Salicylic acid

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Fine, needle-like crystals of salicylic acid produced through recrystallization.

Salicylic acid is the organic compound with the chemical formula C6H4(OH)COOH. It is an example of a phenolic acid. It is the active metabolite of aspirin and is also used in many other medications such as skin-care products.

Properties

Chemical

Physical

Salicylic acid is most often encountered as a fine, fluffy crystalline powder or as needle-like crystals which are difficult to compact. It has a somewhat minty and irritating odor. Salicylic acid is quite soluble in alcohols, acetone, ether, and nonpolar solvents such as benzene and toluene, has low solubility in cold to warm water, and high solubility in boiling water.

Preparation

Salicylic acid can be easily synthesized using either methyl salicylate or acetylsalicylic acid (aspirin) as the primary precursor, but because methyl salicylate may not be as cheaply or easily obtained as aspirin, it is usually synthesized from the latter, which is also typically seen as the easier process.

Production from aspirin requires the acetylsalicylic acid to be refluxed in the presence of a stronger acid(often hydrochloric acid) for a period of time, hydrolyzing it to salicylic acid and acetic acid. The product can then be washed and recrystallized.

A reflux setup for producing salicylic acid by Alexleyenda's process.

Salicylic acid can also be prepared from store-bought oil of wintergreen (methyl salicylate); a detailed write-up for this process can be found here.

Availability

Salicylic acid, while usually produced using aspirin as a precursor with ease, can also be purchased in solution as various skin-care products for treating warts or acne, those these can be both expensive and impure, as well as highly diluted.

Projects

Safety

Salicylic acid has the ability to break down lipids in the skin, causing symptoms ranging from dryness and irritation at low concentrations to mild acid burns at higher ones. When ingested orally in large amounts it can cause salicylate intoxication, which may produce very serious side effects.

Gallery

File:Salicylic acid recrystallized.JPG
Hair-like crystals of salicylic acid in the bottom of a beaker. (Click to see up close)