Salicylic acid

Salicylic acid

Salicylic acid
Skeletal formula of salicylic acid Ball-and-stick model of salicylic acid
Salicylic acid
CAS number  YesY
ChemSpider  YesY
EC number
RTECS number VO0525000
ATC code A01,B01 D01 N02 S01
Jmol-3D images Image 1
Molecular formula C7H6O3
Molar mass 138.12 g mol−1
Appearance colorless to white crystals
Odor odorless
Density 1.443 g/cm3 (20 °C)[1]
Melting point 158.6 °C (317.5 °F; 431.8 K)
Boiling point 200 °C (392 °F; 473 K)
211 °C (412 °F; 484 K)
at 20 mmHg[1]
Sublimation conditions sublimes at 76 °C[3]
Solubility in water 1.24 g/L (0 °C)
2.48 g/L (25 °C)
4.14 g/L (40 °C)
17.41 g/L (75 °C)[2]
77.79 g/L (100 °C)[4]
Solubility soluble in ether, CCl4, benzene, propanol, acetone, ethanol, oil of turpentine, toluene
Solubility in benzene 0.46 g/100 g (11.7 °C)
0.775 g/100 g (25 °C)
0.991 g/100 g (30.5 °C)
2.38 g/100 g (49.4 °C)
4.4 g/100 g (64.2 °C)[2][4]
Solubility in chloroform 2.22 g/100 mL (25 °C)[4]
2.31 g/100 mL (30.5 °C)[2]
Solubility in methanol 40.67 g/100 g (−3 °C)
62.48 g/100 g (21 °C)[2]
Solubility in olive oil 2.43 g/100 g (23 °C)[2]
Solubility in acetone 39.6 g/100 g (23 °C)[2]
log P 2.26
Vapor pressure 10.93 mPa[3]
Acidity (pKa) 1 = 2.97 (25 °C)[5]
2 = 13.82 (20 °C)[2]
λmax 210 nm, 234 nm, 303 nm (4 mg % in ethanol)[3]
Refractive index (nD) 1.565 (20 °C)[1]
Std enthalpy of
-589.9 kJ/mol
Std enthalpy of
3.025 MJ/mol[6]
GHS pictograms The corrosion pictogram in the Globally Harmonized System of Classification and Labelling of Chemicals (GHS)The exclamation-mark pictogram in the Globally Harmonized System of Classification and Labelling of Chemicals (GHS)[7]
GHS signal word Danger
GHS hazard statements H302, H318[7]
GHS precautionary statements P280, P305+351+338[7]
EU Index 200-712-3
EU classification Harmful Xn
R-phrases R22, R38, R41, R61
S-phrases S22, S26, S36, S37, S39
Eye hazard Severe irritation
Skin hazard Mild irritation
NFPA 704
Flash point 157 °C (315 °F; 430 K)
closed cup[3]
Autoignition temperature 540 °C (1,004 °F; 813 K)[3]
LD50 480 mg/kg (mice, oral)
Related compounds
Related compounds Methyl salicylate,
Benzoic acid,
Phenol, Aspirin,
4-Hydroxybenzoic acid,
Magnesium salicylate,
Choline salicylate,
Bismuth subsalicylate,
Sulfosalicylic acid
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
 YesY   YesY/N?)

Salicylic acid (from plant hormone. It is derived from the metabolism of salicin. In addition to being an important active metabolite of aspirin (acetylsalicylic acid), which acts in part as a prodrug to salicylic acid, it is probably best known for its use as a key ingredient in topical anti-acne products. The salts and esters of salicylic acid are known as salicylates.


  • Chemistry 1
  • Plant hormone 2
  • Production 3
  • History 4
  • Dietary sources 5
  • Medicinal and cosmetic uses 6
  • Mechanism of action 7
  • Other uses 8
  • Safety 9
  • See also 10
  • References 11
  • External links 12


Salicylic acid has the formula C6H4(OH)COOH, where the OH group is ortho to the carboxyl group. It is also known as 2-hydroxybenzoic acid. It is poorly soluble in water (2 g/L at 20 °C).[8] Aspirin (acetylsalicylic acid or ASA) can be prepared by the esterification of the phenolic hydroxyl group of salicylic acid with the acetyl group from acetic anhydride or acetyl chloride.

Plant hormone

Salicylic acid (SA) is a phenolic phytohormone and is found in plants with roles in plant growth and development, photosynthesis, transpiration, ion uptake and transport. SA also induces specific changes in leaf anatomy and chloroplast structure. SA is involved in endogenous signaling, mediating in plant defense against pathogens.[9] It plays a role in the resistance to pathogens by inducing the production of pathogenesis-related proteins.[10] It is involved in the systemic acquired resistance (SAR) in which a pathogenic attack on one part of the plant induces resistance in other parts. The signal can also move to nearby plants by salicylic acid being converted to the volatile ester, methyl salicylate.[11]


Salicylic acid is biosynthesized from the amino acid phenylalanine. In Arabidopsis thaliana it can also be synthesized via a phenylalanine-independent pathway.

Sodium salicylate is commercially prepared by treating sodium phenolate (the sodium salt of phenol) with carbon dioxide at high pressure (100 atm) and high temperature (390K) -a method known as the Kolbe-Schmitt reaction. Acidification of the product with sulfuric acid gives salicylic acid:

It can also be prepared by the hydrolysis of aspirin (acetylsalicylic acid)[12] or methyl salicylate (oil of wintergreen) with a strong acid or base.


White willow (Salix alba) is a natural source of salicylic acid

The Cherokee and other Native Americans used an infusion of the bark for fever and other medicinal purposes for centuries.[13] The medicinal part of the plant is the inner bark and was used as a pain reliever for a variety of ailments. In 2014, archaeologists identified traces of salicylic acid on 7th century pottery fragments found in east central Colorado.[14] The Reverend Edward Stone, a vicar from Chipping Norton, Oxfordshire, England, noted in 1763 that the bark of the willow was effective in reducing a fever.[15]

The active extract of the bark, called salicin, after the Latin name for the white willow (Salix alba), was isolated and named by the German chemist Johann Andreas Buchner in 1828.[16] A larger amount of the substance was isolated in 1829 by Henri Leroux, a French pharmacist.[17] Raffaele Piria, an Italian chemist, was able to convert the substance into a sugar and a second component, which on oxidation becomes salicylic acid.[18][19]

Salicylic acid was also isolated from the herb meadowsweet (Filipendula ulmaria, formerly classified as Spiraea ulmaria) by German researchers in 1839.[20] While their extract was somewhat effective, it also caused digestive problems such as gastric irritation, bleeding, diarrhea, and even death when consumed in high doses.

Dietary sources

Unripe fruits and vegetables are natural sources of salicylic acid, particularly blackberries, blueberries, cantaloupes, dates, grapes, kiwi fruits, guavas, apricots, green pepper, olives, tomatoes, radish and chicory; also mushrooms. Some herbs and spices contain quite high amounts, while meat, poultry, fish, eggs and dairy products all have little to no salicylates.[21] Of the legumes, seeds, nuts, and cereals, only almonds, water chestnuts and peanuts have significant amounts.[22]

Medicinal and cosmetic uses

Salicylic acid is known for its ability to ease aches and pains and reduce fevers.[23] These medicinal properties, particularly fever relief, have been known since ancient times, and it is used as an anti-inflammatory drug.[23]

In modern medicine, salicylic acid and its derivatives are used as constituents of some rubefacient products. For example, methyl salicylate is used as a liniment to soothe joint and muscle pain, and choline salicylate is used topically to relieve the pain of mouth ulcers.

Cotton pads soaked in salicylic acid can be used to chemically exfoliate skin

As with other hydroxy acids, salicylic acid is a key ingredient in many skin-care products for the treatment of seborrhoeic dermatitis, acne, psoriasis, calluses, corns, keratosis pilaris, acanthosis nigricans, ichthyosis, and warts.[23] The standard treatment for calluses is a 6% aspirin suspension in petroleum jelly, applied on the callus for one hour and then removed with washing. Salicylic acid works as a keratolytic, comedolytic, and bacteriostatic agent, causing the cells of the epidermis to shed more readily, opening clogged pores and neutralizing bacteria within, preventing pores from clogging up again by constricting pore diameter, and allowing room for new cell growth.[23][24][25] Because of its effect on skin cells, salicylic acid is used in several shampoos to treat dandruff. Use of concentrated solutions of salicylic acid may cause hyperpigmentation on unpretreated skin for those with darker skin types (Fitzpatrick phototypes IV, V, VI), as well as with the lack of use of a broad spectrum sunblock.[26][27]

Bismuth subsalicylate, a salt of bismuth and salicylic acid, is the active ingredient in stomach relief aids such as Pepto-Bismol, is the main ingredient of Kaopectate, and “displays anti-inflammatory action (due to salicylic acid) and also acts as an antacid and mild antibiotic.”[28]

A 2004 article in New Scientist discussing the controversial idea of treating salicylates as micronutrients, “akin to vitamins and antioxidants,” suggested that “perhaps in future we might even call salicylate ‘vitamin S’.”[29][30]

Mechanism of action

Salicylic acid has been shown to work through several different pathways. It produces its anti-inflammatory effects via suppressing the activity of cyclooxygenase (COX), an enzyme that is responsible for the production of pro-inflammatory mediators such as the prostaglandins. It does this not by direct inhibition of COX like most other non-steroidal anti-inflammatory drugs (NSAIDs) but instead by suppression of the expression of the enzyme (via a yet-unelucidated mechanism).[31] Salicylic acid has also been shown to activate adenosine monophosphate-activated protein kinase (AMPK), and it is thought that this action may play a role in the anticancer effects of the compound and its prodrugs aspirin and salsalate. In addition, the antidiabetic effects of salicylic acid are likely mediated by AMPK activation primarily through allosteric conformational change that increases levels of phosphorylation.[32] Salicylic acid also uncouples oxidative phosphorylation, which leads to increased ADP:ATP and AMP:ATP ratios in the cell. As a consequence, salicylic acid may alter AMPK activity and subsequently exert its anti-diabetic properties through altered energy status of the cell. Even in AMPK knock-out mice, however, there is an anti-diabetic effect, demonstrating that there is at least one additional, yet-unidentified action of the compound.[33]

Other uses

Although toxic in large quantities, salicylic acid is used as a food preservative, a bactericidal, and an antiseptic.[34]

Sodium salicylate is a useful phosphor in the vacuum ultraviolet, with nearly flat quantum efficiency for wavelengths between 10 to 100 nm.[35] It fluoresces in the blue at 420 nm. It is easily prepared on a clean surface by spraying a saturated solution of the salt in methanol followed by evaporation.


As a topical agent, and as a beta-hydroxy acid (and unlike alpha-hydroxy acids), salicylic acid is capable of penetrating and breaking down fats and lipids, making it capable of causing moderate chemical burns of the skin if at very high concentrations. It is capable of damaging the lining of pores in such cases if the solvent is alcohol, acetone, or an oil. Over-the-counter limits are set at 2% for topical left on the face and 3% for those expected to be washed off, such as acne cleansers or shampoo. Caution should be exercised when handling large volumes of salicylic acid, and protective gloves are recommended for any repeat, prolonged exposure. 17% and 27% salicylic acid, which is often sold for wart removal, should not be applied to the face and should not be used for acne treatment. Even for wart removal, such a solution should be applied only twice a day – more frequent use may lead to an increase in side-effects without an increase in efficacy.[36]

When ingested, salicylic acid has a possible ototoxic effect by inhibiting prestin.[37] It can induce transient hearing loss in zinc-deficient individuals. This finding is based on clinical studies with rats. An injection of salicylic acid induced hearing loss in zinc-deficient rats, while a simultaneous injection of zinc reversed the hearing loss. An injection of magnesium in the zinc-deficient rats did not reverse the salicylic acid-induced hearing loss.

There are no studies specifically looking at topical salicylic acid in pregnancy. Oral salicylic acid has not been associated with an increase in malformations if used during the first trimester, but use in late pregnancy has been associated with bleeding, especially intracranial bleeding.[38] The risks of aspirin late in pregnancy are probably not relevant for a topical exposure to salicylic acid, even late in the pregnancy, because of its low systemic levels. Topical salicylic acid is common in many over-the-counter dermatological agents, and the lack of adverse reports suggests a low teratogenic potential.[39]

Salicylic acid overdose can lead to salicylate intoxication, which often presents clinically in a state of metabolic acidosis with compensatory respiratory alkalosis. In patients presenting with an acute overdose, a 16% morbidity rate and a 1% mortality rate are observed.[40]

Some people are hypersensitive to salicylic acid and related compounds.

The United States Food and Drug Administration (FDA) recommends the use of sun protection when using skincare products containing salicylic acid (or any other BHA) on sun-exposed skin areas.[41]

There is data that supports an association between exposure to salicylic acid and Reye's Syndrome. The National Reye's Syndrome Foundation cautions against the usage of these substances, and other substances similar to aspirin, on children and adolescents.

Epidemiological research has shown an association between the development of Reye's Syndrome and the use of aspirin (a salicylate compound) for treating the symptoms of influenza-like illnesses, chicken pox, colds, etc.

The U.S. Surgeon General, the FDA, the Centers for Disease Control and Prevention, and the American Academy of Pediatrics recommend that aspirin and combination products containing aspirin not be given to children under 19 years of age during episodes of fever-causing illnesses, because of a concern about Reye's Syndrome.[42]

See also


  1. ^ a b c Haynes, William M., ed. (2011).  
  2. ^ a b c d e f g h
  3. ^ a b c d e CID 338 from PubChem
  4. ^ a b c Seidell, Atherton; Linke, William F. (1952). [Google Books Solubilities of Inorganic and Organic Compounds]. Van Nostrand. Retrieved 2014-05-29. 
  5. ^ Salicyclic acid. Retrieved on 2012-06-03.
  6. ^
  7. ^ a b c Sigma-Aldrich Co., Salicylic acid. Retrieved on 2014-05-23.
  8. ^ "Salicilyc acid". Retrieved 2008-10-13. 
  9. ^ Hayat, S. and Ahmad, A. (2007). Salicylic acid – A Plant Hormone.  
  10. ^ Van Huijsduijnen, R. A. M. H.; Alblas, S. W.; De Rijk, R. H.; Bol, J. F. (1986). "Induction by Salicylic Acid of Pathogenesis-related Proteins and Resistance to Alfalfa Mosaic Virus Infection in Various Plant Species". Journal of General Virology 67 (10): 2135.  
  11. ^ Taiz, L. and Zeiger, Eduardo (2002) Plant Physiology, 3rd Edition, Sinauer Associates, p. 306, ISBN 0878938230.
  12. ^ "Hydrolysis of ASA to SA". Retrieved July 31, 2007. 
  13. ^ Hemel, Paul B. and Chiltoskey, Mary U. Cherokee Plants and Their Uses – A 400 Year History, Sylva, NC: Herald Publishing Co. (1975); cited in Dan Moerman, A Database of Foods, Drugs, Dyes and Fibers of Native American Peoples, Derived from Plants. [1] A search of this database for "salix AND medicine" finds 63 entries.
  14. ^
  15. ^ Stone, Edmund (1763). "An Account of the Success of the Bark of the Willow in the Cure of Agues". Philosophical Transactions of the Royal Society of London 53: 195–200.  
  16. ^ A. Buchner (1828) "Ueber das Rigatellische Fiebermittel und über eine in der Weidenrinde entdeckte alcaloidische Substanz" (On Rigatelli's antipyretic [i.e., anti-fever drug] and on an alkaloid substance discovered in willow bark), Repertorium für die Pharmacie, 29: 405–420. From page 419: "Noch ist es mir aber nicht geglückt, den bittern Bestandtheil der Weide, den ich Salicin nennen will, ganz frei von allem Färbestoff darzustellen." (I have still not succeeded in preparing the bitter component of willow, which I will name salicin, completely free from colored matter.)
  17. ^ See:
    • (Leroux, H.) (1830) "Mémoire relatif à l'analyse de l'écorce de saule et à la découverte d'un principe immédiat propre à remplacer le sulfate de quinine" (Memoir concerning the analysis of willow bark and the discovery of a substance immediately likely to replace quinine sulfate), Journal de chimie médicale, de pharmacie et de toxicologie, 6: 340–342.
    • A report on Leroux's presentation to the French Academy of Sciences also appeared in: Histoire de Mémoires de l'Académie des sciences de l'Institut de France. Analyse des travaux de Mémoires de l'Académie des sciences pendant l'année 1830 (History of the Memoirs of the Academy of Sciences of the Institute of France. Analysis of the works of the Memoirs of the Academy of Sciences during the year 1830) in: Mémoires de l'Académie des sciences de l'Institut de France, 16: xx–xxi (1838).
  18. ^ Piria (1838) "Sur de neuveaux produits extraits de la salicine" (On new products extracted from salicine), Comptes rendus6: 620–624. On page 622, Piria mentions "Hydrure de salicyle" (hydrogen salicylate, i.e., salicylic acid).
  19. ^ Jeffreys, Diarmuid (2005). Aspirin: the remarkable story of a wonder drug. New York: Bloomsbury. pp. 38–40.  
  20. ^ C. Löwig & S. Weidmann (1839) "Beiträge zur organischen Chemie" (Contributions to organic chemistry) Annalen der Physik und Chemie, 46: 45–91; see pages 57–83: "Spiraea Ulmaria"III. Untersuchungen mit dem destillierten Wasser der Blüthen von (III. Investigations of the water distilled from the blossoms of Spiraea ulmaria). Löwig and Weidman called salicylic acid Spiräasaure (spiraea acid).
  21. ^ "Salicylates in foods". Retrieved 2014-05-09. 
  22. ^ What is Salicylic acid and in which foods does it occur?. Retrieved on 2012-06-03.
  23. ^ a b c d Madan RK, Levitt J (April 2014). "A review of toxicity from topical salicylic acid preparations". J Am Acad Dermatol 70 (4): 788–92.  
  24. ^ "Salicylic Acid."
  25. ^ Bosund, I., I. Erichsen, and N. Molin. "The Bacteriostatic Action of Benzoic and Salicylic Acids.. VI. Influence of Amino Acids and Related Substances on the Growth Inhibition." Physiologia Plantarum 13.4 (1960): 800-11.
  26. ^ Grimes, P. E. (1999). "The Safety and Efficacy of Salicylic Acid Chemical Peels in Darker Racial-ethnic Groups". Dermatologic Surgery 25 (1): 18–22.  
  27. ^ Roberts, W. E. (2004). "Chemical peeling in ethnic/dark skin". Dermatologic Therapy 17 (2): 196–205.  
  28. ^ "Bismuth subsalicylate". PubChem. United States  
  29. ^ Morgan, G. P. (7 February 2004). "An aspirin a day...".  
  30. ^ McMullen, M. (2005). "Aspirin for everyone over 50?: Are we treating a nutritional deficiency?".  
  31. ^ "Anti-Inflammatory Activity of Aspirin – It's All About Salicylic Acid". American Chemical Society. 
  32. ^ Hawley, S. A.; Fullerton, M. D.; Ross, F. A.; Schertzer, J. D.; Chevtzoff, C.; Walker, K. J.; Peggie, M. W.; Zibrova, D. et al. (2012). "The Ancient Drug Salicylate Directly Activates AMP-Activated Protein Kinase". Science 336 (6083): 918–22.  
  33. ^ Raffensperger, Lisa (2012-04-19). "Clues to aspirin's anti-cancer effects revealed". New Scientist
  34. ^ "Definition of Salicylic acid". 
  35. ^ Samson, James (1976). Techniques of Vacuum Ultraviolet Spectroscopy. Wiley, .
  36. ^ salicylic acid 17 % Topical Liquid. Kaiser Permanente Drug Encyclopedia. Accessed 28 Sept 2011.
  37. ^ Wecker, H.; Laubert, A. (2004). "Reversible hearing loss in acute salicylate intoxication".  
  38. ^ Rumack, CM; Guggenheim, MA; Rumack, BH; Peterson, RG; Johnson, ML; Braithwaite, WR (1981). "Neonatal intracranial hemorrhage and maternal use of aspirin". Obstetrics and gynecology 58 (5 Suppl): 52S–6S.  
  39. ^ Acne and Pregnancy. Retrieved on 2012-06-03.
  40. ^ Salicylate Toxicity. Retrieved on 2012-06-03.
  41. ^ "Beta Hydroxy Acids in Cosmetics". Archived from the original on 2007-12-21. Retrieved 2007-11-23. 
  42. ^ "Aspirin / Salicylates and Reye's Syndrome". Retrieved 2009-05-22. 

External links

  • Salicylic acid MS Spectrum
  • Safety MSDS data
  • International Chemical Safety Cards | CDC/NIOSH