Benzoic Acid
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Product Description
Benzoic acid (C₇H₆O₂), the simplest aromatic carboxylic acid, is a white crystalline solid that occurs naturally in some fruits and resins and is synthesized industrially by oxidizing toluene. Best known as a food-grade preservative—especially as its more water-soluble sodium salt—it inhibits yeasts, molds and many bacteria in acidic products such as fruit juices, pickles and carbonated drinks. Beyond the kitchen cupboard the molecule is a synthetic cornerstone: it serves as a feedstock for benzoyl chloride, phenol, plasticizers, alkyd resins, dyes and fragrances, and is increasingly used as an antibiotic-free feed additive that improves gut health and reduces ammonia emissions in swine production. Benzoic acid is also a familiar laboratory standard for calibrating bomb calorimeters and a common reagent in undergraduate organic-chemistry sequences. Although classified as GRAS at low concentrations, it can irritate skin and eyes and, under harsh heat-and-light conditions in the presence of vitamin C, may yield trace benzene, so its levels in foods and beverages are tightly regulated.
Other Information
Natural Occurrence:
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Benzoic acid occurs naturally in many plants and berries. Significant amounts are found in gum benzoin, cranberries (300-1300 mg/kg), and also in plums, cinnamon, and cloves. It is a byproduct of phenylalanine metabolism in bacteria and is produced when gut bacteria process polyphenols.
History:
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Discovered in the 16th century by Prospero Alpini from gum benzoin. It was later isolated and characterized in the 19th century. The name is derived from "gum benzoin."
Synthesis and Production
Industrial Synthesis:
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Partial Oxidation of Toluene: The primary commercial method involves the liquid-phase oxidation of toluene with oxygen. This process is catalyzed by cobalt or manganese naphthenates and is favored due to its high yield and use of abundant raw materials.
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Hydrolysis: Benzoic acid can be produced by hydrolyzing benzonitrile or benzamide.
Emerging and Alternative Production Methods:
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Upcycling of Waste Polystyrene: Waste polystyrene plastics can be efficiently converted into benzoic acid through an oxidative upgrade process using nitric acid. This method achieves high yields (nearly 90%) and purity (>95%), offering a promising route for plastic waste valorization.
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From Lignin (Biorefinery): A two-step biorefinery process can produce benzoic acid from wood lignin:
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Lignin monomers are obtained via reductive-catalytic-fractionation.
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These monomers are hydrodeoxygenated to alkylbenzenes and then oxidized to benzoic acid using O₂ as the oxidant. This method yields approximately 54 mg of benzoic acid per gram of pine wood lignin.
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Photocatalytic Upcycling of Polystyrene: Acridinium-based photoredox catalysts can be used to upcycle polystyrene into benzoic acid. Computational design has identified optimized, non-intuitive catalyst structures (e.g., fluorinated acridinium salts) that achieve useful yields even with real-life polystyrene waste containing dyes and additives.
Uses and Applications
Food and Feed Industry:
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Preservative: Benzoic acid and its salts (sodium benzoate, potassium benzoate) are widely used as antimicrobial food preservatives, effective against yeast, bacteria, and molds. They are assigned E numbers E210, E211, E212, and E213.
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Feed Additive: Used in animal nutrition (e.g., swine diets) as a zootechnical additive to improve growth performance, feed efficiency, and gut health. It reduces post-weaning diarrhea in piglets and lowers the risk of urinary tract infections in sows. It also acidifies urine, leading to reduced ammonia emissions from manure.
Pharmaceuticals and Cosmetics:
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Medicinal Agent: Functions as an antifungal agent, antiseptic, and preservative in topical medications and pharmaceutical preparations.
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Drug Precursor: Serves as a key intermediate in the synthesis of various drugs. It is also a metabolite of certain pharmaceuticals, such as Lisdexamfetamine.
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Cosmetic Ingredient: Used as a preservative in cosmetic products, reviewed and approved by the Cosmetic Ingredient Review (CIR).
Industrial and Chemical Synthesis:
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Chemical Precursor: A vital starting material for producing a wide range of chemicals, including:
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Plasticizers (e.g., for PVC)
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Pesticides
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Benzoate Esters (e.g., methyl benzoate), used in perfumes and flavors.
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Organic Synthesis Intermediate: Used in various reactions, such as decarboxylative sulfoximination for drug discovery, enabled by photoinduced processes.
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Modulator in Materials Science: Acts as a monocarboxylic acid modulator in the synthesis of metal-organic frameworks (MOFs), like Al₂O₃/UiO-66 composites, to engineer defects, create mesopores, and control crystallinity.
Environmental Applications:
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Wastewater Treatment: Benzoic acid-modified adsorbents (e.g., Al₂O₃/UiO-66 composites) show high efficiency in removing anionic dyes, such as Remazol Red 5B, from industrial wastewater.
Chemical and Physical Properties
Physical Properties:
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Molecular Formula: C₇H₆O₂
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Molecular Weight: 122.12 g/mol
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Appearance: White (or colorless) crystalline solid, often as scales, needles, or leaflets.
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Odor: Faint, pleasant odor, sometimes described as having a slight benzaldehyde or almond-like scent.
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Melting Point: 122.4 °C
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Boiling Point: 249.2 °C
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Density: 1.316 g/cm³
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Solubility: Slightly soluble in cold water (approx. 3.4 g/L at 25°C); highly soluble in organic solvents like alcohol, ether, chloroform, and acetone.
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Flash Point: 121 °C (closed cup)
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Vapor Pressure: 0.00083 mmHg at 25 °C
Chemical Properties:
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pKa: 4.19 - 4.20 (at 25 °C), making it a weak acid.
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Antimicrobial Activity: Most effective in its undissociated form at acidic pH (2.5–4.5).
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Stability: Stable under normal conditions, but when heated to decomposition, it emits acrid smoke and irritating fumes.
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Reactivity: Can undergo typical carboxylic acid reactions (e.g., esterification) and electrophilic aromatic substitution reactions on the benzene ring (e.g., halogenation, nitration).
Computational and Spectroscopic Properties:
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XLogP3: 1.9 (indicates moderate lipophilicity)
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Topological Polar Surface Area (TPSA): 37.3 Ų
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Collision Cross Section (CCS): ~122 Ų (for [M+H]+ ion in ion mobility spectrometry)
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Kovats Retention Index: Values provided for gas chromatography under standard non-polar, semi-standard non-polar, and standard polar conditions.
Safety and Handling
Hazards:
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GHS Pictograms: Corrosive (GHS05) and Health Hazard (GHS08).
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Primary Hazards: Potential for environmental damage if released. Can be corrosive and pose health hazards upon exposure.
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Decomposition: When heated, it decomposes to emit acrid smoke and irritating fumes.
Exposure and Handling:
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Handling: Immediate steps should be taken to limit environmental spread if released. Use appropriate personal protective equipment.
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Storage: Store in a cool, dry place.
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Stability/Shelf Life: A 0.1% w/v aqueous solution is stable for at least 8 weeks when stored in polyvinyl chloride bottles at room temperature.
Toxicology and Regulatory Status:
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Feed Additive: Approved for use in animal feed (e.g., up to 0.5% in complete swine feeds in the USA; registered in the EU and Brazil).
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General Safety: Generally recognized as safe at approved levels. High doses (≥2.5–5% of diet in animal studies) have been linked to adverse effects like growth retardation and organ injury. It contributes to metabolic acid load, so acid-base balance should be monitored in specific applications.