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4-Hydroxybenzoic acid
4-Hydroxybenzoic acid
4-Hydroxybenzoic acid
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4-Hydroxybenzoic acid
Skeletal formula
Skeletal formula
Ball-and-stick model
Ball-and-stick model
Names
Preferred IUPAC name
4-Hydroxybenzoic acid
Other names
p-Hydroxybenzoic acid
para-Hydroxybenzoic acid
PHBA
4-hydroxybenzoate
Identifiers
3D model (JSmol)
ChEBI
ChEMBL
ChemSpider
DrugBank
ECHA InfoCard 100.002.550 Edit this at Wikidata
EC Number
  • 202-804-9
KEGG
UNII
  • InChI=1S/C7H6O3/c8-6-3-1-5(2-4-6)7(9)10/h1-4,8H,(H,9,10) checkY
    Key: FJKROLUGYXJWQN-UHFFFAOYSA-N checkY
  • InChI=1/C7H6O3/c8-6-3-1-5(2-4-6)7(9)10/h1-4,8H,(H,9,10)
    Key: FJKROLUGYXJWQN-UHFFFAOYAQ
  • O=C(O)c1ccc(O)cc1
  • c1cc(ccc1C(=O)O)O
Properties
C7H6O3
Molar mass 138.122 g·mol−1
Appearance White crystalline solid
Odor Odorless
Density 1.46 g/cm3
Melting point 214.5 °C (418.1 °F; 487.6 K)
Boiling point N/A, decomposes[1]
0.5 g/100 mL
Solubility
log P 1.58
Acidity (pKa) 4.54
Hazards
Occupational safety and health (OHS/OSH):
Main hazards
 Irritant
NFPA 704 (fire diamond)
NFPA 704 four-colored diamondHealth 2: Intense or continued but not chronic exposure could cause temporary incapacitation or possible residual injury. E.g. chloroformFlammability 0: Will not burn. E.g. waterInstability 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazards (white): no code
2
0
0
250 °C (482 °F; 523 K)
Lethal dose or concentration (LD, LC):
2200 mg/kg (oral, mouse)
Safety data sheet (SDS) HMDB
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
checkY verify (what is checkY☒N ?)

4-Hydroxybenzoic acid, also known as p-hydroxybenzoic acid (PHBA), is a monohydroxybenzoic acid, a phenolic derivative of benzoic acid. It is a white crystalline solid that is slightly soluble in water and chloroform but more soluble in polar organic solvents such as alcohols and acetone. 4-Hydroxybenzoic acid is primarily known as the basis for the preparation of its esters, known as parabens, which are used as preservatives in cosmetics and some ophthalmic solutions. It is isomeric with 2-hydroxybenzoic acid, known as salicylic acid, a precursor to aspirin, and with 3-hydroxybenzoic acid.

Natural occurrences

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It is found in plants of the genus Vitex such as V. agnus-castus or V. negundo, and in Hypericum perforatum (St John's wort). It is also found in Spongiochloris spongiosa, a freshwater green alga.

The compound is also found in Ganoderma lucidum, a medicinal mushroom with the longest record of use.

Cryptanaerobacter phenolicus is a bacterium species that produces benzoate from phenol via 4-hydroxybenzoate.[2]

Occurrences in food

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4-Hydroxybenzoic acid can be found naturally in coconut.[3] It is one of the main catechins metabolites found in humans after consumption of green tea infusions.[4] It is also found in wine,[5] in vanilla, in Macrotyloma uniflorum (horse gram), carob[6] and in Phyllanthus acidus (Otaheite gooseberry).

Açaí oil, obtained from the fruit of the açaí palm (Euterpe oleracea), is rich in p-hydroxybenzoic acid (892±52 mg/kg).[7] It is also found in cloudy olive oil[citation needed] and in the edible mushroom Russula virescens (green-cracking russula).[citation needed]

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p-Hydroxybenzoic acid glucoside can be found in mycorrhizal and non-mycorrhizal roots of Norway spruces (Picea abies).[8]

Violdelphin is an anthocyanin, a type of plant pigments, found in blue flowers and incorporating two p-hydroxybenzoic acid residues, one rutinoside and two glucosides associated with a delphinidin.

Agnuside is the ester of aucubin and p-hydroxybenzoic acid.[9]

Biosynthesis

[edit]

Chorismate lyase is an enzyme that transforms chorismate into 4-hydroxybenzoate and pyruvate. This enzyme catalyses the first step in ubiquinone biosynthesis in Escherichia coli and other Gram-negative bacteria.

Benzoate 4-monooxygenase is an enzyme that utilizes benzoate, NADPH, H+ and O2 to produce 4-hydroxybenzoate, NADP+ and H2O. This enzyme can be found in Aspergillus niger.

4-Hydroxybenzoate also arises from tyrosine.[10]

Metabolism

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As an intermediate

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The enzyme 4-methoxybenzoate monooxygenase (O-demethylating) transforms 4-methoxybenzoate, an electron acceptor AH2 and O2 into 4-hydroxybenzoate, formaldehyde, the reduction product A and H2O. This enzyme participates in 2,4-dichlorobenzoate degradation in Pseudomonas putida.

The enzyme 4-hydroxybenzaldehyde dehydrogenase uses 4-hydroxybenzaldehyde, NAD+ and H2O to produce 4-hydroxybenzoate, NADH and H+. This enzyme participates in toluene and xylene degradation in bacteria such as Pseudomonas mendocina. It is also found in carrots (Daucus carota).

The enzyme that 2,4'-dihydroxyacetophenone dioxygenase transforms 2,4'-dihydroxyacetophenone and O2 into 4-hydroxybenzoate and formate. This enzyme participates in bisphenol A degradation. It can be found in Alcaligenes species.

The enzyme 4-chlorobenzoate dehalogenase uses 4-chlorobenzoate and H2O to produce 4-hydroxybenzoate and chloride. It can be found in Pseudomonas species.

The enzyme 4-hydroxybenzoyl-CoA thioesterase utilizes 4-hydroxybenzoyl-CoA and H2O to produce 4-hydroxybenzoate and CoA. This enzyme participates in 2,4-dichlorobenzoate degradation. It can be found in Pseudomonas species.

The enzyme 4-hydroxybenzoate polyprenyltransferase uses a polyprenyl diphosphate and 4-hydroxybenzoate to produce diphosphate and 4-hydroxy-3-polyprenylbenzoate. This enzyme participates in ubiquinone biosynthesis.

The enzyme 4-hydroxybenzoate geranyltransferase utilizes geranyl diphosphate and 4-hydroxybenzoate to produce 3-geranyl-4-hydroxybenzoate and diphosphate. Biosynthetically, alkannin is produced in plants from the intermediates 4-hydroxybenzoic acid and geranyl pyrophosphate. This enzyme is involved in shikonin biosynthesis. It can be found in Lithospermum erythrorhizon.

The enzyme 3-hydroxybenzoate—CoA ligase uses ATP, 3-hydroxybenzoate and CoA to produce AMP, diphosphate and 3-hydroxybenzoyl-CoA. The enzyme works equally well with 4-hydroxybenzoate. It can be found in Thauera aromatica.

Biodegradation

[edit]

The enzyme 4-hydroxybenzoate 1-hydroxylase transforms 4-hydroxybenzoate, NAD(P)H, 2 H+ and O2 into hydroquinone, NAD(P)+, H2O and CO2. This enzyme participates in 2,4-dichlorobenzoate degradation. It can be found in Candida parapsilosis.

The enzyme 4-hydroxybenzoate 3-monooxygenase transforms 4-hydroxybenzoate, NADPH, H+ and O2 into protocatechuate, NADP+ and H2O. This enzyme participates in benzoate degradation via hydroxylation and 2,4-dichlorobenzoate degradation. It can be found in Pseudomonas putida and Pseudomonas fluorescens.

The enzyme 4-hydroxybenzoate 3-monooxygenase (NAD(P)H) utilizes 4-hydroxybenzoate, NADH, NADPH, H+ and O2 to produce 3,4-dihydroxybenzoate (protocatechuic acid), NAD+, NADP+ and H2O. This enzyme participates in benzoate degradation via hydroxylation and 2,4-dichlorobenzoate degradation. It can be found in Corynebacterium cyclohexanicum and in Pseudomonas sp.

The enzyme 4-hydroxybenzoate decarboxylase uses 4-hydroxybenzoate to produce phenol and CO2. This enzyme participates in benzoate degradation via coenzyme A (CoA) ligation. It can be found in Klebsiella aerogenes (Aerobacter aerogenes).

The enzyme 4-hydroxybenzoate—CoA ligase transforms ATP, 4-hydroxybenzoate and CoA to produce AMP, diphosphate and 4-hydroxybenzoyl-CoA. This enzyme participates in benzoate degradation via CoA ligation. It can be found in Rhodopseudomonas palustris.

Coniochaeta hoffmannii is a plant pathogen that commonly inhabits fertile soil. It is known to metabolize aromatic compounds of low molecular weight, such as p-hydroxybenzoic acid.

Glycosylation

[edit]

The enzyme 4-hydroxybenzoate 4-O-beta-D-glucosyltransferase transforms UDP-glucose and 4-hydroxybenzoate into UDP and 4-(beta-D-glucosyloxy)benzoate. It can be found in the pollen of Pinus densiflora.

Chemistry

[edit]

The Hammett equation describes a linear free-energy relationship relating reaction rates and equilibrium constants for many reactions involving benzoic acid derivatives with meta- and para-substituents.

Chemical production

[edit]

4-Hydroxybenzoic acid is produced commercially from potassium phenoxide and carbon dioxide in the Kolbe-Schmitt reaction.[11] It can also be produced in the laboratory by heating potassium salicylate with potassium carbonate to 240 °C, followed by treating with acid.[12]

Chemical reactions

[edit]

4-Hydroxybenzoic acid has about one tenth the acidity of benzoic acid, having an acid dissociation constant Ka = 3.3×10−5 M at 19 °C.[citation needed] Its acid dissociation follows this equation:

HOC6H4CO2H ⇌ HOC6H4CO−2 + H+

Chemical use

[edit]

Vectran is a manufactured fiber, spun from a liquid crystal polymer. Chemically it is an aromatic polyester produced by the polycondensation of 4-hydroxybenzoic acid and 6-hydroxynaphthalene-2-carboxylic acid. The fiber has been shown to exhibit strong radiation shielding used by Bigelow Aerospace and produced by StemRad.[13]

4,4′-Dihydroxybenzophenone is generally prepared by the rearrangement of p-hydroxyphenylbenzoate. Alternatively, p-hydroxybenzoic acid can be converted to p-acetoxybenzoyl chloride. This acid chloride reacts with phenol to give, after deacetylation, 4,4′-dihydroxybenzophenone.

Examples of drugs made from PHBA include nifuroxazide, orthocaine, ormeloxifene and proxymetacaine.

Bioactivity and safety

[edit]

4-Hydroxybenzoic acid is a popular antioxidant in part because it is rather nontoxic. The LD50 is 2200 mg/kg in mice (oral).[14]

4-Hydroxybenzoic acid has weak estrogenic activity both in vitro and in vivo,[15] and stimulates the growth of human breast cancer cell lines.[16][17] It is a common metabolite of paraben esters, such as methylparaben.[15][16][17] The compound is a relatively weak estrogen, but can produce uterotrophy with sufficient doses to an equivalent extent relative to estradiol, which is unusual for a weakly estrogenic compound and indicates that it may be a full agonist of the estrogen receptor with relatively low binding affinity for the receptor.[16][18][19] It is about 0.2% to 1% as potent as an estrogen as estradiol.[18]

Research

[edit]

4-Hydroxybenzoic acid has been used as a precursor to co-enzyme Q10 as an experimental treatment for mitochondrial encephalopathy caused by an inherited deficiency in 4-hydroxyphenylpyruvate dioxygenase-like protein.[20]

See also

[edit]

References

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

4-Hydroxybenzoic acid

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4-Hydroxybenzoic acid is a monohydroxy derivative of benzoic acid characterized by a hydroxy group at the 4-position of the benzene ring, with the molecular formula C₇H₆O₃ and a molecular weight of 138.12 g/mol. It appears as a white, crystalline solid with a melting point of 214.5 °C and low solubility in water (approximately 0.5 g/100 mL at room temperature). The compound's structure enables it to participate in hydrogen bonding, contributing to its physical properties and reactivity as both a weak acid (pKa ≈ 4.54 for the carboxylic group) and a phenolic moiety. In industrial applications, 4-hydroxybenzoic acid serves primarily as the precursor for paraben esters, which function as effective antimicrobial preservatives in cosmetics, pharmaceuticals, and food products due to their broad-spectrum activity against bacteria and fungi. It is also employed in the synthesis of liquid crystal polymers and polyesters, enhancing material properties such as thermal stability and rigidity in applications ranging from electronics to textiles. Additionally, the compound acts as an intermediate for producing dyes, fungicides, and antioxidants, leveraging its aromatic framework for further derivatization. Biochemically, 4-hydroxybenzoic acid occurs as a metabolite in microbial catabolic pathways, where it is degraded via protocatechuate or gentisate routes, and has demonstrated , , and effects in various studies, though its physiological roles in higher organisms remain limited. Synthetic production often involves microbial from renewable feedstocks like glucose, offering sustainable alternatives to traditional methods.

Chemical Properties

Molecular Structure and Physical Characteristics

4-Hydroxybenzoic acid possesses the molecular formula C₇H₆O₃ and a structure derived from benzoic acid, featuring a hydroxyl group attached to the benzene ring at the para position relative to the carboxylic acid substituent. The molecule's planarity arises from the sp²-hybridized carbon atoms in the aromatic ring, with bond angles approximately 120°, and it exhibits polarity due to the dipole moments of the electron-withdrawing -COOH and hydrogen-bond donor/acceptor -OH groups, facilitating intermolecular hydrogen bonding in the solid state. As a white crystalline solid with a molar mass of 138.12 g/mol, it has a density of 1.46 g/cm³. Its melting point ranges from 214 to 217 °C, while the boiling point is approximately 334 °C at standard pressure, though decomposition may occur prior to boiling. The compound demonstrates limited solubility in water, approximately 5 g/L at 20 °C, attributable to hydrogen bonding with water molecules despite the hydrophobic aromatic core. It possesses two ionizable protons: the carboxylic acid with pKₐ 4.54 and the phenolic hydroxyl with pKₐ approximately 9.4, influencing its acidity and behavior in aqueous environments.
PropertyValue
Molecular formulaC₇H₆O₃
Molar mass
AppearanceWhite crystalline solid
Melting point
Boiling point
Water solubility (20 °C)~5 g/L
pKₐ (carboxylic)4.54
pKₐ (phenolic)~9.4
Spectroscopic characterization confirms its identity: infrared (IR) spectra display a characteristic carbonyl stretch for the dimer around 1670 cm⁻¹ and broad O-H stretching above 3000 cm⁻¹; ¹H NMR reveals symmetric aromatic doublets at approximately 6.9 ppm (2H) and 7.9 ppm (2H), with variable OH signals; UV-Vis absorption shows a maximum near 255 nm due to π-π* transitions enhanced by the para-hydroxy substituent.

Reactivity and Derivatives

4-Hydroxybenzoic acid undergoes reactions, directed primarily by the strongly activating hydroxyl group at the para position relative to the , favoring ortho positions (3 and 5) on the benzene ring despite the deactivating, meta-directing influence of the -COOH group. , such as bromination, occurs under these conditions, with the hydroxyl directing addition. The functionality readily undergoes esterification with alcohols in the presence of acid catalysts, yielding alkyl esters known as parabens, such as (methyl 4-hydroxybenzoate), , , and butylparaben, which exhibit properties. These reactions typically proceed via Fischer esterification, where the acid is activated by , facilitating nucleophilic attack by the alcohol. Decarboxylation of 4-hydroxybenzoic acid produces , often requiring catalytic conditions such as complexes or high temperatures; for instance, electron-rich Pd catalysts enable selective decarboxylation of hydroxybenzoic acids to substituted . This process involves beta-keto acid-like tautomerism facilitated by the ortho relationship in the ionized form, though para substitution alters the kinetics compared to the ortho isomer . Under oxidative conditions, such as (Fe²⁺/H₂O₂), 4-hydroxybenzoic acid generates -like byproducts that participate in redox cycling, enhancing iron reduction and radical propagation, though direct formation requires strong oxidants or enzymatic catalysis like . The compound exhibits hydrolytic stability across 4–9 at 25°C, resisting in neutral to mildly acidic or basic aqueous environments.
Common Paraben DerivativesFormulaApplication Note
MethylparabenC₈H₈O₃Preservative in cosmetics
EthylparabenC₉H₁₀O₃Antimicrobial agent
PropylparabenC₁₀H₁₂O₃Used in pharmaceuticals
ButylparabenC₁₁H₁₄O₃Food and cosmetic preservative

Natural Occurrence

In Plants and Foods

4-Hydroxybenzoic acid occurs naturally in numerous plant-based foods, where it acts as a phenolic compound aiding in antioxidant protection and defense against environmental stresses such as pathogens and UV radiation. Concentrations vary by source and form, often present as free acid, glycosides, or esters; for instance, in berries like blackberries, total hydroxybenzoic acids range from 8 to 27 mg per 100 g fresh weight, with 4-hydroxybenzoic acid contributing notably after hydrolysis. In spices from the Apiaceae family, such as anise, p-hydroxybenzoic acid-O-glucoside levels attain 730–1080 mg/kg fresh weight, representing a substantial reservoir of the compound. Specific examples include green tea leaves (), containing approximately 6.6 mg/kg of 4-hydroxybenzoic acid, and grapes (), where it is identified in the berry's solid components, contributing to the phenolic profile transferred to wine. Traces appear in and red wines, though at lower levels compared to berries and spices; in red wines, total hydroxybenzoic acids may reach up to 218 mg/L, with 4-hydroxybenzoic acid as a component. Unlike synthetic variants employed industrially, naturally occurring forms in plants are predominantly conjugated, influencing upon consumption. Estimated dietary intake of hydroxybenzoic acids, encompassing 4-hydroxybenzoic acid from these sources, averages around 11 mg per day in populations with typical European diets, varying with , , and spice consumption.

Biosynthesis

Pathways in Organisms

In bacteria such as and other proteobacteria, 4-hydroxybenzoic acid is biosynthesized from chorismate, an intermediate of the , via the chorismate pyruvate-lyase (encoded by ubiC), which catalyzes the elimination of pyruvate to form 4-hydroxybenzoic acid as part of ubiquinone (coenzyme Q) . This reaction proceeds without coenzyme A involvement, distinguishing it from certain eukaryotic routes, and is regulated by feedback mechanisms tied to cellular demands. In the phytopathogen , genes encoding enzymes for 4-hydroxybenzoic acid synthesis are clustered with those for its transport and utilization, forming a coordinated operon-like structure that supports both endogenous production and scavenging from the environment, as identified in genomic analyses from 2015. Similar genetic organization occurs in other gamma-proteobacteria like Lysobacter enzymogenes, where 4-hydroxybenzoic acid links shikimate flux to pathways, such as antifungal heat-stable antifungals, via diffusible signaling. Plants employ a CoA-dependent pathway for 4-hydroxybenzoic acid production, diverging from the bacterial lyase mechanism; chorismate is first converted to isochorismate or routed through tyrosine-derived intermediates like 4-hydroxyphenylpyruvate within the shikimate network, followed by ligation to and subsequent or oxidation steps to yield the free acid for ubiquinone or phenolic precursor roles. Anthranilate-derived routes have been proposed in some but lack widespread enzymatic confirmation, contrasting with the direct lyase in microbes. Across species, pathway efficiency varies due to and precursor availability; for instance, Pseudomonas species primarily utilize the ubiC-like lyase but can incorporate under nutrient stress, enhancing flux. In biotechnological applications, engineering microbes like Corynebacterium glutamicum by overexpressing genes (aro cluster) and a resistant ubiC variant achieves titers exceeding 10 g/L from glucose, demonstrating scalable variations not native to wild-type organisms.

Industrial Production

Synthetic Methods

The principal industrial synthesis of 4-hydroxybenzoic acid employs the Kolbe-Schmitt carboxylation, involving the reaction of potassium phenoxide with at elevated temperatures (typically 150–200 °C) and pressures (up to 100 atm), followed by acidification to yield the product alongside the ortho isomer (), which is separated via fractional or . This method, commercialized since the 1870s, achieves overall yields of approximately 70–90% after purification, leveraging inexpensive phenol and CO2 feedstocks for economic viability on multi-ton scales. Alternative chemical routes include the oxidation of p-cresol using molecular oxygen or air in the presence of cobalt-based catalysts (e.g., CoCl2 or Co3O4) under alkaline conditions, which can proceed stepwise via p-hydroxybenzaldehyde to the carboxylic acid, though selectivity and catalyst recovery pose challenges for large-scale adoption. Another approach entails hydrolysis of esters such as methyl 4-hydroxybenzoate, sourced from electrophilic aromatic substitution on phenol followed by esterification and saponification, offering high purity (>99%) but higher costs due to multi-step processing and byproduct management. Commercial products from these chemical methods routinely meet purity standards exceeding 99%, with scalability favored by the Kolbe-Schmitt process owing to its direct carboxylation and established infrastructure. Emerging bio-based alternatives utilize of microorganisms, such as Pseudomonas taiwanensis VLB120, to ferment renewable feedstocks like glucose, , or into 4-hydroxybenzoic acid via overexpression and , attaining titers up to 10 g/L and molar yields around 20–30% in fed-batch processes as reported in 2021 studies. These microbial routes prioritize by avoiding inputs but lag in economic competitiveness due to lower productivity (e.g., 0.1–0.5 g/L/h), higher purification demands, and unproven industrial scalability relative to .

Metabolism and Biodegradation

In Human and Animal Systems

In mammals, 4-hydroxybenzoic acid is rapidly absorbed following , with predicted human intestinal absorption exceeding 98% based on models validated against experimental data. Distribution is primarily to plasma and tissues, facilitated by its moderate (log Kow ≈1.6), though it does not readily cross the blood-brain barrier. In hepatic tissues, it undergoes phase II conjugation, predominantly to glucuronides and sulfates, which enhance water solubility for elimination; this process mirrors the of related phenolic acids. Excretion occurs mainly via , with over 70% of an oral dose recovered as conjugated metabolites within 24 hours in models, indicating efficient clearance. The plasma half-life is short, typically 1–2 hours for phenolic acids like 4-hydroxybenzoic acid in humans, reflecting rapid and renal elimination. A intervention study in humans showed significantly elevated urinary levels of 4-hydroxybenzoic acid following consumption of an organic diet (>80% organic products for 4 days), with increasing up to 4-fold, attributed to higher dietary phenolic precursors from plant sources. In animal systems, pharmacokinetics align with human patterns, featuring quick absorption and conjugation; for instance, in mice, oral dosing yields an LD50 of 2200 mg/kg, consistent with low systemic retention due to metabolic efficiency. The compound serves as a minor intermediate in gut microbial catabolism of or lignin-derived aromatics in ruminants, where ruminal convert complex phenolics to 4-hydroxybenzoic acid before host absorption and further processing. No occurs in mammals, owing to its polarity, short , and conversion to polar conjugates that preclude tissue partitioning.

Microbial and Environmental Degradation

Bacteria such as and species degrade 4-hydroxybenzoic acid primarily through the protocatechuate pathway, initiating with to protocatechuate via 4-hydroxybenzoate 3-hydroxylase, followed by extradiol ring cleavage by protocatechuate 3,4-dioxygenase, and subsequent funneling into central for complete mineralization to CO₂ and H₂O. This process enables aerobic soil isolates like Acinetobacter johnsonii FZ-5 to utilize 4-hydroxybenzoic acid as a sole carbon source, achieving substantial breakdown under both aerobic and anaerobic conditions. Specific strains demonstrate high efficiency; Herbaspirillum aquaticum KLS-1, isolated from tailing soil, degrades p-hydroxybenzoic acid (synonymous with 4-hydroxybenzoic acid) via protocatechuate ortho-cleavage, with optimal rates at pH 6.0–8.0, 30–35 °C, and 180 rpm shaking, supporting acquisition and removing over 90% within days under favorable conditions. Similarly, Pseudarthrobacter phenanthrenivorans Sphe3 employs versatile catabolic routes for 4-hydroxybenzoic acid, including meta- and ortho-cleavage variants, allowing growth on it as the sole carbon source and >90% degradation in short-term cultures as detailed in 2024 analyses of growth profiles and metabolites. In plant-pathogen contexts, competitively inhibits 4-hydroxybenzoic acid degradation in , reducing virulence by blocking uptake and catabolism, as observed in host interactions in 2024 studies. Fungi contribute to degradation, with endophytic Phomopsis liquidambari B3 capable of metabolizing 4-hydroxybenzoic acid through similar aromatic ring-opening mechanisms, and white-rot fungi like those funneling lignin-derived aromatics exhibiting enzyme-mediated conversion to central metabolites. 4-Hydroxybenzoic acid exhibits ready biodegradability per 301 guidelines, with 100% degradation in 28 days under aerobic conditions, reflecting complete microbial mineralization. In environmental matrices, aerobic half-lives in soil and water are short, typically under one week, with 34–70% mineralization reported in sediments within 6 days.

Applications

Preservative and Antimicrobial Uses

Ester derivatives of 4-hydroxybenzoic acid, known as parabens (e.g., , ), are employed as broad-spectrum preservatives in , pharmaceuticals, and certain food products at concentrations typically ranging from 0.1% to 0.4% for individual esters, with mixtures limited to 0.8% to inhibit microbial growth and extend . These compounds demonstrate efficacy against , such as (with minimum inhibitory concentrations often 200–1000 ppm at pH 6), and fungi, achieving substantial reductions in microbial load through disruption of transport processes and integrity. The antimicrobial action of parabens operates via partitioning into lipid membranes, altering fluidity and inhibiting energy-dependent transport, which is more pronounced in their undissociated form prevalent at pH 4–8. This pH range provides an advantage over alternatives like sorbic acid, which exhibits optimal activity at lower pH (3.0–6.5) and reduced efficacy in neutral or alkaline formulations. Parabens were first utilized as preservatives in the , following demonstrations of their inhibitory effects by researchers like Theodor Sabalitschka, and have since been integral to preventing spoilage in water-based products. Additionally, 4-hydroxybenzoic acid itself exhibits properties suitable for stabilizing food and beverage formulations against oxidative degradation.

Other Industrial Applications

4-Hydroxybenzoic acid functions as a chemical intermediate in the manufacture of polymers, where it contributes to the structural properties enabling applications in displays and optical films. It is also incorporated into polyester resins, enhancing thermal stability and mechanical performance in industrial polymers. In the dyes sector, the compound serves as a precursor for pigments and oil-soluble azo pigments, which are employed in inks, textiles, and coatings for colorfastness. Thermosensitive developers derived from 4-hydroxybenzoic acid find use in production for receipts and labels. For agrochemicals, it acts as an intermediate in synthesizing fungicides and organophosphorus insecticides, supporting crop protection formulations. In pharmaceuticals, derivatives are utilized in antiseptics and other drug intermediates, leveraging its phenolic structure for targeted synthesis. Additionally, it contributes to absorbers in coatings and plastics, providing photostability against UV degradation. Other roles include inhibition in industrial fluids and emulsification in formulations, owing to its and surface-active properties. Global market analyses indicate production supports diverse sectors, with estimated values around USD 80-150 million annually as of recent years, reflecting steady demand for these non-preservative applications.

Toxicology and Safety

Biological Activity and Health Effects

4-Hydroxybenzoic acid (4-HBA) demonstrates antioxidant activity primarily through its phenolic hydroxyl group, which facilitates donation to neutralize free radicals such as peroxyl species. Studies report its (ORAC) comparable to other hydroxybenzoic acids, with values reflecting moderate peroxyl radical scavenging efficiency relative to equivalents. This bioactivity contributes to reducing in cellular models, though structure-activity analyses indicate lower potency than multi-hydroxylated analogs like protocatechuic acid. In terms of anti-inflammatory and anti-allergic effects, 4-HBA inhibits inflammasome priming and activation, thereby attenuating release and in lipopolysaccharide-challenged models. A 2025 murine study on allergic further showed that hydroxybenzoic acid suppresses eosinophil-driven inflammation and interleukin-5 production, highlighting selective modulation of allergic responses without broad . These effects occur at concentrations achievable via dietary or supplemental intake, supporting potential roles in mitigating inflammation-related conditions. 4-HBA exhibits antimicrobial activity against , including pathogens like species, with minimum inhibitory concentrations in the range of those for related phenolic acids; this is attributed to membrane disruption and enzyme inhibition. In contexts, such activity enhances safety by inhibiting microbial growth without requiring esterification to parabens. Human and animal exposure data indicate low , with no observed adverse effects in repeated oral dosing up to 1000 mg/kg/day in , corresponding to substantial margins over typical dietary levels. However, topical application can induce mild skin sensitization in susceptible individuals, classified as a weak dermal sensitizer due to potential hapten formation with skin proteins, though absorption is limited and irritation is generally minimal at concentrations below 1%. Dose-response evidence from animal studies confirms safety at high systemic doses but underscores the need for caution in prolonged cutaneous exposure. Overall, while 4-HBA's bioactivities offer benefits for antioxidant defense and microbial control, its health effects are context-dependent, with benefits outweighing risks at low environmental or dietary exposures.

Regulatory Status and Risk Assessments

The U.S. (FDA) approves 4-hydroxybenzoic acid for use as a at concentrations up to 0.1%, reflecting its established safety profile in direct contact applications. It is also affirmed for use as a agent or adjuvant under FDA listings for substances added to , with no requirement for pre-market approval beyond these parameters due to its low . In the , 4-hydroxybenzoic acid and its salts and are regulated as permitted preservatives in under Annex V of Regulation (EC) No 1223/2009, with maximum concentrations of 0.4% for a single or 0.8% for mixtures of , as amended by Commission Regulation (EU) No 1004/2014. These limits are based on toxicological data indicating minimal risk at approved levels, including rapid to the parent acid, which undergoes complete and excretion without significant accumulation. The Joint FAO/WHO Expert Committee on Food Additives (JECFA) evaluated 4-hydroxybenzoic acid in 2001 and concluded no safety concern at current levels of intake when used as a agent, supported by acute oral LD50 values exceeding 2,000 mg/kg body weight in rats and mice. Screening Information Data Set () assessments similarly classify it as having low , with no observed adverse effects in repeated oral dosing studies at or above 1,000 mg/kg body weight in rats (NOAEL ≥1,000 mg/kg), and no evidence of carcinogenicity in rodent models. Chronic exposure studies confirm no reproductive or developmental toxicity below 500 mg/kg body weight, aligning with regulatory thresholds that prioritize empirical no-effect levels over speculative risks.

Environmental Impact

Fate in the Environment

4-Hydroxybenzoic acid exhibits low volatility, with an extrapolated of 1.9 × 10^{-7} mm Hg at 25 °C, limiting its partitioning into the atmosphere and aerial transport. Its moderate solubility of approximately 6 g/L at 25 °C facilitates dissolution in aqueous environments, while the low (log K_{ow} ≈ 1.5) indicates minimal sorption to in sediments or soils. In aquatic and soil systems, 4-hydroxybenzoic acid undergoes rapid aerobic , achieving complete mineralization within 28 days under standardized conditions ( 301C), with half-lives typically under 10 days in adapted environments. This contrasts with more persistent esters, as the free acid form supports faster microbial uptake and transformation, reducing overall environmental persistence. The compound shows no significant potential due to its low log K_{ow}, with predicted factors below levels of concern. Runoff from industrial or agricultural sites is minimal, as rapid metabolic degradation in receiving waters and soils limits long-range transport.

Ecotoxicological Effects

4-Hydroxybenzoic acid demonstrates low acute toxicity to representative aquatic species across trophic levels. The 96-hour LC50 for the fish Oryzias latipes is 92.8 mg/L, while the 14-day LC50 is 66.5 mg/L. For the crustacean Daphnia magna, the 48-hour EC50 for immobilization is 135.7 mg/L. Algal growth inhibition, measured as the 72-hour EC50 for Selenastrum capricornutum, is 68.5 mg/L. Chronic exposure yields similarly low effect levels. The no-observed-effect concentration (NOEC) for algal growth is 32.0 mg/L over 72 hours, and for Daphnia magna reproduction, the 21-day NOEC exceeds 100 mg/L. These values, all exceeding typical thresholds for high toxicity (e.g., <10 mg/L), support assessments of minimal risk to aquatic populations at predicted environmental concentrations.
OrganismEndpointValue (mg/L)DurationReference
Oryzias latipes (fish)LC5092.896 hours
Daphnia magnaEC50135.748 hours
Selenastrum capricornutum (algae)EC5068.572 hours
Selenastrum capricornutum (algae)NOEC32.072 hours
Daphnia magnaNOEC (reproduction)>10021 days
Effects on microorganisms are mild at high concentrations but are offset by rapid biodegradability, with 100% degradation achieved in 28 days under 301C conditions. The compound's low (log Kow = 1.37) indicates negligible potential. Data on terrestrial non-target species, such as earthworms, are limited, with no reported chronic adverse effects in available empirical assessments. Overall, the predicted environmental concentration to predicted no-effect concentration ratio (PEC/PNEC = 0.003) falls well below 1, affirming low ecotoxicological risk under the High Production Volume chemicals program. No causal evidence links 4-hydroxybenzoic acid directly to endocrine disruption in aquatic or soil organisms, distinguishing it from certain derivatives.

Controversies

Endocrine Disruption and Paraben Associations

Parabens, alkyl esters of 4-hydroxybenzoic acid, exhibit weak estrogenic activity in vitro by binding to estrogen receptors with affinities roughly 10,000- to 1,000,000-fold lower than 17β-estradiol, prompting investigations into endocrine disruption potential. This binding can activate receptor-dependent pathways at high concentrations, but potency diminishes rapidly with shorter alkyl chains, and effects are not consistently replicated in vivo due to metabolic barriers. Upon absorption, parabens hydrolyze swiftly via ubiquitous esterases to 4-hydroxybenzoic acid, a metabolite with substantially reduced estrogenic activity relative to the parent compounds. A pivotal 2004 study detected intact in 19 of 20 human breast tumor samples, averaging 20.6 ng/g tissue, fueling claims of accumulation and cancer promotion via mimicry. Critics of this work, including the European Commission's Scientific Committee on Consumer Products in its 2005 assessment, emphasized methodological flaws: lack of comparison to healthy tissue, unidentified exposure routes (e.g., dermal vs. dietary), and absence of dose-response or causal data, rendering it correlative rather than evidentiary. Independent analyses further note that paraben levels in tumors were orders of magnitude below those required for proliferative effects in cell models. Epidemiological evidence has not corroborated causation; nested case-control studies, such as one involving over 400 cases, found no positive association between urinary paraben metabolites and risk, with some showing weak inverse trends potentially attributable to factors like detection limits or reverse causation. These null findings align with toxicological thresholds, where no-observed-adverse-effect levels exceed typical human exposures by factors of 100-1,000, underscoring that estrogenicity does not equate to systemic disruption absent exceeding metabolic capacity. Proponents of safety, drawing from industry-sponsored but peer-reviewed data, highlight approved use limits—0.4% for single parabens and 0.8% mixtures in —as protective, enabling microbial control that averts greater harms like bacterial outbreaks in products. Alarmist viewpoints persist on , yet parabens biodegrade rapidly in environments, with >90% dissipation in 3 days and ready aerobic degradation, limiting persistence and ecological carryover. For 4-hydroxybenzoic acid itself, isolated estrogenic signals in specific assays (e.g., plant-derived contexts) lack translation to human risk at trace levels, prioritizing empirical null over precautionary extrapolation.

Debates on Safety and Alternatives

In , the implemented restrictions on propylparaben and butylparaben, prohibiting their use in leave-on cosmetic products applied to the nappy area of children under three years old, citing precautionary concerns over potential absorption and endocrine effects despite limited direct evidence of harm at typical exposure levels. This measure, enacted via Regulation (EU) No 1004/, extended from Denmark's national ban on these parabens in child products under three, reflecting a precautionary approach amid ongoing debates rather than conclusive . Safety assessments, including those by the Cosmetic Ingredient Review panel, have affirmed that parabens derived from 4-hydroxybenzoic acid exhibit low acute and in studies, with rapid and minimizing systemic risks at preservative concentrations up to 0.1-0.4%. Critics of such bans argue they prioritize hypothetical risks over empirical evidence of efficacy in preventing microbial spoilage, which has historically reduced food waste by extending shelf life and curbing emissions from discarded products; for instance, methylparaben and propylparaben additions at 0.1% levels effectively inhibit bacteria, molds, and yeasts in items like olives and desserts. Alternatives such as sodium benzoate, often combined with potassium sorbate, provide antimicrobial activity but are less effective in neutral or alkaline pH environments common in many formulations, where parabens maintain broad-spectrum protection. Natural preservatives, including essential oils or plant extracts, face limitations in efficacy, requiring higher concentrations that may alter sensory properties or fail to achieve equivalent microbial inhibition, resulting in shorter shelf lives. Proponents of continued 4-hydroxybenzoic acid derivative use emphasize causal evidence from reviews showing no practical impacts at approved levels, contrasting with the inefficiencies of substitutes that could increase spoilage-related by 20-30% in perishable without optimized systems. Regulatory bodies like the U.S. FDA permit parabens in food at levels supporting safety, underscoring that bans in regions like the may reflect bias toward precaution over data-driven , potentially overlooking preservatives' role in global .

Recent Developments

Biotechnological Advances

Recent metabolic engineering efforts have focused on microbial hosts like Escherichia coli to produce 4-hydroxybenzoic acid (4-HBA) from renewable feedstocks such as glucose, aiming to displace petroleum-derived chemical synthesis. In a 2024 study, an engineered E. coli strain achieved a titer of 21.35 g/L 4-HBA in a 5-L fermenter, with a yield of 0.19 g/g glucose and productivity of 0.44 g/L/h, representing a significant improvement over prior benchmarks like 12 g/L reported in earlier E. coli systems. These advances leverage shikimate pathway modifications and cofactor balancing to enhance carbon flux from sugars to 4-HBA, reducing reliance on fossil fuels and enabling integration with lignocellulosic biomass hydrolysates as feedstocks. Parallel developments in and other bacteria have explored aromatic feedstocks like L-tyrosine, derived from renewables, yielding up to 128 mM (approximately 17.7 g/L) 4-HBA in optimized shake-flask cultures as of 2021. Despite these titers, scalability remains a hurdle compared to chemical routes, which achieve near-theoretical yields but generate more waste; biotechnological processes offer environmental benefits through milder conditions and byproduct minimization, though fermentation costs and downstream purification efficiencies must improve for industrial viability. For , engineered and native strains have been advanced post-2020 to degrade 4-HBA in contaminated environments, such as industrial effluents. In 2023, Herbaspirillum aquaticum KLS-1 demonstrated efficient PHBA breakdown under aerobic conditions, initiating catabolism via protocatechuate pathways suitable for and remediation. By 2024, microbiome engineering incorporated 4-HBA-degrading , including specialized strains like those utilizing hydroxybenzoate hydroxylase, enhancing consortium robustness for polluted sites while highlighting challenges in maintaining degradation rates at scale against abiotic factors. Economic analyses project the 4-HBA market expanding to USD 195–400 million by 2035, driven partly by demands for bio-based preservatives and polymers, with biotechnological routes poised to capture share through premiums despite current production costs.

Emerging Research on Bioactivity

Recent studies indicate that 4-hydroxybenzoic acid (4-HBA) exhibits effects in allergic models. In a 2025 murine study, hydroxybenzoic acid administration significantly reduced airway hyperresponsiveness, infiltration, and pro-inflammatory cytokines such as TNF-α and IL-6 in fluid, while peroxybenzoic acid showed no such benefits; notably, neither compound altered IgE, IL-4, or IL-13 levels, suggesting targeted suppression of innate rather than Th2-driven responses. Similarly, 4-HBA restrains priming and activation by disrupting PU.1 DNA-binding activity and direct antioxidative mechanisms, attenuating lipopolysaccharide-induced and increasing levels in preclinical models. Dietary intake influences 4-HBA bioavailability as an phenolic metabolite. A 2019 human intervention trial demonstrated that switching from a conventional to an organic and diet for one week increased urinary 4-HBA excretion by approximately 40%, correlating with higher phenolic compound levels in organic produce and potential enhancements in systemic capacity, though long-term health outcomes remain unestablished. Emerging evidence supports 4-HBA's role in adjunct cancer therapies via modulation of . In human cell lines, 4-HBA at concentrations of 1-5 mM enhanced adriamycin by inhibiting P-glycoprotein-mediated efflux, reducing values by up to 50% and promoting without inherent . Complementary findings show 4-HBA and its derivative bind plasma proteins in cancer cells, potentially altering of chemotherapeutics like pirarubicin. In microbial interactions, 4-HBA influences phytopathogen . A 2024 study on (Xcc) revealed that host-derived competitively binds the regulator PobR, inhibiting 4-HBA degradation genes and reducing bacterial lesion sizes on by 60-70%, thereby attenuating virulence without direct toxicity to the pathogen. This suggests 4-HBA's involvement in plant defense signaling, with implications for sustainable biocontrol strategies. Additional preclinical data highlight 4-HBA's therapeutic potential in metabolic and mitochondrial disorders. In high-fat diet-induced obese mice, 4-HBA promoted browning via AMPK-Drp1 pathway activation, increasing thermogenic markers like by 2-3 fold and reducing body weight gain. In a 2024 mouse model of , oral 4-HBA supplementation rescued perinatal lethality, improved multisystemic symptoms, and boosted coenzyme Q biosynthesis as a precursor substrate. Despite these empirical findings, human trials are limited, emphasizing gaps in long-term safety, dosing, and translational efficacy beyond and models.

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