Salicylic acid (from Latin salix, willow tree) is a lipophilic monohydroxybenzoic acid, a type of phenolic acid, and a beta hydroxy acid (BHA). It has the formula C7H6O3. This colorless crystalline organic acid is widely used in organic synthesis and functions as a plant hormone. It is derived from the metabolism of salicin. In addition to serving as 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.
|Preferred IUPAC name|
3D model (JSmol)
CompTox Dashboard (EPA)
|Molar mass||138.122 g·mol−1|
|Appearance||Colorless to white crystals|
|Density||1.443 g/cm3 (20 °C)|
|Melting point||158.6 °C (317.5 °F; 431.8 K)|
|Boiling point|| 200 °C (392 °F; 473 K) decomposes|
211 °C (412 °F; 484 K)
at 20 mmHg
|Sublimes at 76 °C|
|Solubility||Soluble in ether, CCl4, benzene, propanol, acetone, ethanol, oil of turpentine, toluene|
|Solubility in benzene|
|Solubility in chloroform|
|Solubility in methanol|
|Solubility in olive oil||2.43 g/100 g (23 °C)|
|Solubility in acetone||39.6 g/100 g (23 °C)|
|Vapor pressure||10.93 mPa|
|UV-vis (λmax)||210 nm, 234 nm, 303 nm (4 mg % in ethanol)|
Refractive index (nD)
|1.565 (20 °C)|
Std enthalpy of
Std enthalpy of
|A01AD05 (WHO) B01AC06 (WHO) D01AE12 (WHO) N02BA01 (WHO) S01BC08 (WHO)|
|Safety data sheet||MSDS|
|GHS Signal word||Danger|
|Eye hazard||Severe irritation|
|Skin hazard||Mild irritation|
|NFPA 704 (fire diamond)|
|Flash point|| 157 °C (315 °F; 430 K) |
|540 °C (1,004 °F; 813 K)|
|Lethal dose or concentration (LD, LC):|
LD50 (median dose)
|480 mg/kg (mice, oral)|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Similar to other hydroxy acids, salicylic acid is a key ingredient in many skincare products for the treatment of seborrhoeic dermatitis, acne, psoriasis, calluses, corns, keratosis pilaris, acanthosis nigricans, ichthyosis and warts.
Uses in manufacturing
Salicylic acid was one of the original starting materials for making acetylsalicylic acid (aspirin) in 1897.
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".
Sodium salicylate is a useful phosphor in the vacuum ultraviolet spectral range, with nearly flat quantum efficiency for wavelengths between 10 and 100 nm. 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.
Mechanism of action
Salicylic acid modulates COX2 gene expression to decrease the formation of pro-inflammatory prostaglandins. Salicylate may competitively inhibit prostaglandin formation. Salicylate's antirheumatic (nonsteroidal anti-inflammatory) actions are a result of its analgesic and anti-inflammatory mechanisms. Salicylic acid works by causing the cells of the epidermis to slough off more readily, preventing pores from clogging up, and allowing room for new cell growth. Salicylic acid inhibits the oxidation of uridine-5-diphosphoglucose (UDPG) competitively with nicotinamide adenosine dinucleotide (NAD) and noncompetitively with UDPG. It also competitively inhibits the transferring of glucuronyl group of uridine-5-phosphoglucuronic acid (UDPGA) to the phenolic acceptor. The wound-healing retardation action of salicylates is probably due mainly to its inhibitory action on mucopolysaccharide synthesis.
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, causing moderate chemical burns of the skin at very high concentrations. It may damage the lining of pores if the solvent is alcohol, acetone or an oil. Over-the-counter limits are set at 2% for topical preparations expected to be left on the face and 3% for those expected to be washed off, such as acne cleansers or shampoo; up to a 6% concentration can be used if necessary under prescription for these uses.
17% to 27% salicylic acid used in the form of a paint, and 20% to 50% in plaster form, which are sold for wart and corn 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 once or twice a day – more frequent use may lead to an increase in side-effects without an increase in efficacy.
Some people are hypersensitive to salicylic acid and related compounds.
If high concentrations of salicylic ointment are applied to a large percentage of body surface, high levels of salicylic acid can enter the blood, requiring hemodialysis to avoid further complications.
Chemistry and production
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 (390 K) – a method known as the Kolbe-Schmitt reaction. Acidification of the product with sulfuric acid gives salicylic acid:
Thermal decomposition of salicylic acid to phenol and carbon dioxide proceeds at 200 °C to 230 °C with no need for catalysts, due to the electron donating effect of the OH group. Since salicylic acid is easily made from over-the-counter acetylsalicylic acid in aspirin tablets, this is a common laboratory route to phenol among amateur chemists:
Hippocrates, Galen, Pliny the Elder and others knew that willow bark could ease pain and reduce fevers. It was used in Europe and China to treat these conditions. This remedy is mentioned in texts from ancient Egypt, Sumer and Assyria. The Cherokee and other Native Americans use an infusion of the bark for fever and other medicinal purposes.
In 2014, archaeologists identified traces of salicylic acid on 7th century pottery fragments found in east central Colorado. 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.
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. A larger amount of the substance was isolated in 1829 by Henri Leroux, a French pharmacist. Raffaele Piria, an Italian chemist, was able to convert the substance into a sugar and a second component, which on oxidation becomes salicylic acid.
Salicylic acid was also isolated from the herb meadowsweet (Filipendula ulmaria, formerly classified as Spiraea ulmaria) by German researchers in 1839. 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.
Salicylic acid occurs in plants as free salicylic acid and its carboxylated esters and phenolic glycosides. Several studies suggest that humans metabolize salicylic acid in measurable quantities from these plants. High-salicylate beverages and foods include beer, coffee, tea, numerous fruits and vegetables, sweet potato, nuts, and olive oil, among others. Meat, poultry, fish, eggs, dairy products, sugar, and breads and cereals have low salicylate content.
Some people with sensitivity to dietary salicylates may have symptoms of allergic reaction, such as bronchial asthma, rhinitis, gastrointestinal disorders, or diarrhea, and so may need to adopt a low-salicylate diet.
Salicylic acid is a phenolic phytohormone and is found in plants with roles in plant growth and development, photosynthesis, transpiration, ion uptake and transport. SA is involved in endogenous signaling, mediating in plant defense against pathogens. It plays a role in the resistance to pathogens by inducing the production of pathogenesis-related proteins. It is involved in the systemic acquired resistance 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.
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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
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