- Benzoic acid
Benzoic acid crystals
Identifiers CAS number PubChem ChemSpider UNII EC number DrugBank KEGG MeSH ChEBI ChEMBL RTECS number DG0875000 Beilstein Reference 636131 Gmelin Reference 2946 3DMet Jmol-3D images Image 1
Properties Molecular formula C7H6O2 Molar mass 122.12 g mol−1 Appearance Colorless crystalline solid Density 1.27 g/cm3 Melting point
122.38 °C, 396 K, 252 °F (source)
250 °C, 523 K, 482 °F ()
Solubility in water 2.9 g/L Acidity (pKa) 4.21 Refractive index (nD) 1.5397 Structure Crystal structure Monoclinic Molecular shape planar Dipole moment 1.72 D in Dioxane Hazards MSDS JT Baker EU Index Not listed Main hazards Irritating NFPA 704 Flash point 121 °C (250 °F) Autoignition
570 °C (1,058 °F) Related compounds Related carboxylic acids Hydroxybenzoic acids
Related compounds Benzaldehyde,
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Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Benzoic acid (pronunciation: //), C7H6O2 (or C6H5COOH), is a colorless crystalline solid and the simplest aromatic carboxylic acid. The name derived from gum benzoin, which was for a long time the only source for benzoic acid. Its salts are used as a food preservative and benzoic acid is an important precursor for the synthesis of many other organic substances. The salts and esters of benzoic acid are known as benzoates (pronunciation: //).
- 1 History
- 2 Production
- 3 Uses
- 4 Biology and health effects
- 5 Chemistry
- 6 References
- 7 Further reading
- 8 External links
Benzoic acid was discovered in the sixteenth century. The dry distillation of gum benzoin was first described by Nostradamus (1556), and subsequently by Alexius Pedemontanus (1560) and Blaise de Vigenère (1596).
Pioneer work in 1830 through a variety of experiences based on amygdalin, obtained from bitter almonds (the fruit of Prunus dulcis) oil by Pierre Robiquet and Antoine Boutron-Charlard, two French chemists, had produced benzaldehyde  but they failed in working out a proper interpretation of the structure of amygdalin that would account for it, and thus missed the identification of the benzoyl radical C7H5O. This last step was achieved some few months later (1832) by Justus von Liebig and Friedrich Wöhler, who determined the composition of benzoic acid. These latter also investigated how hippuric acid is related to benzoic acid.
Benzoic acid is produced commercially by partial oxidation of toluene with oxygen. The process is catalyzed by cobalt or manganese naphthenates. The process uses cheap raw materials, proceeds in high yield, and is considered environmentally green.
U.S. production capacity is estimated to be 126,000 tonnes per year (139,000 tons), much of which is consumed domestically to prepare other industrial chemicals.
Benzoic acid is cheap and readily available, so the laboratory synthesis of benzoic acid is mainly practiced for its pedagogical value. It is a common undergraduate preparation.
For all syntheses, benzoic acid can be purified by recrystallization from water because of its high solubility in hot water and poor solubility in cold water. The avoidance of organic solvents for the recrystallization makes this experiment particularly safe. Other possible recrystallization solvents include acetic acid (anhydrous or aqueous), benzene, acetone, petroleum ether, and a mixture of ethanol and water.
- C6H5MgBr + CO2 → C6H5CO2MgBr
- C6H5CO2MgBr + HCl → C6H5CO2H + MgBrCl
From benzyl alcohol
Benzyl alcohol is refluxed with potassium permanganate or other oxidizing reagents in water. The mixture is hot filtered to remove manganese dioxide and then allowed to cool to afford benzoic acid.
From benzyl chloride
Benzoic acid can be prepared by oxidation of benzyl chloride in the presence of alkaline KMnO4:
- C6H5CH2Cl + 2 KOH + 2 [O] → C6H5COOK + KCl + H2O
The first industrial process involved the reaction of benzotrichloride (trichloromethyl benzene) with calcium hydroxide in water, using iron or iron salts as catalyst. The resulting calcium benzoate is converted to benzoic acid with hydrochloric acid. The product contains significant amounts of chlorinated benzoic acid derivatives. For this reason, benzoic acid for human consumption was obtained by dry distillation of gum benzoin. Food-grade benzoic acid is now produced synthetically.
Benzoic acid is the most commonly used chemical standard to determine the heat of capacity of a bomb calorimeter.
Benzoic acid is used to make a large number of chemicals; important examples of which are:
- Benzoyl chloride, C6H5C(O)Cl, is obtained by treatment of benzoic with thionyl chloride, phosgene or one of the chlorides of phosphorus. C6H5C(O)Cl is an important starting material for several benzoic acid derivates like benzyl benzoate, which is used in artificial flavours and insect repellents.
- Benzoate plasticizers, such as the glycol-, diethylengylcol-, and triethyleneglycol esters, are obtained by transesterification of methyl benzoate with the corresponding diol. Alternatively these species arise by treatment of benzoylchloride with the diol. These plasticizers are used similarly to those derived from terephthalic acid ester.
- Phenol, C6H5OH, is obtained by oxidative decarboxylation at 300-400°C. The temperature required can be lowered to 200°C by the addition of catalytic amounts of copper(II) salts. The phenol can be converted to cyclohexanol, which is a starting material for nylon synthesis.
Benzoic acid and its salts are used as a food preservatives, represented by the E-numbers E210, E211, E212, and E213. Benzoic acid inhibits the growth of mold, yeast and some bacteria. It is either added directly or created from reactions with its sodium, potassium, or calcium salt. The mechanism starts with the absorption of benzoic acid in to the cell. If the intracellular pH changes to 5 or lower, the anaerobic fermentation of glucose through phosphofructokinase is decreased by 95%. The efficacy of benzoic acid and benzoate is thus dependent on the pH of the food. Acidic food and beverage like fruit juice (citric acid), sparkling drinks (carbon dioxide), soft drinks (phosphoric acid), pickles (vinegar) or other acidified food are preserved with benzoic acid and benzoates.
Typical levels of use for benzoic acid as a preservative in food are between 0.05–0.1%. Foods in which benzoic acid may be used and maximum levels for its application are laid down in international food law.
Benzoic acid is a constituent of Whitfield's ointment which is used for the treatment of fungal skin diseases such as tinea, ringworm, and athlete's foot. As the principal component of benzoin resin, benzoic acid is also a major ingredient in both tincture of benzoin and Friar's balsam. Such products have a long history of use as topical antiseptics and inhalant decongestants.
Biology and health effects
Benzoic acid occurs naturally free and bound as benzoic acid esters in many plant and animal species. Appreciable amounts have been found in most berries (around 0.05%). Ripe fruits of several Vaccinium species (e.g., cranberry, V. vitis idaea; bilberry, V. macrocarpon) contain as much as 0.03-0.13% free benzoic acid. Benzoic acid is also formed in apples after infection with the fungus Nectria galligena. Among animals, benzoic acid has been identified primarily in omnivorous or phytophageous species, e.g., in viscera and muscles of the Rock Ptarmigan (Lagopus muta) as well as in gland secretions of male muskoxen (Ovibos moschatus) or Asian bull elephants (Elephas maximus).
Gum benzoin contains up to 20% of benzoic acid and 40% benzoic acid esters.
Benzoic acid is present as part of hippuric acid (N-benzoylglycine) in urine of mammals, especially herbivores (Gr. hippos = horse; ouron = urine). Humans produce about 0.44 g/L hippuric acid per day in their urine, and if the person is exposed to toluene or benzoic acid it can rise above that level.
For humans, the World Health Organization's International Programme on Chemical Safety (IPCS) suggests a provisional tolerable intake would be 5 mg/kg body weight per day. Cats have a significantly lower tolerance against benzoic acid and its salts than rats and mice. Lethal dose for cats can be as low as 300 mg/kg body weight. The oral LD50 for rats is 3040 mg/kg, for mice it is 1940–2263 mg/kg.
In Taipei, Taiwan, a city health survey in 2010 found 30% of tested dried and pickled food products failed a test having too much benzoic acid, which is known to affect the liver and kidney, along with more serious issues like excessive cyclamate.
The second substitution reaction (on the right) is slower because the first nitro group is deactivating. Conversely, if an activating group (electron-donating) was introduced (e.g., alkyl), a second substitution reaction would occur more readily than the first and the disubstituted product might not accumulate to a significant extent.
All the reactions mentioned for carboxylic acids are also possible for benzoic acid.
- Benzoic acid esters are the product of the acid catalysed reaction with alcohols.
- Benzoic acid amides are more easily available by using activated acid derivatives (such as benzoyl chloride) or by coupling reagents used in peptide synthesis like DCC and DMAP.
- The more active benzoic anhydride is formed by dehydration using acetic anhydride or phosphorus pentoxide.
- Highly reactive acid derivatives such as acid halides are easily obtained by mixing with halogenation agents like phosphorus chlorides or thionyl chloride.
- Orthoesters can be obtained by the reaction of alcohols under acidic water free conditions with benzonitrile.
- Reduction to benzaldehyde and benzyl alcohol is possible using DIBAL-H, LiAlH4 or sodium borohydride.
- The copper catalysed decarboxylation of benzoate to benzene may be effected by heating in quinoline. Also, Hunsdiecker decarboxylation can be achieved by forming the silver salt and heating. Benzoic acid can also be decarboxylated by heating with an alkali hydroxide or calcium hydroxide.
- ^ a b c d e Record in the GESTIS Substance Database from the IFA
- ^ Melting point of benzoic acid
- ^ Neumüller O-A (1988). Römpps Chemie-Lexikon (6 ed.). Stuttgart: Frankh'sche Verlagshandlung. ISBN 3-440-04516-1. OCLC 50969944.
- ^ Nouvelles expériences sur les amandes amères et sur l'huile volatile qu'elles fournissent Robiquet, Boutron-Charlard, Annales de chimie et de physique, 44 (1830), 352–382,
- ^ Liebig J, Wöhler F (1832). "Untersuchungen über das Radikal der Benzoesäure". Annalen der Chemie, 3 (3): 249–282. doi:10.1002/jlac.18320030302.
- ^ Salkowski E (1875). Berl Klin Wochenschr 12: 297–298.
- ^ D. D. Perrin; W. L. F. Armarego (1988). Purification of Laboratory Chemicals (3rd ed.). Pergamon Press. pp. 94. ISBN 0-08-034715-0.
- ^ Donald L. Pavia (2004). Introduction to Organic Laboratory Techniques: A Small Scale Approach. Thomson Brooks/Cole. pp. 312–314. ISBN 0534408338.
- ^ Experiment 2: Using Bomb Calorimetry to Determine the Resonance Energy of Benzene
- ^ A D Warth (1 December 1991). "Mechanism of action of benzoic acid on Zygosaccharomyces bailii: effects on glycolytic metabolite levels, energy production, and intracellular pH". Appl Environ Microbiol. 1991 December 1 (12): 3410–4. PMC 183988. PMID 1785916. http://aem.asm.org/cgi/content/abstract/57/12/3410.
- ^ Pastrorova I, de Koster CG, Boom JJ (1997). "Analytic Study of Free and Ester Bound Benzoic and Cinnamic Acids of Gum Benzoin Resins by GC-MS HPLC-frit FAB-MS". Phytochem Anal 8 (2): 63–73. doi:10.1002/(SICI)1099-1565(199703)8:2<63::AID-PCA337>3.0.CO;2-Y.
- ^ GSFA Online Food Additive Group Details: Benzoates (2006)
- ^ EUROPEAN PARLIAMENT AND COUNCIL DIRECTIVE No 95/2/EC of 20 February 1995 on food additives other than colours and sweeteners (Consleg-versions do not contain the latest changes in a law)
- ^ BfR article Indications of the possible formation of benzene from benzoic acid in foods, BfR Expert Opinion No. 013/2006, 1 December 2005
- ^ Whitfield Ointment
- ^ Charles Owens Wilson; Ole Gisvold; John H. Block (2004). Wilson and Gisvold's Textbook of Organic Medicinal and Pharmaceutical. Lippincott Williams & Wilkins. pp. 234. ISBN 0781734819.
- ^ http://books.google.com/books?id=cs3mAAAAMAAJ&pg=PA25&lpg=PA25&dq=beef+extract+analgesic&source=bl&ots=dy2HVpaeRy&sig=Zvxh4LgsMYHOkl2dlHGXozPQxOY&hl=en&ei=g7-oTpmREuyDsgKd-qzODw&sa=X&oi=book_result&ct=result&resnum=6&ved=0CDIQ6AEwBQ#v=onepage&q=beef%20extract%20analgesic&f=false
- ^ a b c Concise International Chemical Assessment Document 26: BENZOIC ACID AND SODIUM BENZOATE
- ^ Tomokuni K, Ogata M (1972). "Direct Colorimetric Determination of Hippuric Acid in Urine". Clin Chem 18 (4): 349–351. PMID 5012256.
- ^ Krebs HA, Wiggins D, Stubbs M (1983). "Studies on the mechanism of the antifungal action of benzoate". Biochem J 214 (3): 657–663. PMC 1152300. PMID 6226283. http://www.biochemj.org/bj/214/0657/2140657.pdf.
- ^ Bedford PG, Clarke EG (1972). "Experimental benzoic acid poisoning in the cat". Vet Rec 90 (3): 53–58. doi:10.1136/vr.90.3.53. PMID 4672555.
- ^ Nearly 30% dried, pickled foods fail safety inspections, The China Post, January 18, 2010
- ^ Bromelain, mpbio.com
- ^ Brewster, R. Q.; Williams, B.; Phillips, R. (1955), "3,5-Dinitrobenzoic Acid", Org. Synth., http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=cv3p0337 ; Coll. Vol. 3: 337
- Cosmetic Ingredient Review Expert Panel Bindu Nair (2001). "Final Report on the Safety Assessment of Benzyl Alcohol, Benzoic Acid, and Sodium Benzoate". Int J Tox 20 (Suppl. 3): 23–50. doi:10.1080/10915810152630729. PMID 11766131.
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