“A rose by any other name would smell as sweet” – William Shakespeare
A rose is the quintessential symbol of beauty and love.
It has been immortalized in both ancient and modern cultures, so that we now associate the rose with an almost unattainable level of attractiveness. They appear throughout art and history, time and again with an effortless, ethereal beauty.
As the national flower of England, roses are used to represent sports teams and have been permanently entwined with English history. Following the epic 15th Century Wars of the Roses, the red rose of Lancashire fused with the white rose of York, giving the Tudor rose; a striking emblem that has been carved deep into the heart of English culture.
However, it is not only for aesthetic reasons that the rose is held in such high regard. Today, we have a variety of applications for this most famous of flowers. From flavoring food dishes, to the development of perfumes, the many uses of roses all stem from the fact that they have one of the most desirable scents known to man.
It is this addictive smell that has accelerated the popularity of the rose and allowed it to become the pinnacle of romantic icons.
But what is the source of this heavenly aroma?
There are several chemical molecules responsible for the distinctive rose smell.
The first molecule is called 2-phenylethanol. Two different types of phenylethanol exist: 1-phenylethanol and 2-phenylethanol. These two molecules are isomers of each other. This means that they have exactly the same atoms in their structure (in this case 8 carbons, 10 hydrogens and 1 oxygen – C8H10O) but the atoms are arranged slightly differently. However, while 2-phenylethanol causes a rose odor, 1-phenylethanol is not present in roses at all, but is found in tea leaves instead. This is the beauty of chemistry, very subtle differences, like where an -OH is placed, can have a huge effect on the properties of the molecule.
The next three molecules that contribute to the smell of roses are β-ionone, β-damascone and β-damascenone. These three molecules all have a very similar structure – β-ionone and β-damascone are isomers of each other like the phenylethanol molecules above – and are found in many plants, not just roses.
Citronellol can have two different stereoisomers, because one of its carbons is assymetric, which means that it has four different groups bonded to it. The two enantiomers are labelled -R and -S. In this case, both the -R and -S enantiomers have been found to influence the smell of the rose. In the drawing below the molecule has been represented without any stereochemistry.
Another major constituent of the rose scent is Rose Oxide. Due to the fact that Rose Oxide has two stereocentres (see citronellol) and a double bond, there are several possible enantiomers of the molecule. The rose oxide enantiomer shown here is the one that adds to the aroma of the plant.
Geraniol and Nerol are another two molecules that are found in rose petals. They are a further example of a pair of isomers in the flower. In this case the isomerism shown is called cis-trans isomerism. This type of isomerism is caused by the restricted rotation around the double bond between two carbon atoms. When the big groups are on the same side of the double bond, the molecule is labelled cis (Nerol in this example) and if the two big groups are on the opposite side, the molecule is labelled trans (Geraniol)
There are other molecules in the plant, present in lower concentrations, that also contribute to its aroma. Different types of roses will have a different concentration of these molecules, which is why they smell slightly different to each other.
For a long time, it was been believed that the way the human nose interprets the smell of a molecule was based on its shape. However, a recent study has provided new evidence for the theory that our sense of smell may instead be linked to molecular vibrations on the smallest scale.
Regardless of the method used to filter scents, one thing we do know is that though they may seem innocuous by themselves, the molecules in rose flowers have the potential to become something extraordinary. When combined, they join forces to make a smell so enticing that it has enchanted human beings from generation to generation.
If you would like to know more about the molecules found in roses, check out this article. Also, for those of you more chemically minded Picture it Chemistry did a really great post about the synthesis of two molecules found in rose petal oil: Limonene and Rose Oxide.