|Name ||salicylic acid (Mr = 132.12) |
|Systematic name ||2-hydroxybenzoic acid |
|Formula ||C6H4(OH)CO2H |
|Melting point ||159°C (318°F) |
|Boiling point ||211°C (412°F) |
|Color ||white, crystalline |
|Structure || |
Salicylic acid is a colorless, crystalline organic carboxylic acid. It is usually prepared by Kolbe synthesis (aka Kolbe-Schmitt reaction), which works by heating sodium phenolate (the sodium salt of phenol) with carbon dioxide under pressure (100 atm, 125°C), then treating it with sulfuric acid.
Salicylic acid is also found in plants, especially in fruit, in the form of methyl salicylate. It functions as a plant hormone; see Salicylic acid (plant hormone).
It is toxic if digested 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 hydroxyl group (–OH) can react with acetic acid to form acetylsalicylic acid. Salicylic acid can also trap oxygen (O2) and initiate free radical reactions.
Salicylic acid is the active ingredient in many skin-care products for the treatment of acne and warts. It treats acne by causing skin cells to slough off more readily, preventing pores from clogging up.
The medicinal properties of salicylate (mainly the lowering of fever) have been known since ancient times. The substance occurs in the bark of willow trees and the name Salicylic acid is derived from the Latin name for the Willow tree - Salix.
Acetylsalicylic acid (Aspirin or ASA) can be prepared by the esterification of the phenolic hydroxyl group of salicylic acid.
"Vitamin S" is a proposed alternative name for salicylate, which would have the effect of classifying it as a vitamin. There is no generally agreed definition of what constitutes a vitamin, though salicylate meets at least one of the requirements:
- it is present in our natural diet in fruit and vegetables, particularly if they have had to defend themselves against damage or disease
- it isn't produced by our bodies
- trace amounts, it has been suggested, are required for the maintenance of life
This last point is controversial because salicylate deficiency, unlike, say, Vitamin C deficiency, which causes scurvy, does not result in any particular known symptoms. A low salicylate intake seems to be related to longer term problems, resulting in higher risks of age-related chronic diseases.
Whereas most vitamins are enzyme cofactors, promoting cellular biochemical reactions - Vitamin C boosts production of collagen - salicylate is not, but then nor is Vitamin E, which is an antioxidant, like salicylate.
Lack of salicylate appears to predispose humans to:
It can be argued that salicylate has a better claim to be called a vitamin than vitamins A and D.
Whether or not you choose to call aspirin a vitamin, there is still a case to be made that salicylate is an important micronutrient.
There is evidence that salicylates in past ages were much more common in human diets than they are now. Salicylates are produced by fruit as a defense mechanism: inducing damaged and diseased cells to commit suicide. Modern man's predilection for fruit and vegetables in a pristine condition - with shoppers often rejecting fruit with bumps of bruises - means that we eat less salicylates than in the past. A study has shown that organic vegetable soups contain nearly six times as much salicylate as non-organic equivalents (European Journal of Nutrition, vol. 40 p 289).