Carboxyl Group: Definition And Function

Carboxyl is an organic functional group that is made out of a single carbon atom double bonded to an oxygen atom, and single bonded to a hydroxyl group. The chemical formula for the carboxyl group is:

R[COOH]

where R refers to the rest of the molecule that the carboxyl group is attached to. Another way to write this formula is:

R[C(=O)OH]

which represents the double bond between the carbon atom and the terminal oxygen atom. One can think of a carboxyl group as a carbonyl group (C=O) whose carbon atom is bonded to a hydroxyl group (OH). The carboxyl group is an important component of organic molecules such as amino acids, fatty acids, and acetic acids, all of which play essential roles in biosynthesis and cellular respiration. An organic compound that binds to a carboxyl group is called a carboxylic acid.

Carboxylic compounds readily dissolve in neutral solvents to form weak acids. Carboxylic acids are Brønsted-Lowry acids because they donate a single proton (H⁺) to form acidic solutions. A deprotonated carboxylic acid is called a carboxylate ion. They are polar compounds and the presence of a hydrogen atom at the terminal end of the hydroxyl group allows them to engage in hydrogen bonding. Carboxylic acids are known for their strong odor and are what gives vinegar its pungent smell. Esters of carboxylic acids are known for their sweet smell and are often used in perfumes and fragrances.

What Are Functional Groups in Chemistry?

In chemistry, functional groups are specific subunits of larger chemical compounds that are responsible for that compound’s bonding behavior. One can think of functional groups as recurring patterns of atoms that play consistent functional roles (bonding and properties) across the molecules they show up in. Functional groups tend to have similar bonding behavior, no matter which molecule they show up in, so functional groups can be used to predict chemical reactions and the behavior of compounds that contain a functional group. The presence of functional groups can also be used to predict the physical properties of an organic compound. A molecule’s functional groups affect its structure, reactivity, bonding behavior, and solubility,

The simplest example of a functional group is the hydroxyl group. A hydroxyl group simple has a chemical formula of OH. Many organic and inorganic compounds contain a hydroxyl group, including water (H₂O) and most acids. Hydroxyl groups tend to act as a “cap” on chains of organic molecules that can be removed to let them bond with other functional groups. For instance, fatty acids are a lipid that is formed by a chain of hydrocarbons attached to a hydroxyl group. The hydroxyl groups are removed from fatty acid bases when forming triglycerides. The removal of a hydroxyl group allows the fatty acid to bond with the glycerol backbone. When fatty acids are removed from a triglyceride, a hydroxyl group is reattached to the end.

Compounds that contain hydroxyl groups tend to form acids in aqueous solutions. The polarity of the hydroxyl group allows the hydrogen to be removed easily, leaving behind a hydroxyl anion. This polarity also allows hydroxyl compounds to be soluble in water. The presence of a polar hydrogen end allows compounds with hydroxyl groups to engage in hydrogen bonding. As such, hydroxyl compounds tend to have higher boiling and evaporating points than structurally similar molecules that lack a hydroxyl group.

The presence of functional groups explains the maxim in chemistry that “like dissolves like.” The mutual solubility of compounds is due to the interaction of shared functional groups. Water readily dissolves sugar because both sugar and water contain a hydroxyl group and hydroxyl groups interact strongly with each other. If a functional group is more electronegative than its base then the functional group becomes polar and makes the molecule soluble in aqueous solutions.

Functional groups themselves can also contain functional groups. The carboxyl group is a perfect example. The carboxyl group itself contains two functional groups: a carbonyl group formed by a double bonded carbon and oxygen (C=O), and a hydroxyl group formed from a single bonded oxygen and a hydrogen (O–H). The presence of these two functional groups explains the bonding behavior of carboxyl groups.

Carboxyl As A Functional Group

As stated before, carboxyl is a functional group with a formula of R[COOH]. Carboxyl groups are ubiquitous in biological systems because they give organic compounds the polar and solvent properties that are necessary for life.

Carboxyl is extremely polar due to the presence of a double bond between the oxygen and carbon in the carbonyl base. As a result, the attachment of a carboxyl group makes a compound very soluble in other polar solvents. This polarity, coupled with the hydroxyl group at the other end of the molecule, allows carboxyl compounds to engage in a strong form of hydrogen bonding. The highly polar carbonyl base attracts the hydroxyl base of nearby molecules. Because hydrogen is so small, the hydroxyl base can get very close to the carbonyl base, which creates a strong electrostatic attraction.

As a consequence of this hydrogen bonding, carboxyl compounds tend to have very high melting and boiling points. For instance, both aspartate and glutamate have carboxyl groups in their side chains. The hydrogen bonding between carboxyl groups stabilizes the structure of the proteins. Carboxyl groups also tend to facilitate the linking of larger macromolecules. Carboxyl groups are also crucial for the formation of peptide bonds. Peptide bond form when the amino group of an amino acid bonds to the carbonyl group of a carboxyl group.

Examples Of Compounds With Carboxyl Groups

Formic Acid (HCOOH): The simplest possible carboxylic acid. Formic acid is what causes the pain from bee stings and ant bites.

Acetic Acid (CH₃COOH): Acetic acid forms from the joining of a carboxyl and a methyl group (CH₃). It is known for its sour taste and has wide-spread uses in commercial and textile industries.

Citric Acid (C₆H₈O₇): Citric acid is a weak acid that is most commonly associated with sour fruits, like lemons or lime. Citric acid actually has 3 individual carboxyl groups. Citric acid is used by all aerobic organisms and is an important element of the Krebs cycle—the process by which ATP is produced from the oxidation of acetyl-CoA.

Butyric Acid (C₃H₇COOH): Butyric acid is a carboxylic acid found in esters of animal fats and plant oils. Butyric acid is the chemical that is responsible for the characteristic “buttery” taste of butter.

Oxalic Acid (HOOCCOOH): Oxalic acid is formed by the conjunction of two carboxyl groups via their central carbon atoms. It is the simplest possible dicarboxylic acid. In humans, it is most commonly found in the form of calcium oxalate, the main component of kidney stones.

Benzoic Acid (C₆H₅COOH): Benzoic acid comes mostly from plants is used in food preservation and medical technologies. Benzoic acid inhibits the growth of fungi, molds, and bacteria, so it has historically been used as a means of preserving perishable good. For the same reason, benzoic acid has found some use in the medical field as a treatment for fungal skin infections. Benzoic acid is also an important precursor to compound that make up insect repellents and certain artificial flavors.

Fatty Acids: Fatty acids are biological lipids that are made out of a carboxyl base attached to a linear hydrocarbon chain. Fatty acids are the simplest kind of lipid and are the building block for more complex lipids and lipo-proteins. The unique combination of the hydrophilic carboxyl head and hydrophobic hydrocarbon chain allows them to bond to glycerides. Fatty acids are an important source of energy for cellular respiration.

Other Kinds of Functional Groups

There are many other kinds of functional groups, each characterized by their distinct bonding properties. Some examples include:

Alkyls: R[CH₂]: Alkyls are the base constituents of most single-bonded hydrocarbons, like methane, propane, ethane, octane, etc. Compounds made form alkyls tend to readily combust.

Benzene R[C₆H₅]: benzene is a special kind of alkyl composed of hydrogen and carbon arranged in a hexagonal ring-like structure. Benzene is a primary component of crude oils and petroleum.

Peroxy R[OO]R: The peroxy group consists of two single bonded oxygen molecules. The most well-known occurrence of the peroxy group is in hydrogen peroxide (H₂O₂) Because it is a powerful oxidizer, compounds with the peroxy group have been used as bleaching and antiseptic agents.

Nitrate R[ONO₂]: Nitrates are a functional group with 3 oxygen atoms bonded to a central nitrogen atom. Nitrate has a delocalized double bond between nitrogen and oxygen. It is used mainly in agricultural sectors as a fertilizer in the form of ammonium nitrate, calcium nitrate, and sodium nitrate. Compounds containing nitrate groups tend to be good oxidizers, so they can combust rapidly and form large explosions.

Phosphate R[OPO(OH)₂]: Phosphates are a polyatomic ion made out of 4 oxygen atoms bonded to a central phosphorus atom. Phosphate is very biologically important functional group, as the removal of phosphate groups from ATP generates the energy necessary for biological activity.