PHENYLACETIC ACID: STRUCTURE, PROPERTIES, USES, EFFECTS - CHEMISTRY - 2025

The phenylacetic acid is a solid organic compound with the chemical formula C ₈ H ₈ O ₂ or C ₆ H ₅ CH ₂ CO ₂ H. It is a monocarboxylic acid, that is, has a single carboxyl group -COOH.

It is also known as benzeneacetic acid or phenylethanoic acid. It is a white crystalline solid with an unpleasant odor, however, its taste is sweet. It is present in some flowers, fruits and plants, in fermented beverages such as tea and cocoa. It is also found in tobacco and wood smoke.

Crystals of phenylacetic acid. Tmv23. Source: Wikipedia Commons.

Phenylacetic acid is a compound that is formed by the transformation of endogenous molecules of some living beings, that is, of molecules that are a natural part of these.

It fulfills important functions which depend on the type of organism in which it is found. For example, in plants it is involved in their growth, while in humans it is involved in the release of important molecular messengers from the brain.

Its effects as an antifungal agent and as an inhibitor of bacterial growth have been studied.

Structure

The phenylacetic or benzeneacetic acid molecule has two functional groups: the carboxyl -COOH and the phenyl C ₆ H ₅ -.

It is like an acetic acid molecule to which a benzene ring or phenyl group C ₆ H ₅ - has been added to the methyl group -CH ₃.

It can also be said to be like a toluene molecule in which a hydrogen H of the methyl group -CH ₃ has been replaced by a carboxyl group -COOH.

Structure of phenylacetic acid. Author: Marilú Stea.

Nomenclature

- Phenylacetic acid

- Benzeneacetic acid

- 2-phenylacetic acid

- Phenylethanoic acid

- Benzylformic acid

- Alpha-toluic acid

- Benzylcarboxylic acid.

Properties

Physical state

White to yellow solid in the form of crystals or flakes with an unpleasant, pungent odor.

Molecular weight

136.15 g / mol

Melting point

76.7 ºC

Boiling point

265.5 ºC

Flashpoint

132 ºC (closed cup method)

Auto-ignition temperature

543 ºC

Density

1.09 g / cm ³ at 25 ºC

Solubility

Very soluble in water: 17.3 g / L at 25 ºC

Very soluble in ethanol, ethyl ether, and carbon disulfide. Soluble in acetone. Slightly soluble in chloroform.

pH

Its aqueous solutions are weakly acidic.

Dissociation constant

pK _a = 4.31

Other properties

It has a very unpleasant odor. When diluted in water, it has a sweetish smell similar to honey.

Its flavor is sweet, similar to honey.

When heated to decomposition, it emits acrid and irritating smoke.

Synthesis

It is prepared by reacting benzyl cyanide with dilute sulfuric or hydrochloric acid.

Also through the reaction of benzyl chloride and water in the presence of a Ni (CO) ₄ catalyst.

Synthesis of phenylacetic acid. Claudio Pistilli. Source: Wikipedia Commons.

Role in the biochemistry of living beings

It acts as a metabolite (a molecule that participates in metabolism, either as a substrate, intermediate compound or as a final product) in living beings, for example, in humans, in plants, in Escherichia coli, in Saccharomyces cerevisiae, and in the Aspergillus. However, it seems that it is not generated in the same way in all of them.

Function in humans

Phenylacetic acid is the main metabolite of 2-phenylethylamine, which is an endogenous constituent of the human brain and is involved in cerebral transmission.

The metabolism of phenylethylamine leads to its oxidation through the formation of phenylacetaldehyde, which is oxidized to phenylacetic acid.

Phenylacetic acid acts as a neuromodulator by stimulating the release of dopamine, which is a molecule that performs important functions in the nervous system.

It has been reported that in affective disorders, such as depression and schizophrenia, there are changes in the levels of phenylethylamine or phenylacetic acid in biological fluids.

Variation in the concentration of these compounds has also been suspected of influencing the attention deficit hyperactivity syndrome suffered by some children.

Brain of a person with attention deficit and hyperactivity syndrome where the prefrontal area is highlighted, which is where the disease has the greatest effect. Manu5. Source: Wikipedia Commons.

Function in plants

Several researchers have shown that phenylacetic acid is widely distributed in vascular and non-vascular plants.

For more than 40 years it has been recognized as a natural phytohormone or auxin, that is, a hormone that regulates plant growth. It has a positive effect on the growth and development of plants.

It is generally located on the shoots of plants. It is known for its beneficial action on corn plants, oats, beans (peas or beans), barley, tobacco and tomato.

Pea or bean plant sprout. Bijay chaurasia. Source: Wikipedia Commons.

However, its mechanism of action in plant growth has not yet been well clarified. It is also not known with certainty how it is formed in plants and vegetables. It has been suggested that in them it is produced from phenylpyruvate.

Others suggest that it is a deamination product of the amino acid phenylalanine (2-amino-3-phenylpropanoic acid) and that phenylalanine-producing plants and microorganisms can generate phenylacetic acid from it.

Function in some microorganisms

Some microbes can use it in their metabolic processes. For example, the fungus Penicillium chrysogenum uses it to produce penicillin G or natural penicillin.

Structure of the Penicillin G molecule where the component provided by phenylacetic acid is observed on the left side. Cacycle. Source: Wikipedia Commons.

Others use it as the sole source of carbon and nitrogen, such as Ralstonia solanacearum, a bacterium in the soil that causes wilting plants such as tomatoes.

Applications

In agriculture

Phenylacetic acid has proven to be an effective antifungal agent for agricultural applications.

In some studies, the phenylacetic acid produced by the bacterium Streptomyces humidus and isolated in the laboratory has been found to be effective in inhibiting the germination of zoospores and the mycelial growth of the Phytophthora capsici fungus that attacks pepper plants.

It can induce resistance in these plants against P. capsici infection, as it works as well as other commercial fungicides.

Pepper plantation. PJeganathan. Source: Wikipedia Commons.

Other studies show that the phenylacetic acid produced by various types of Bacillus exerts a toxic effect against the nematode that attacks pine wood.

In the food industry

It is used as a flavoring agent, as it has a caramel, floral, honey flavor.

In the production of other chemical compounds

It is used to make other chemicals and perfumes, to prepare esters that are used as perfumes and flavorings, pharmaceutical compounds and herbicides.

Phenylacetic acid has a use that can be highly harmful, which is in obtaining amphetamines, stimulant drugs that generate addiction, for which it is subject to strict control by the authorities of all countries.

Potential use against pathogenic germs

In some studies, the accumulation of phenylacetic acid has been found to decrease the cytotoxicity of Pseudomonas aeruginosa in human and animal cells and tissues. This bacterium causes pneumonia.

This accumulation of phenylacetic acid occurs when a high concentration of these microorganisms is inoculated into the human test cells.

The findings suggest that the P. aeruginosa bacterium, under the conditions of the experiments, produces and accumulates this inhibitor, which counteracts the infection.

Negative effects due to accumulation in humans

It has been determined that the accumulation of phenylacetic acid that occurs in patients with chronic renal failure contributes to increasing atherosclerosis and the rate of cardiovascular disease in them.

Phenylacetic acid strongly inhibits the enzyme that regulates the formation of nitric oxide (NO) from L-arginine (an amino acid).

This generates imbalances at the level of the walls of the arteries, since in normal conditions nitric oxide exerts a protective effect against the formation of atherogenic plaques in the vascular walls.

This imbalance leads to high plaque generation and cardiovascular disease in these at-risk patients.

References

1. US National Library of Medicine. (2019). Phenylacetic acid. Recovered from: pubchem.ncbi.nlm.nih.gov
2. Sugawara, S. et al. (2015). Distinct Characteristics of Indole-3-Acetic Acid and Phenylacetic Acid, Two Common Auxins in Plants. Plant Cell Physiol. 2015 Aug; 56 (8): 1641-1654. Recovered from ncbi.nlm.nih.gov.
3. Wang, J. et al. (2013). Pseudomonas aeruginosa Cytotoxicity Is Attenuated at High Cell Density ans Associated with the Accumulation of Phenylacetic Acid. PLoS One. 2013; 8 (3): e60187. Recovered from ncbi.nlm.nih.gov.
4. Mangani, G. et al. (2004). Gas chromatographic-mass spectrometric determination of phenylacetic acid in human blood. Ann. Chim. 2004 Sep-Oct; 94 (9-10): 715-9. Recovered from pubfacts.com.
5. Byung Kook Hwang, et al. (2001). Isolation and In Vivo and In Vitro Antifungal Activity of Phenylacetic Acid and Sodium Phenylacetate from Streptomyces humidus. Appl Environ Microbiol. 2001 Aug; 67 (8): 3739-3745. Recovered from ncbi.nlm.nih.g
6. Jankowski, J. et al. (2003). Increased plasma phenylacetic acid in patients with end-stage renal failure inhibits iNOS expression. Clin. Invest. 2003 Jul15; 112 (2): 256-264. Recovered from ncbi.nlm.nih.gov.