Introduction

As we explore the world of perfumery, we come across one of the most fascinating aspects of fragrance chemistry: aromatic ring. You might be wondering why they’re called aromatic when there are so many other compounds that also have smells. What makes these rings so special that it took scientists almost a century to fully understand their structure? This chapter will take you through the chemical backbone found in beloved scents like vanilla, cinnamon, clove, and almond, among others. Join me as we explore why aromatic rings are not just essential but truly fascinating in the art of fragrance making.

Scents and Science. Chapter 1: Organic Chemistry Behind Fragrances
Scents and Science. Chapter 2: Atoms and Elements
Scents and Science. Chapter 3 Chemical Bonds
Scents and Science. Chapter 4: Electron Configuration
Scents and Science. Chapter 5: Molecular Orbitals
Scents and Science. Chapter 6: Cis Trans Isomers
Scents and Science. Chapter 7: Hydrocarbons

The History of Aromatic Rings

In everyday conversation, the word “aromatic” often brings to mind pleasant, mouth-watering scents – a fresh cup of coffee or a cinnamon roll straight from the oven, perhaps. Or maybe it makes you think of a romantic candlelit dinner. The chemists were just as driven when they first stumbled across the aromatic ring. This discovery paved the way for what we now know as the benzene ring, which led to the creation of a whole family of aromatic compounds.

The journey began in 1825 with English chemist Michael Faraday, who isolated a new hydrocarbon while working with whale oil. Almost a decade later, in 1834, the German chemist Eilhardt Mitscherlich synthesised this molecule in his lab. The results were pretty incredible: the new hydrocarbon had as many hydrogen atoms as carbon atoms. You might be wondering why this was surprising. Well, it could have been an unsaturated hydrocarbon like 1,2,4,5-hexatetraene – a 6-carbon molecule with 4 double bonds. The catch was that this new molecule didn’t act like typical unsaturated hydrocarbons. It was surprisingly unreactive and didn’t undergo the usual reactions expected of its saturated counterparts. At this point, the chemists realised they were onto something completely new.

It’s not always easy to understand a molecule, but that doesn’t mean you can’t categorise its properties. As the Kekulé, Couper, and Butlerov team made progress in developing new theories about electrons, they also introduced a basic categorisation of organic compounds into two types: aliphatic and aromatic.

In the early days of organic chemistry, if a compound was classified as “aliphatic”, it meant that it had characteristics similar to fats. This is captured in the term’s root from the Greek “aleiphar”, which means fat. These days, though, this classification covers compounds that act like alkanes, alkenes, alkynes, or their derivatives. Basically, that means those with open chains or non-aromatic rings. On the other hand, being labelled ‘aromatic’ originally meant that the compounds had a low hydrogen-to-carbon ratio and a unique smell – that’s where the name comes from. This dual classification not only highlighted how the compounds behaved chemically, but also the sensory experience they gave off.

One of the most fascinating stories in the history of chemistry is how Kekulé came to understand the structure of benzene. It’s said that the answer came to him in a dream. Kekulé had this image in his mind of a snake seizing its own tail, forming a continuous loop. It’s a bit like the ancient symbol Ouroboros, which represents the eternal cycle of renewal and the unity of all things. This dream made him think that the benzene molecule might not be a straight chain of carbon atoms, but rather a closed ring. This idea proved to be a turning point in the study of aromatic compounds. It led to the eventual acceptance of the benzene ring as a six-carbon ring with alternating double and single bonds. This structure explained many of the ring’s unusual properties.

It wasn’t until the 1920s that we started to understand the strange behaviour and stability of benzene thanks to the advent of quantum mechanics. Just imagine for a moment that for generations of chemists, aromatic compounds were a complete mystery. This mysterious chapter in chemistry shows just how much we’ve learned about the secrets of scents.

The Chemistry of Aromatic Rings

We’re going to have to leave out some of the finer details of every aromatic compound for now, because our focus here is on the role they play in the world of fragrances. So, what makes aromatic rings different from other types of hydrocarbons? Many of you will probably know about the 1865 model of the aromatic ring proposed by Kekulé. This model showed alternating double and single bonds, which was a big step forward in structural theory.

However, this isn’t quite the actual structure of an aromatic ring. Further tests showed that the carbon bonds in benzene – the classic aromatic compound – are all the same length, somewhere between a single and a double bond. This discovery showed that the bonds in aromatic compounds aren’t typical double or single bonds. After more experiments, and yes, perhaps hundreds of broken beakers, we finally got a clearer understanding of what truly occurs within an aromatic ring.

It turns out that the pi bonds, which in typical double bonds are shared between just two carbon atoms, are instead delocalised across all six carbon atoms in the benzene ring. Each carbon atom contributes one electron from its p orbital, and these orbitals overlap with each other, creating a highly stable pi bond that encompasses the entire ring. Basically, benzene doesn’t just have a few double bonds, it has one single, strong pi bond shared by all its carbon atoms, which makes it very stable.

It’s worth mentioning that the world of aromatic compounds isn’t limited to six-carbon benzene rings. In 1931, German physicist Erich Hückel did some maths and found that rings with 4n + 2 pi electrons – like rings with 6, 10, 14, etc. pi electrons – also have the same properties as benzene. This discovery broadened the definition of aromaticity, showing that the stability and distinctive chemical behaviour associated with benzene could be found in a wider variety of ring structures.

Unlike in alkenes, where you can usually “break” a double bond to add elements or functional groups, benzene doesn’t usually allow such direct additions. Instead of addition reactions, benzene usually goes through substitution reactions, where one of the hydrogen atoms is replaced with something different. This special reaction shows us more about the different chemical behaviour of aromatic rings compared to other hydrocarbons. It makes them a really interesting topic in both theoretical and applied chemistry, especially when it comes to creating fragrances.

Aromatic Compounds in Perfumery

Now we’re going to get to the main point: molecules with aromatic rings in perfumery. There are so many different types of these compounds in the world of fragrance chemistry, and each one has its own unique scent. It’s incredible how these molecules, even though they have the same basic structure, can give off such a wide range of smells. Some are well-known for their distinctive scents and are used in lots of everyday products, while others are essential tools for perfumers, helping to create complex and beautiful fragrances. Let’s take a closer look at some of the most popular molecules with aromatic rings and their roles in perfumery:

1. Benzyl acetate – Carries a floral scent and is often used to mimic jasmine, a staple in many perfumes for its sweet, rich aroma.
2. Vanillin – The primary component of vanilla bean extract, vanillin is beloved for its warm, creamy scent that forms the backbone of many fragrances and flavorings.
3. Eugenol – Known for its clove aroma, eugenol is found in clove oil, nutmeg, cinnamon, and bay leaf, adding a spicy touch to fragrance blends.
4. Anethole – This compound has a sweet, anise-like flavor and is found in anise and fennel, used both for its scent and flavor properties.
5. Coumarin – Evokes the scent of fresh-mown hay and is naturally occurring in tonka beans and sweet clover, used extensively in perfumery for its sweet, herbaceous aroma.
6. Benzyl salicylate – With a mild floral scent, this chemical is a frequent additive in cosmetics, providing a subtle floral note.
7. Cinnamaldehyde – Responsible for the warm, inviting scent of cinnamon, derived from cinnamon bark and cassia oil and used in both flavoring and fragrance.
8. Methyl anthranilate – Offers a grape-like scent and is also noted for its similarity to neroli and orange flower, used in both flavors and fragrances to add a fruity flourish.
9. Phenethyl alcohol – A floral, rose-like scent found naturally in rose, carnation, and hyacinth, and widely used in perfumery to create lush, floral bouquets.
10. Benzaldehyde – Known for its almond-like scent, this compound is found in bitter almonds and is commonly used in perfumes for its rich, comforting aroma.

These examples show just how much potential and versatility there is in aromatic compounds when it comes to creating fragrances. Each molecule has a different scent profile, even though they all have the same aromatic ring structure. This shows how chemistry works: small differences in molecular structure can lead to very different sensory experiences. This diversity is what makes aromatic compounds so valuable in perfumery.

Conclusion

As we’ve looked at aromatic rings, we’ve learned more about the molecular details that make these compounds so important in the world of perfumery. Aromatic rings are pretty special because of their delocalised pi electrons. This makes them last a long time and gives perfumers a huge range of different scents to work with, from floral to spicy to earthy.

Every aromatic compound, whether it’s the floral benzyl acetate or the comforting vanilla aroma of vanillin, plays a big part in the creation of perfumes. It’s pretty amazing that they can embody such varied and distinct scents while sharing a common chemical backbone. This versatility is what makes aromatic compounds the perfect tools for perfumers, allowing them to create complex and appealing fragrances that really capture the imagination.

As we wrap up this section on aromatic rings, I’d like you to take a moment to think about the complex chemistry behind the scents that delight you. Whether it’s your favourite cologne, a perfume you love, or the smell of spices wafting through your kitchen, remember that there’s a whole world of intricate chemistry that makes these delightful aromas possible. Take a closer look, find out more, and most importantly, keep appreciating the fascinating science that perfumes our world, one molecule at a time.

Take care of yourselves and your noses.

References and Further Reading

For those eager to delve deeper into the world of perfumery, here are some resources for further exploration:

Books:

• Chemistry and the Sense of Smell by Charles S. Sell
• Fundamentals of Fragrance Chemistry by Charles S. Sell
• Chemical Bonds.An Introduction to Atomic and Molecular Structure by Harry B. Gray