The Synthesis of Aspirin
Pharmaceutical research labs perform chemical reactions between organic molecules that contain mostly carbon. Since the atoms in organic molecules tend to be nonmetals, then the bonds that hold them together are covalent bonds. Only a few elements on the periodic table are nonmetals: C, N, O, S, Cl, and F. This small number of elements can be bonded together in different quantities, bonding types (single, double, triple bonds), and structural patterns to form over 10 million known molecules! The billions of dollars generated by the pharmaceutical industry is a prime example of how important organic chemistry is to our healthcare industry. In this lab, you will perform an organic synthesis to make aspirin. Aspirin is the trade name for the molecule acetylsalcylic acid. The earliest known use of this molecule has been traced back to fifth century B.C. The Greek physician Hippocrates described an extract of willow tree bark, a bitter powder that could be used to reduce fevers. In 1829, salicin was isolated from willow bark and used as a pain reliever. Unfortunately, salicin was not very popular, since it was found to be very acidic and a stomach irritant. In 1897, a German chemist named Felix Hoffman was working for the Bayer chemical company. Hoffman was looking for a less acidic pain reliever that his father could take for his arthritis. His research led to the synthesis of acetylsalicylic acid or aspirin. Bayer patented the name and marketed the product in 1899. It was a huge success and sales grew rapidly. In fact, the company set up by Friedrich Bayer & Company is generally reckoned to have been the first pharmaceutical company, and the production of aspirin is generally accepted to have laid the foundation of the modern pharmaceutical industry. Interestingly enough it wasn’t until the 1970’s that scientists began to understand how aspirin actually worked as a pain reliever. Today 80 billion aspirin tablets are consumed every year across the globe to reduce fevers, relieve pain, and even help prevent heart attacks. In commercial aspirin products, a small amount of aspirin (300 to 400 mg) is bound together with a starch binder and sometimes caffeine and buffers to make an aspirin tablet. The basic conditions in the small intestine break down the aspirin to yield salicylic acid, which is absorbed into the bloodstream. The addition of a buffer reduces the irritation caused by the carboxylic acid group of the aspirin molecule. Aspirin can be produced in a one-step chemical process by reacting salicylic acid with acetyl chloride, according to the reaction:
Aspirin is a white solid that is almost completely insoluble in water. We will use this physical property of our product to separate it from the final solution. If time allows, we will synthesize methyl salicylate, which is another ester of salicylic acid. It occurs in a wide range of plants and is known as ‘oil of wintergreen’. It is still used in candies and in ointments for joint and muscle pains.
Thin layer chromatography (TLC) is used to separate and identify aspirin. Small amounts of the synthesized product, starting material (salicylic acid) and commercial aspirin are placed along one edge of a chromatography plate. The plate is then placed in a container with solvent. With the plate acting like a wick, the solvent flows up the chromatogram, carrying samples with it. Molecules that are more soluble in the solvent will move higher on the paper; the molecules that are more attracted to the plate will remain closer to the original line. After removing the plate, the samples can be detected with UV light. Safety Notes 1. The acetyl chloride and pyridine should only be dispensed in a fume hood. 2. Always wear appropriate eye protection and gloves while handling the chemicals. Materials Gloves goggles suction...