What are the chemical compounds behind these scents? That’s the question that this graphic tries to answer, with a more detailed discussion of each below.
Firstly, it’s important to realise that aroma chemistry is complex, and the smell of any flower is never really the consequence of a single chemical compound. Flowers give off a complex mix of volatile organic chemicals, and whilst not all of these will contribute to the aroma, a significant number will impact it to varying degrees. Whilst we can’t point to single compounds as being the cause of flowers’ scents, we can identify those that have a major impact on the aroma that our noses detect; in many cases, there will be molecules that make a large contribution, and it’s those we’ll discuss here.
Their scent is majorly influenced by compound named after the flower, (-)-cis-rose oxide. This molecule is a particular isomer of rose oxide (which has 4 different isomers), and the one which contributes the typical floral rose fragrance. It’s detectable by our noses at very low concentrations in air – down as low as 5 parts per billion. To give this some perspective, one part per billion is equivalent to one second in thirty-two years.
Another compound that contributes to the scent of roses is beta-damascenone. This compound belongs to a family of chemical entities known as rose ketones. It also has an even lower odour threshold than rose oxide, with its aroma being detectable at just 0.009 parts per billion. Another compound with a comparably low odour threshold, beta-ionone, is also an important contributor; both of these compounds are minor constituents of the plant’s essential oil, but of great importance to its perceived fragrance.
Other compounds that make minor contributions to the aroma include geraniol, nerol, (-)-citronellol, farnesol, and linalool.
Carnations, too, are a common component of floral bouquets. In comparison to roses, their scent is much fainter; the major aroma chemicals that make up this scent are eugenol, beta-caryophyllene, and benzoic acid derivatives. Scents can be very variable between different species of carnations; one study correlated this with the varying proportions of eugenol and methyl salicylate in the aroma volatiles. Eugenol is actually a compound that’s been discussed before on the site, in the context of its occurrence in cloves. Methyl salicylate is also found in many other plants, and is more commonly known as oil of wintergreen.
Violets are perhaps less common in bouquets than roses and carnations, but perhaps much more interesting from an aroma perspective. Their scent is primarily caused by the presence of compounds called ionones, of which there are a range of forms with subtly different structures. On the face of it, this might not seem that interesting – but these ionones have a peculiar interaction with our olfactory receptors.
We become accustomed to most persistent smells, as our brain registers them as constants and phases them out. This is why you can get used to the smell of a perfume, to the extent that you no longer notice it. This is where the ionones in violets’ aroma differ. They essentially short-circuit our sense of smell, binding to the receptors and temporarily desensitising them. As this shut-down is only temporary, the ionones can soon be detected again, and are registered as a new smell. Consequently, the scent of the violet appears to disappear – then reappear!
Lilies are flowers more commonly associated with funerals in some countries. Their composition is varied across different species, but across the genus (E)-beta-ocimene and linalool are major components of the aroma. Lilies aren’t particularly unique in producing linalool – in fact, it’s produced by over 200 other species of plants. A large number of personal hygiene products include the compound as a perfume, and it’s also found in perfumes themselves.
Other aroma-contributing compounds in lilies include myrcene, a compound also found in hops used to brew beers. Additionally, some varieties of lily contain eucalyptol (also referred to as 1,8-cineole), so called because it’s also a major component of the essential oil of the eucalyptus tree.
Three compounds are particular contributors to the scent of hyacinth. Ocimenol has a scent described as fresh and citrusy, whilst cinnamyl alcohol has a balsamic odour – its name is derived from the fact that it also occurs in cinnamon. Another compound, ethyl 2-methoxybenzoate, adds a floral, fruity aspect to the scent.
Again, the variability between members of the chrysanthemum genus is considerable. Terpene compounds such as alpha-pinene, eucalyptol, camphor, and borneol have all been found in varying quantities in different species. Alpha-pinene is another molecule discussed previously on the site, in the context of its contribution to the aroma of Christmas trees. Compounds named after chrysanthemums themselves are also present; chrysanthenone and chrysanthenyl acetate are both contributors. Beta-caryophyllene has also been detected in some varieties.
Lilacs are another flower which lend their name to the chemical compounds that their aroma contains. Whilst (E)-beta-ocimene is the major component of their fragrance, it is also contributed to by lilac aldehyde and lilac alcohol.