Salicylic acid

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Salicylic acid
Sali22.jpg
Hair-like crystals of salicylic acid in the bottom of a beaker. (Click to see up close)
Salicylic Acid structure.jpg
Names
IUPAC name
2-Hydroxybenzoic acid
Other names
2-Carboxyphenol
O-Carboxyphenol
O-hydroxybenzoic acid
Properties
C7H6O3
Molar mass 138.12 g/mol
Appearance Colorless crystalline solid
Odor Odorless
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)
0.124 g/100 ml (0 °C)
0.248 g/100 ml (25 °C)
0.414 g/100 ml (40 °C)
1.741 g/100 ml (75 °C)
7.779 g/100 ml (100 °C)
Solubility Soluble in acetone, chloroform, diethyl ether, ethanol, methanol, propanol, oil of turpentine
Slightly soluble in benzene, CCl4, toluene
Solubility in acetone 39.6 g/100 g (23 °C)
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)
Solubility in chloroform 2.22 g/100 ml (25 °C)
2.31 g/100 ml (30.5 °C)
Solubility in methanol 40.67 g/100 g (−3 °C)
62.48 g/100 g (21 °C)
Solubility in olive oil 2.43 g/100 g (23 °C)
Vapor pressure 8.2·10-5 mmHg (25 °C)
Acidity (pKa) 1 = 2.97 (25 °C)
2 = 13.82 (20 °C)
Thermochemistry
-589.9 kJ/mol
Hazards
Safety data sheet Sigma-Aldrich
Flash point 157 °C (315 °F; 430 K) (closed cup)
Lethal dose or concentration (LD, LC):
480 mg/kg (mice, oral)
Related compounds
Related compounds
Acetylsalicylic acid
Phenol
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

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

Decarboxylation of salicylic acid via heating will yield phenol.

C6H4(OH)COOH → C6H5OH + CO2

Esterification with acetic anhydride gives aspirin.

C6H4(OH)COOH + (CH3O)2O → C6H4(CH3OCO)COOH + CH3COOH

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 slightly lower in nonpolar solvents, such as benzene and toluene, has low solubility in cold to warm water, and high solubility in boiling water.

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.

It can also be found in food stores as preservative, usually as a ester or salt, or sometimes as free acid. Sodium salicylate is more commonly available. The obtain the free acid, simply add a stronger acid to the sodium salt and purify the compound.

Salicylic acid can be extracted from willow tree bark, by boiling said bark in water, precipitate the impurities, filter, then hydrolyze the resulting salicin to obtain crude salicylic acid. The crude salicylic acid is dissolved in hot water and recrystallized from the solution. Repeated recrystallizations may be needed to remove impurities, namely tannins. This is how salicylic acid was historically prepared.

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.

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.

Projects

Handling

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.

Storage

No special storage is required, salicylic acid can be stored in any clean container.

Disposal

Salicylic acid can be safely poured down the drain or dumped in the trash, as it does not pose any danger for the environment.

Gallery

References

Relevant Sciencemandess threads