Synthesis of Salicylic Acid from Oil of Wintergreen


The previous experiment illustrated how one kind of structural parameter, the length of a carbon chain, can alter a fundamental physical property like solubility in water. This new experiment features several kinds of interesting solubility phenomena, and it also takes us over to the pharmacy counter. Both our starting material – methyl salicylate (oil of wintergreen) – and our product – salicylic acid – are naturally occurring compounds that have been sought out by humans for centuries as medicines. Salicylic acid, a component of willow bark, was used to reduce fever and pain, but don’t even consider trying this for yourself. Salicylic acid is highly caustic. Swallowing it will most assuredly burn the sensitive lining of your gums and mouth, and rubbing it on your skin will have equally unpleasant consequences.

This experimental procedure is also interesting for another practical reason. It is essentially identical to the traditional procedure for making soap in which natural fats (triglycerides) are boiled with lye (aqueous NaOH) to release the salts of the fatty acids. These “ester + lye -> acid” procedures are referred to as saponifications, from the Latin, “making soap.” Unfortunately, your chemical product, salicylic acid, does not make a useful soap. First, it is a brittle solid composed of abrasive, thin, needle-like crystals. Second, as mentioned above, it is rather caustic and can damage your skin.

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