Wintergreen in summerwhite

Delighting the senses with the power of chemistry

Not only can science satisfy our deeply ingrained human desire to learn about the world around us, but it can also please the finest of tastes. Today we will be synthesising two chemicals that know just right which receptors to hijack in our nose: methyl salicylate and butyl salicylate.

Esters

Oh, gummy bananas! How can the copy smell better and stronger than the real thing!? One word: chemistry. You might be disappointed to know that with a lab and a few chemicals, one could very easily trick you without setting foot on a banana plantation.

Esters are famous for their strong smells and are commonplace in fragrances and aromas in almost everything you can imagine, from cosmetics to food products. They are a type of compound in the form R-COO-R’, which generally form from the condensation of a carboxylic acid (R-COOH) with an alcohol (R’-OH), giving water (H₂O) as a side product. This specific process for forming an ester is known as Fischer esterification. The mechanism by which this process occurs is illustrated below.

Now, depending on the alcohol (R-OH) and carboxylic acid (R-COOH) used, different esters, and therefore different aromas, can be formed, as shown in the table below.

Like archaeologists sifting through layers of soil, my friend Máximo and I painstakingly combed through the safety cabinet in our lab, choosing methanol and n-butanol as our alcohol protagonists. To accompany them in their theatre performance, we decided to use salicylic acid, a key precursor to aspirin, as their partner-in-esterification. This would give us two compounds: butyl salicylate (ascribed to an intense raspberry aroma) and methyl salicylate (famously known as wintergreen).

Methyl salicylate is one of the most well-known esters, as it’s used to aromatise gum, root beer and medicines, amongst many others. It has a strong minty smell and was first extracted from the plant pictured at the beginning of this article, American wintergreen (Gaultheria procumbens), which can be used to make “oil of wintergreen”. Hence, while dressed in a white lab coat and at the dawn of a hot Spanish summer, we got straight to work on creating this refreshing aroma.

We only needed two test tubes and a beaker of hot water. In equal quantities, we added butanol to one test tube and methanol to the other. Then, we added a few grams of salicylic acid and stirred it around. We added a dash of 96% sulphuric acid because otherwise the esterification would have taken ages: lowering the pH of the solution catalyses the reaction, saving precious time. Tempus fugit! We put the test tubes in a beaker of hot water and waited about an hour. Unless you want to experience the dizzying and overwhelmingly wonderful mix of raspberry, mint, sulfuric acid, methanol, and butanol smells, then please do this under a fume hood.

A punch in the face

We took out the test tubes and probed the magic by pouring their contents into a beaker with sodium hydroxide (NaOH) to neutralise the pesky sulfuric acid.

WOW!

Unfortunately, we (still) can’t transmit smells digitally, but believe me, both the delightful wintergreen and raspberry smells were so intensely intoxicating that they gave both of us a (nice?) headache, and even people in other parts of the lab could notice their presence.