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Page: Contraindications and Warnings
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* Aspirin should be avoided by those known to be allergic to ibuprofen or naproxen.
* Caution should be exercised in those with asthma or NSAID-precipitated bronchospasm.
* It is generally recommended that one seek medical help if symptoms do not improve after a few days of therapy.
* Caution should be taken in patients with kidney disease, peptic ulcers, mild diabetes, gout or gastritis; manufacturers recommend talking to one's doctor before using this medicine.
* Taking aspirin with alcohol increases the chance of gastrointestinal hemorrhage (stomach bleeding).
* Children, including teenagers, are discouraged from using aspirin in cold or flu symptoms as this has been linked with Reye's syndrome.
* Patients with hemophilia or other bleeding tendencies should not take salicylates.
* Some sources recommend that patients with hyperthyroidism avoid aspirin because it elevates T4 levels.
Aspirin
Aspirin is commercially synthesized using a two-step process. First, phenol (generally extracted from coal tar) is treated with a sodium base which generates sodium phenoxide, which is then reacted with carbon dioxide under high temperature and pressure to yield salicylate, which is acidified, yielding salicylic acid. This process is known as the Kolbe-Schmitt reaction.
Salicylic acid is then acetylated using acetic anhydride, yielding aspirin and acetic acid as a byproduct. It is a common experiment performed in undergraduate organic chemistry practicals, and generally tends to produce low yields due to the relative difficulty of its extraction from an aqueous state. The trick to getting the reaction to work is to acidify with phosphoric acid and heat the reagents under reflux with a boiling water bath for between 40 to 60 minutes.
The original synthesis of aspirin from salicylic acid involved acetylation with acetyl chloride. Unfortunately, the byproduct from this is hydrochloric acid, which is corrosive and environmentally hazardous. As described above, it was then later found that acetic anhydride was a better acylating agent, with the byproduct acetic acid formed, which does not have the unwanted properties of hydrochloric acid and can also be recycled. The salicylic acid/acetic anhydride method is commonly employed in undergraduate teaching labs.
Formulations containing high concentrations of aspirin often smell like vinegar. This is because aspirin can undergo autocatalytic degradation to salicylic acid in moist conditions, yielding salicylic acid and acetic acid.
The acid dissociation constant (pKa) for Acetylsalicylic acid is 3.5 at 25 °C. ASA, being a weak acid, dissociates as shown by the following reaction equation:
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Important notice:
The content is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other
qualified health provider with any questions you may have regarding a medical condition.
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