The Chemistry Column

The chemistry of smells

Claire Gormley

We have all experienced a moment when a particular scent evokes an emotion or a memory. For me, the combination of salty ocean air, fresh White Pine (Pinus strobus) and sweet Lupins (Lupinus perennis) transports me to my grandparents' house in Prince Edward Island, Canada. Whether it be spices in a dish we ate as a child, or the familiar blooms of home, at the core of each scent are esters. Esters are chemical compounds made from the reaction between an alcohol and a carboxylic acid. For example, if butanol (an alcohol) and ethanoic acid (a carboxylic acid) are reacted together, butyl ethanoate (an ester) is produced.

Esters are found naturally in flowers, fruits, herbs, and spices— giving them their unique smells. Our earlier example, butyl ethanoate, is one of the esters that contributes to the aroma of Apples (Malus domestica) (EspinoDíaz et al., 2016). Different combinations of alcohols and carboxylic acids yield different scents. For example, methyl octanoate gives Coconuts (Cocos nucifera) their scent, and nonyl hexanoate gives dirt its unique smell (Kennedy, 2013). Some esters have an ambiguous odour that resembles a few familiar scents, such as pentyl 2-methyl propanoate which smells of both butter and Apricots (Prunus armeniaca). James Kennedy, a VCE Chemistry teacher in Melbourne, has created a really useful infographic showing which esters contribute to different smells. He has very kindly allowed us to reproduce it here.

We learn the scents of Strawberries (Fragaria x ananassa), Cherries (Prunus avium), wood, etc., and use them to describe more complex aromas like wine, places, or even people. Perfumes, soaps, candles, and food flavourings all utilise ester chemistry to recreate the familiar scents we love— such as 'Mum's homemade apple pie', which is likely a mixture of ethyl cinnamate and a few of the many Apple-associated esters. Sometimes these fragrances don't smell quite right, though. That's because sometimes the esters used in products are not the same as those found in nature. There are some exceptions. Pentyl ethanoate, though it does not originate from Bananas (Musa spp.), can be used as banana flavouring because it has a similar structure to the ester that does occur naturally in Bananas, ethyl ethanoate (Ouelette and Rawn, 2014).

Re-creating the complex combinations of esters that make up our favourite smells is no easy feat, so when you find your memory provoked, cherish that blend of esters and, if possible, stock up!

Images

Kennedy, J. (2013) Infographic: Table of Esters and their Smells. Reproduced with kind permission.

References

Espino-Díaz, M., Sepúlveda, D. R., GonzálezAguilar, G. and Olivias, G. I. (2016) 'Biochemistry of Apple Aroma: A Review', in Food Technology and Biotechnology, 54(4): 375-397

Kennedy, J. (2013) Infographic: Table of Esters and their Smells. Also available from: jameskennedymonash.wordpress.com

Ouelette, R. J., and Rawn, J. D. (2014) 'Carboxylic Acid Derivatives' in Organic Chemistry: Structure, Mechanism and Synthesis. Pp. 699-745