Salicylic acid


Chemical name	2-Hydroxybenzoic acid
Chemical formula	C₇H₆O₃
Molecular mass	138.12 g/mol
Melting point	159 °C
Boiling point	211 °C (2666 Pa)
Density	1.44 g/cm³ (at 20 °C)
pKa	2.97
CAS number	^*
SMILES	c1(O)ccccc1C(O)=O
Chemical infobox

Salicylic acid is a colorless, crystalline organic carboxylic acid. It is usually prepared by Kolbe synthesis (aka Kolbe-Schmitt reaction). Salicylic acid functions as a plant hormone.

Properties

Salicylic acid is toxic if ingested in large quantities, but in small quantities is used as a food preservative and antiseptic in toothpaste. The carboxyl group (–COOH) can react with alcohols, forming several useful esters. The ester with methanol is methyl salicylate, also known by the name oil of wintergreen. The hydroxyl group (–OH) can be acetylated to form acetylsalicylic acid (aspirin). Salicylic acid can also trap oxygen (O₂) and initiate free radical reactions.

Medicinal and cosmetic uses

Also known as Beta Hydroxy Acid (compare to AHA), salicylic acid is the key additive in many skin-care products for the treatment of acne, psoriasis, callouses, corns, keratosis pilaris and warts. It treats acne by causing skin cells to slough off more readily, preventing pores from clogging up. This effect on skin cells also makes salicylic acid an active ingredient in several shampoos meant to treat dandruff. Use of straight salicylic solution 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.

The medicinal properties of salicylate (mainly for fever relief) have been known since ancient times. The substance occurs in the bark of willow trees; the name salicylic acid is derived from salix, the Latin name for the willow tree.

Aspirin (acetylsalicylic acid or ASA) can be prepared by the esterification of the phenolic hydroxyl group of salicylic acid.

Subsalicylate in combination with bismuth form the popular stomach relief aid known commonly as Pepto-Bismol. When combined the two key ingredients help control diarrhea, nausea, heartburn, and even gas. It is also very mildly anti-biotic.

Salicylic acid as a plant hormone

Here is a list of some of salicylic acid's properties when acting as a plant hormone. It is interesting to note, in light of these properties, that there is a popular belief that cut flowers will last longer if aspirin (acetylsalicylic acid) is dissolved in their water.

Location, characteristics and occasions for synthesis induction

Salicylic Acid is the primary immune hormone in plants
Cells returning from water stress
Released by cells secure in having more than enough nutrients and environmental conditions locally to survive at its current metabolic level (speculative)
All cells capable of synthesizing
Has its effect or acts by rapid local increases followed by rapid decreases in levels

Effects

Triggers systemic immune response
Acts as a mild antibiotic
Inhibits genes necessary for pathogenesis in some soil microbes, such as P. aeruginosa.
Retards senescence (regulatory role) – probably by inhibiting Ethylene biosynthesis
Induces flowering
Inhibits seed germination – by inhibiting ABA synthesis
May also block the wound response and act antagonistically to ABA, preventing the wound response from spreading further than necessary (speculative)
After a survival threat has passed SA quickly removes a plant, organ, tissue or cell from a defensive posture and returns it to normal "relaxed" functioning (speculative)
Increases cell metabolism rate to take advantage of new complete more advantageous nutrient and environmental conditions (speculative)
A climactic or sustained level of SA may occur if a cell has reached its peak metabolic levels and may signal that a plant's resources can be turned to growth (speculative)
This climactic or sustained level of SA may be a prerequisite for the synthesis of Auxin and/or Cytokinin, because only then does a plant know that it has enough resources to turn them to growing bigger (highly speculative)

References

Carboxylic acids | Phenols | Plant hormones | Antiseptics | Over-the-counter substances