid group with lone electron pairs

Quick Facts on the Carboxylic Acid Functional Group

  • Structure: A carbon double bound to one oxygen and single bonded to another oxygen, which is bound to a hydrogen.
  • General Formula: R-COOH or RCOOH
  • Atomic weight: 45.02
  • pH: Generally acidic. Acidity depends on the R group.
  • pKa: Generally below 5
    • Acetic Acid: (CH3COOH): 4.76
    • Formic Acid: (HCOOH): 3.75
    • Benzoic Acid: (PheCOOH): 4.20

Naming Conventions

Depending on the structure of the R group, the formal IUPAC names of carboxylic acids end in “-oic acid,” such as in benzoic acid if the R group is a phenyl group. If a substance is not a monocarboxylic acid, but instead has multiple COOH groups, prefixes such as “di” or “tri” may be inserted before “-oic acid,” such as in hexanedioic acid. Other naming conventions, which chemists commonly use for structures with three or more carboxylic acid groups, “carboxylic acid” is used instead of “-oic acid,” with numbers indicating the carbon atoms to which the carboxylic acid groups are attached, such as in butane-1,2,4-tricarboxylic acid.

Outside of naming conventions, informal names for many carboxylic acids are often used in place of conventional names. One such example is ethanoic acid, the main component of vinegar, which chemists often call acetic acid. Other examples include methanoic acid, more commonly known as formic acid, and hexanediotic acid, more commonly known as adipic acid.

Let’s examine several carboxylic acid examples:

Methanoic Acid (or Formic Acid)
Ethanoic Acid (or Acetic Acid)
Benzoic Acid
Hexanediotic Acid (or Adipic Acid)
Butane-1,2,4-Tricarboxylic Acid

Carboxylic Acid Group Reactions

General Reaction Trends

Since three of the four bonds formed by the central carbon involve oxygen, carbon is highly electron-deficient, which makes it reactive with electron-rich molecules. Additionally, due to the acidity of the hydrogen in the carboxylic acid, basic or neutral conditions often remove the hydrogen, forming a negatively charged carboxylate ion. Consequently, the electron richness of the carboxylate oxygens allows for reaction with electron-deficient molecules.

When the R group is an aromatic substance, such as benzene, the carboxylic acid acts as a deactivating group, due to the electron deficiency of the central carbon.

Reactions forming the Carboxylic Acid Group

To form a carboxylic acid, chemists typically need to oxidize a carbon molecule. Primarily, they do this using a chromic acid and a primary alcohol or aldehyde under aqueous conditions.

Also, other carbonyl-containing groups such as esters, amides, and acid halides convert into carboxylic acids in conditions of water or hydroxide.

Additionally, carboxylic acids frequently form as a product of a Claisen Condensation reaction, with a carbonyl group two carbons away from the carboxylic acid. However, this molecule tends to be thermally unstable, and under high temperatures, the carboxylic acid cleaves from the molecule through a decarboxylation reaction, which forms carbon dioxide.

Reactions using a Carboxylic Acid Group

Carboxylic acids have remarkable versatility, and convert into many different functional groups, making them very useful in chemical synthesis. Let’s look at some of the most useful reactions.

Carboxylic acids convert into acid halides using thionyl chloride or phosphorus halides, like phosphorus tribromide or phosphorus pentachloride.

Also, carboxylic acids convert into esters with an acid catalysis. Chemists call this reaction the Fischer Esterification.

carboxylic acid functional group reaction to form exters

Additionally, carboxylic acids become reduced using reagents such as lithium aluminum hydride to form aldehyde groups.

carboxylic acid functional group with lithium aluminum hydride

Unlike other functional groups with oxidized carbon, organometallic reagents such as Grignards and organolithiums, do not add a hydrocarbon group to a carboxylic acid, but instead deprotonate the alcohol, due to the acidity of carboxylic acids. For instance, a methyl lithium organolithium reagent would not add a methyl group to the carboxylic acid.

carboxylic acid functional group grignard reagent reaction
Carboxylic Acid Groups in Amino Acids

Biomolecules frequently have at least one carboxylic acid functional group. For instance, many amino acids have a carboxylic acid group as a fundamental part of the “amino acid backbone.”

These carboxylic acid groups form peptide bonds with amino acid amine groups, linking amino acids together, which provides the structural basis for all proteins. Peptide bond formation is a type of condensation reaction, involving the joining of two molecules which creates a water molecule as a byproduct. This carbonyl group bonded to a nitrogen is called an amide, and is found in many molecules outside of proteins.

carboxylic acid functional group involved in protein synthesis
The newly formed peptide bond is shown in red
Carboxylic Acid Groups in Lipids

Fatty acid molecules also contain a carboxylic acid group, which attaches at one end of a long nonpolar carbon chain. Consequently, the polar nature of the carboxylic acid group gives fatty acids an amphipathic character, which serves their crucial biochemical functions of energy storage and membrane permeability.

Additionally, the carboxylic acid group of fatty acids forms an ester linkage with a glycerol or sphingosine molecule to form larger lipid molecules, such as triglycerides and phospholipids.

carboxylic acid functional group involved in lipid synthesis
The newly formed ester linkage is shown in red.
Example Questions
  1. Identify the compound with the carboxylic acid group. 
compound with carboxylic acid functional group 1
compound with carboxylic acid functional group 2
compound with carboxylic acid functional group 3

2. Draw the structure of the following compounds.

  • Butanoic acid
  • Acetic acid
  • Pentane-1,3,5,-tricarboxylic acid

3. Which of the following reactions yields a carboxylic acid group?

carboxylic acid functional group excercise

4. Which of these reagents converts carboxylic acid into a different functional group? (For the purposes of this question, carboxylate doesn’t count as a “different group”)

  • H2SO4
  • SOCl2
  • H2CrO4
  • NaOH

Topics Covered in Other Articles

The carboxylic acid is a common functional group in many organic molecules and biomolecules. This article describes the basics of the carboxylic acid functional group, as well as some of its reactions and chemical effect.