Scents and Science. Chapter 12: Ketones in Perfumery

Introduction

Ketones might not be as commonly referenced in everyday conversations about perfumes as aldehydes, but their impact on the world of fragrance is equally profound. While aldehydes often bring to mind the crisp, clean scents of fresh laundry and iconic fragrances like Chanel No. 5, ketones in perfumery add their own unique character to the olfactory palette. They offer a wide range of aromas, from the delicate floral notes of violets to the rich, musky undertones that provide depth and complexity to many of our favorite scents.

In this blog post, we continue our exploration of organic compounds that play a pivotal role in perfumery by delving into the world of ketones. These molecules, much like their aldehyde counterparts, have bridged the gap between the science of chemistry and the art of fragrance creation. What exactly are ketones, and how do they differ from aldehydes? What makes ketones so valuable in the creation of both classic and contemporary perfumes?

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Ketones

Ketones are derivatives of the carbonyl group where the carbonyl carbon is bonded to two alkyl groups, which can be identical or different. The key difference between ketones and aldehydes is that in aldehydes, the carbonyl carbon is bonded to at least one hydrogen atom, while in ketones, it is bonded exclusively to carbon atoms from alkyl groups. This structural variation significantly affects both the chemical properties and olfactory characteristics of these compounds.

Nomenclature of Ketones

Ketones are named based on their parent alkane chains. The derived names of ketones are traditionally obtained by listing the names of the two alkyl groups attached to the carbonyl group, followed by the word “ketone” as a separate term. Historically, these alkyl groups were listed in order of increasing complexity, such as “methyl ethyl ketone.” However, according to the modern rules of IUPAC nomenclature, the alkyl groups are listed alphabetically, resulting in names like “ethyl methyl ketone.”

The Origin of the Term “Ketone”

But where does the word “ketone” actually come from? The name “ketone” itself has an intriguing origin, similar to the classic debate of what came first, the chicken or the egg. The story traces back to 1839 when acetone was discovered and named after acetic acid, derived from the Latin word “acetum,” meaning vinegar, and the Greek suffix “-one,” which is a female patronymic similar to words like “anemone” (literally “daughter of the wind,” derived from “anemos”). The French word “acétone” was later adapted into German as “aketon.” In 1848, German chemist Leopold Gmelin coined the term “keton” from “aketon,” thus giving rise to the modern term “ketone.”

The role of ketones in the fragrance industry is as significant as that of aldehydes. Ketones have introduced scents that were previously unattainable using natural components, thereby broadening the scent palette available to perfumers. These compounds can provide depth, richness, and unique character to fragrances, making them invaluable in the creation of complex and sophisticated scent profiles.

When discussing ketones used in perfumery, they can be broadly categorized into classic and special ketones:

Classic Ketones: Compounds like damascones and ionones fall into this category. These ketones are partially present in nature and are known for their ability to mimic natural scents.
Special Ketones: This group includes molecules like, calone, frambinone, and cashmeran, which are prized for their distinctive and often synthetic olfactory profiles.

Classic Ketones

Alpha-damascone (CAS No: 43052-87-5), Trans-beta-damascone (CAS No: 35044-68-9), Delta-damascone (CAS No: 57378-68-4): These three ketones share common olfactory notes such as rose, plum, tea, and blackcurrant but differ in their scent profiles. Alpha-damascone has the “brightest” scent among the three, characterized by an apple-minty-tea aroma. Delta-damascone leans towards darker tones, evoking tobacco, tea buds, and blackcurrant nuances. Trans-beta-damascone strikes a balance between its siblings, being less minty than alpha-damascone but not as dark as delta-damascone, providing a complex, slightly fruity floral note.
Damascenone (CAS No: 23696-85-7): A fruitier and more floral ketone, damascenone is popular in compositions that feature sugared apple and rose notes. It was first isolated from Damask Rose Absolute in 1965 by chemists P. Ruzicka and Dr. Demole, marking a significant milestone in the identification of rose ketones. However, it’s important to note that the use of rose ketones is strictly regulated by the International Fragrance Association (IFRA) due to potential allergenic concerns. As a result, their use in perfumery is limited and may become increasingly restricted in the future.
Alpha-ionone (CAS No: 127-41-3), Beta-ionone (CAS No: 14901-07-6), Alpha-isomethyl-ionone (CAS No: 127-51-5): Ionones share common olfactory characteristics such as violet, powdery, and floral notes but differ in their specific scent profiles. Alpha-ionone has a more woody and floral scent with a subtle powdery aspect, making it the brightest and most prominent of the ionones. Beta-ionone is softer and sweeter, with a more pronounced violet character, often described as closer to a natural violet flower scent. Alpha-isomethyl-ionone is the most versatile, offering a balanced profile that combines woody, floral, and fruity aspects, making it suitable for adding depth to a variety of fragrance compositions. Interestingly, ionones were discovered quite accidentally when a laboratory assistant used sulfuric acid to clean a glass containing an incorrect intermediary in the synthesis of irone.
Alpha-irone (CAS No: 79-69-6): Alpha-irone is one of the most prized ionones due to its rarity and high cost, as it is naturally found only in orris root butter. It has a rich, strong orris (iris root) scent, known for its elegant, powdery, and floral character. Alpha-irone is more stable than other ionones, making it a highly desirable ingredient in luxury perfumes. However, its high price limits its widespread use in the fragrance industry, often making it a key component in high-end or exclusive perfume creations.

Special Ketones

Calone (CAS No: 28940-11-6): Known for introducing “watery” notes to perfumery, Calone has an aquatic and fruity scent, often likened to watermelon. This unique molecule helped pioneer the creation of marine and oceanic fragrances that evoke a sense of freshness and sea breeze, which were not commonly found in earlier perfumery.
Frambinone (CAS No: 5471-51-2): If you’ve ever encountered a raspberry-scented fragrance, there’s a good chance it contained Frambinone. This ketone is a go-to ingredient for adding raspberry notes to compositions, thanks to its realistic and long-lasting fruity scent. Unlike other berry molecules that might evaporate quickly, Frambinone can be used to introduce fruitiness into the base notes, adding depth to a fragrance.
Cashmeran (CAS No: 33704-61-9): Cashmeran offers a distinctive musky scent that has become popular in many fine fragrances. This ketone is often accompanied by woody and slightly spicy undertones, adding warmth and a sensual character to perfumes. Other ketones such as musk ketone, exaltenone, tonalide, exaltolide, crysolide, and exaltone also feature musky notes but with subtle variations, contributing to the rich diversity of musk scents in perfumery.

These classic and special ketones demonstrate the vast range of olfactory experiences that ketones can bring to the table. From the fruity allure of damascones to the musky, woody elegance of cashmeran ketones are indispensable tools for perfumers aiming to create fragrances that captivate the senses and tell a story.

Ketonic Note

If there were a distinct olfactory category known as a “ketonic note,” it would likely resemble the scent profile described above, particularly the group of ketones associated with cashmeran. These scents are characterized by their musky, woody, and sometimes slightly sweet or spicy nuances, which collectively contribute to a warm and sensual fragrance profile. However, in perfumery, such scents are typically categorized under “musky notes.” Much like the diversity seen within aldehydic scents, ketones display a wide range of olfactory characteristics that don’t neatly fit under a single descriptor.

While aldehydes are often linked to fresh, clean, or soapy scents, ketones encompass everything from the bright fruitiness of damascones to the deep, woody warmth of cashmeran. As with aldehydes, attempting to group all ketones under one general “ketonic” category oversimplifies their unique and varied scent profiles. Therefore, rather than using a broad “ketonic” descriptor, perfumers tend to rely on more specific terms like musky, woody, fruity, or floral to capture the intricate nuances of each ketone-based fragrance component.

Ketones in Perfumery

Ketones are widely used in perfumery for their versatile scent profiles and their ability to add depth, warmth, and complexity to fragrances. From fruity and floral damascones to the musky and woody cashmeran, ketones play a vital role in the olfactory landscape of modern perfumery. However, like all fragrance ingredients, ketones come with considerations regarding their stability and how they behave in final products such as perfumes. Understanding their chemical stability and potential reactions is crucial for creating long-lasting and high-quality fragrances.

Stability of Ketones in Perfumes

Oxidation: Ketones are generally more stable than aldehydes due to their molecular structure, which lacks a hydrogen atom directly bonded to the carbonyl carbon. However, ketones can still undergo oxidation, especially when exposed to air, light, or heat over time. Oxidation may lead to the formation of ketone peroxides, which can alter the fragrance profile, often introducing unwanted or off-notes. The rate of oxidation and the effect it has on scent depends on the specific ketone and its chemical environment.
Decolorization: While ketones are less prone to decolorization compared to other reactive fragrance components, certain ketones may still undergo chemical changes that lead to color shifts in the perfume. Exposure to light and air can cause some ketones to break down, leading to discoloration of the fragrance over time. To prevent this, perfumers often use stabilizers, antioxidants, and UV absorbers in their formulations. Storing perfumes in dark or opaque bottles also helps protect ketones from light exposure, preserving both the scent and appearance of the fragrance.
Reactivity with Other Ingredients: Ketones can react with other ingredients in a perfume, especially if the fragrance contains reactive compounds like aldehydes or alcohols. These reactions might form new compounds, which can modify the scent profile. In some cases, these reactions might result in the formation of Schiff bases, although this is more common with aldehydes. Careful formulation and testing are essential to ensure that ketones remain stable and do not react undesirably with other components.
Hydrolysis: In aqueous environments, such as certain types of emulsified perfumes or fragrance mists, ketones can be prone to hydrolysis, although this is less common than with ester-based fragrance components. Hydrolysis can lead to a breakdown of the ketone into its constituent parts, potentially altering the fragrance’s overall character.

Ketones are a vital component in the art of perfumery, providing a wide range of scents from fresh and fruity to deep and musky. Understanding the stability and potential reactivity of ketones is essential for crafting perfumes that not only smell great but also last long and maintain their integrity. Through careful formulation, the use of stabilizers, and proper packaging, perfumers can ensure that the beautiful scents provided by ketones remain stable and consistent, delighting wearers with their complexity and depth.

Conclusion

While ketones can be classified relatively easily based on their chemical structures, categorizing them from an olfactory perspective is much more challenging. Ketones bring a remarkable diversity of scent profiles to the table, ranging from the sweet and floral notes of damascones to the rich, musky, and woody aromas of cashmeran. This variety makes it difficult to group all ketonic scents under a single descriptor, as each ketone contributes its own unique characteristics to a fragrance composition.

Ketones have expanded the perfumer’s palette, introducing new dimensions of depth, warmth, and complexity that were previously unattainable. They play a crucial role in crafting everything from delicate floral bouquets to bold, sensual base notes. However, the successful use of ketones in fragrance compositions depends heavily on their stability within the final product. No matter how captivating a scent may be, if the ketone compounds degrade, react unfavorably, or lose their integrity over time, the overall fragrance experience will be compromised. Therefore, careful formulation, the use of stabilizers, and protective packaging are essential to ensure that these valuable molecules remain effective and true to their intended scent.

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:

Scents and Science. Chapter 12: Ketones in Perfumery

Introduction

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