Salicylic acid is a mildly acidic organic molecule of relatively small size, which possesses both phenol and carboxylic acid functional groups. Salicylic acid is the active portion of the larger molecule salicin, which can be isolated from plants including white willow (Salix alba). It has some application as an analgesic and anti-inflammatory. It is perhaps best known, however, as the synthetic starting material for the popular drug, aspirin.
History
In their book "Napoleon's Buttons," authors Penny Le Couteur and Jay Burreson note that salicin has been used for thousands of years as an analgesic; the Greeks, for instance, used willow bark tea to reduce pain, inflammation and swelling. In the late 19th century, the synthetic chemist Felix Hoffman isolated the molecule responsible for the tea's curative properties---salicylic acid---and proceeded to investigate its potential as a medicinal agent.
Synthesis of Aspirin
According to Le Couteur and Burreson, Hoffman's experiments with salicylic acid revealed it to be a more powerful analgesic drug than the parent tea, but it also led to stomach irritation and ulcers. Hoffman produced a derivative of salicylic acid, working from the principle that it was, in all likelihood, the phenol group that was responsible for salicylic acid's irritating properties.
Aspirin Patent
While Hoffman's derivative of salicylic acid---marketed by the Bayer company as aspirin---proved to be a valuable pharmaceutical, its popularity soon outstripped the supply, due to the difficulty associated with gathering salicin-producing plants and isolating salicylic acid. Instead, a United States branch of the Bayer company developed and patented a synthesis from a simpler, non-biologically derived molecule called, simply, "phenol."
Industrial Aspirin Synthesis
Many modern aspirin syntheses, however, rely directly upon salicylic acid, which is itself easily synthesized from a variety of starting materials. According to United States Patent 3,235,583, aspirin is produced by heating salicylic acid with acetic anhydride, which is a compound very closely related to acetic acid, the molecule responsible for the smell and properties of vinegar. The resulting product is distilled and dried, producing aspirin of sufficient purity to suit pharmaceutical purposes.
Laboratory Aspirin Synthesis
The synthesis of aspirin from salicylic acid is so simple---truly, it's a reaction that takes place in a single step with remarkably high yield---that it's often utilized as a teaching tool in university-level organic chemistry courses. The lab manual "Introduction to Organic Laboratory Techniques," which is designed to serve as an instructional aid in sophomore and junior college courses, includes a preparation of aspirin from salicylic acid very similar to that used on the industrial level---salicylic acid is condensed with acetic anhydride, and the resulting compound isolated and purified.
References
- "Napoleon's Buttons"; Penny Le Couteur and Jay Burreson; 2003.
- "United States Patent 3,235,583"; Preparation of Aspirin; Robert Edmunds; 1964
- "Introduction to Organic Laboratory Techniques"; Donald Pavia et al.; 2007
