C₉H₈O₄ — Aspirin

Aspirin, chemically known as acetylsalicylic acid, is one of the most widely used and studied drugs in medical history. Its molecular formula is \(C_9H_8O_4\). Aspirin belongs to the class of nonsteroidal anti-inflammatory drugs (NSAIDs) and is primarily used for its analgesic (pain-relieving), antipyretic (fever-reducing), and anti-inflammatory properties. It also plays a key role in the prevention of blood clot formation, making it beneficial in reducing the risk of heart attacks and strokes.

The name 'aspirin' is derived from acetyl (referring to the acetyl group) and Spiraea (the plant genus that includes meadowsweet, a natural source of salicylic acid). Aspirin is often called the 'wonder drug' because of its wide range of therapeutic effects and its crucial role in both short-term and long-term medical treatments.

Aspirin is an ester derived from salicylic acid and acetic anhydride. The molecule consists of a benzene ring substituted with two functional groups: a carboxyl group (\(-COOH\)) and an ester group (\(-COOCH_3\)).

The structure of aspirin can be represented as:

Here, the acetyl group (\(-COCH_3\)) is attached to the hydroxyl group of salicylic acid, forming an ester linkage. This modification reduces the irritant effect of salicylic acid on the stomach lining, making aspirin a safer and more tolerable compound for oral administration.

The discovery of aspirin dates back to the 19th century. In 1897, Felix Hoffmann, a chemist working at Bayer in Germany, synthesized acetylsalicylic acid in a pure and stable form. Although salicylic acid had been used earlier for pain relief, its strong acidity caused stomach irritation. Hoffmann’s acetylated form was milder, effective, and soon became a cornerstone of modern medicine. Bayer patented the compound under the trade name 'Aspirin' in 1899.

Today, aspirin is available globally as both an over-the-counter (OTC) and prescription medication, serving as a foundation for many cardiovascular and pain-relief therapies.

Aspirin is synthesized through the esterification reaction between salicylic acid and acetic anhydride, with sulfuric acid or phosphoric acid acting as a catalyst. The reaction proceeds as follows:

In this reaction:

• Salicylic acid (C7H6O3) provides the phenolic hydroxyl group.
• Acetic anhydride ((CH3CO)2O) donates the acetyl group.
• Sulfuric acid (H2SO4) acts as a catalyst.

The resulting product, aspirin, is purified by recrystallization in water or ethanol. The by-product of the reaction is acetic acid.

Aspirin works by inhibiting cyclooxygenase (COX) enzymes, which are involved in the synthesis of prostaglandins — lipid compounds responsible for pain, inflammation, and fever. Aspirin irreversibly acetylates the COX enzyme, preventing the formation of prostaglandins and thromboxanes.

This inhibition reduces inflammation and pain signals, lowers fever, and decreases platelet aggregation (preventing clot formation). Thus, aspirin acts as an anti-inflammatory, analgesic, antipyretic, and antiplatelet agent.

• Pain Relief (Analgesic): Effective against mild to moderate pain such as headaches, toothaches, and muscle pain.
• Anti-inflammatory: Used to treat inflammatory conditions like arthritis and rheumatic fever.
• Antipyretic (Fever Reducer): Helps lower elevated body temperatures by acting on the hypothalamic heat-regulating center.
• Antiplatelet: Prevents blood clots by irreversibly inhibiting platelet aggregation, reducing the risk of heart attacks and strokes.

In low doses, aspirin is prescribed as a preventive measure for cardiovascular diseases, while in higher doses, it acts as an anti-inflammatory drug.

• Appearance: White crystalline solid with a slightly acidic taste.
• Stability: Aspirin is stable under dry conditions but hydrolyzes to salicylic acid and acetic acid in moisture.
• Reactivity: Hydrolyzed by water and bases; reacts with ethanol to form ethyl salicylate.
• Solubility: Sparingly soluble in cold water but highly soluble in alcohol and ether.
• Acidity: Weak acid with a pKa of 3.5, allowing partial ionization in water.

Although widely used, aspirin can cause side effects, especially with prolonged use or high doses:

• Gastrointestinal irritation: May cause stomach ulcers or bleeding due to decreased prostaglandin synthesis.
• Allergic reactions: In sensitive individuals, may cause asthma-like symptoms or rashes.
• Reye’s syndrome: A rare but serious condition affecting children recovering from viral infections — hence aspirin is not recommended for children under 12.
• Bleeding risks: Due to antiplatelet activity, it increases bleeding tendency during surgeries or injuries.

Therefore, aspirin should be used under medical supervision, particularly in individuals with gastrointestinal disorders or those taking anticoagulant therapy.

Apart from its medicinal use, aspirin serves as a reference standard in chemical and pharmacological studies. Industrially, it is used to demonstrate esterification and hydrolysis reactions in academic laboratories. Pharmaceutical formulations include tablets, capsules, and soluble powders, often combined with other drugs like paracetamol and caffeine for enhanced analgesic effects.