Geraniol molecoule on top of a Pelargonium plant.
A memory of warmer days: geraniol is a chemical that makes up most of citronella oil, and which is found in insect repellent sprays, candles, dog spray collars and sweet-smelling perfumes. It can be found in nature or made in a laboratory, and can be converted into many other useful compounds.
1. Geraniol is a sweet-smelling oily liquid that is the main component of citronella oil, and which is also found in the oils of rose and lemongrass (and as the name would suggest, it can also be isolated from Pelargoniums, commonly known as geraniums, although not to be confused with perennial/Cranesbill geraniums (Geranium), see this discussion). It is one of the things that makes them smell nice, and as a consequence it is a common ingredient of perfumes. It is toxic to fish and aquatic life but not to humans, although in rare cases geraniol has caused skin reactions for people when they touch the peel of citrus fruits. In skin tests, a 2% solution of geraniol in petroleum jelly caused an allergic response in 0.13% of people, but geraniol that had oxidised (i.e. gone off) caused a response in 0.55% of patients (Contact Dermatitis, 2012, 67, 20-27). Given that a 1998 study found that 76% of European deodorants contained geraniol (Contact Dermatitis, 1998, 38, 29-35), it is just as well that these low figures lead to it being considered a weak allergen (Chem. Res. Toxicol., 2007, 20, 807-814).
Geraniol is a useful starting material in the production of citronellol and citronellal, the other main components of citronella oil, which is mainly used in insect repellent candles. Tests in laboratories are backed up by in-field tests, showing that these compounds are very effective at repelling lots of mosquito species. (Entomol. Res., 2005, 35, 117-120).
2. The chemical structure of geraniol is shown in Figure 1. Geraniol has three functional groups: two double bonds, making it a diene, and the –OH group on the end, meaning it is also an alcohol. That’s why its name ends in -ol.
There are several possible isomers of geraniol. The position of the double bonds can vary, as can the way that the groups are arranged around them. Strictly speaking, the product known as geraniol is often a mixture of two of these isomers, which have cis and trans arrangements of the terminal –CH2OH group. One of them is actually called geraniol, but the other is called nerol.
A related compound is called citronellol, and this possess a different kind of isomerism, known as chirality. Much like a pair of hands, it comes in two different forms called enantiomers, which are superficially identical but which are in fact mirror images of each other. These are called R-citronellol and S-citronellol, where the R and S are chemical shorthand for the handedness of the isomer.
Figure 1: The terpenoid compounds (geraniol, nerol, citronellol and linalool) mentioned in the text.
As can be seen from figure 1, geraniol, nerol and citronellol all have similar shapes, which means that they all smell similar – in fact, they all smell of roses. Geraniol is described as more ‘flowery’ and nerol more ‘fresh’, and S-citronellol is meant to be ‘more delicate’ than the R isomer. Extending our metaphor from above, this illustrates the way in which the human body recognises scents – the part of the nose that detects molecules is a glove, and the citronellol is the hands. Just like the right and left hands will fit a given glove in different ways, the R– and S-citronellol fit the receptor in the nose in different ways, causing different responses, which we perceive as different smells (J. Chem. Educ., 2011, 88, 1501–1506).
Another compound related to geraniol is called linalool. In this compound the OH group has moved, and this is called a constitutional isomer – it has the same molecular formula as geraniol and nerol, but the atoms are arranged in a different way. Notice again, that it is chiral, and therefore it has two mirror images which smell a bit different: S-linalool is ‘sweet and floral’, whereas R-linalool is lavender-like. See our blogpost on lavender for more details.
White Pelargonium flower.
Figure 2: Nerol and limonene
3. All these compounds belong to a class known as terpenes, which are important in the biosynthesis of many other chemicals, such as limonene. Although at first glance the structure of limonene may seem quite different, drawing nerol in a slightly different way (Figure 2) illustrates its similarity to limonene. Terpenes and their derivatives constitute nearly a third of all the natural compounds isolated and are present in all living organisms, and they obey the biogenetic isoprene rule, the formulation of which helped a Croatian chemist called Leopold Ruzicka (1887–1976) win the 1939 Nobel prize in chemistry. Isoprene is a molecule with formula C5H8 (Figure 3), and his rule is that terpenes can be generally be constructed by joining isoprene units together in a head to tail fashion and then cyclising the results. We have come across this before in our posts on Yew and Euphorbia, and the reason that the nerol and limonene in Figure 2 are coloured is to emphasise the isoprene units (J. Endocrinol., 2019, 242, R9-R22).
Figure 3: isoprene
An important precursor to all these molecules is geranyl pyrophosphate, GPP (Figure 4), which is geraniol where the alcohol has been replaced by a P2O8 unit; when the pyrophosphate falls off and leaves a geranyl cation, the cyclisation reactions can begin (see our post on rosemary for more details). Addition of more isoprene units to GPP generates farnesyl pyrophosphate (FPP) and then geranyl geranyl pyrophosphate, GGPP. Join two GGPP together and you move towards lycopene, which is what makes tomatoes red, or cyclise one and you go towards taxol. Look at the structure of bergamottin in our post on grapefruit to see another geranyl chain.
Figure 4: Top, the biomolecules formed by sequential addition of isoprene units to geranyl pyrophosphate. Bottom is lycopene, formed from two geranyl geranyl units, which is eight isoprenes!
This is not to say that all these compounds are actually formed biologically from isoprene, because they’re not; ultimately they’re derived from glucose. Although isoprene is one of the most copiously produced volatile hydrocarbons on earth, and is the most abundant hydrocarbon in human breath – football crowds cause a spike in isoprene levels when a goal is scored – this is because it comes from terpenes rather than because it makes terpenes. Isoprene is very insoluble in water, which poses the question of how it became incorporated into biological systems in the first place. It has been suggested that reactive phosphate species emitted from volcanoes reacted with hydrocarbons in the primordial atmosphere to generate isopentenyl pyrophosphate, which is water soluble and could thus react in the primordial soup (J. Endocrinol., 2019, 242, R9-R22).
So when you are perfume shopping, remember that ultimately it will all end up smelling like a football crowd.
Contributors: George Stratford (Words), Chris Adams (Words, Editing), Natalie Fey (Images).
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