Scents and Science. Chapter 11: Aldehydes in Perfumery

Introduction

I’m sure most of you have probably heard the term “aldehydic” when someone is trying to describe the scent of a perfume. But what does “aldehydic” actually smell like? And why do we use such a specific descriptor when other scents are typically described by their natural counterparts? In this article, we’re going to look at the special types of molecules: aldehydes. These molecules have brought the language of chemistry and perfumery closer together than any others. No other organic group is used as much to define the essence of scent. In this blog post, we’re continuing our series on organic groups, focusing on the fascinating worlds of aldehydes. We’re examining them from a unique perspective where perfumery meets chemistry. What makes aldehydes so special? And why does Chanel No. 5 often come to mind whenever aldehydes are mentioned? Get ready to explore one of the most aromatic groups in the world of organic compounds.

Carbonyl Group

In a previous blog post, we explored what alkyl and hydroxyl groups are and their roles in alcohols. Similarly, just as phenols and alcohols are classified based on the presence of a hydroxyl group in their structures, aldehydes and ketones are defined by the presence of a carbonyl group. A carbonyl group consists of a carbon atom double-bonded to an oxygen atom (C=O). This distinctive feature is central to the chemistry of both aldehydes and ketones. The carbon atom in the carbonyl group forms two additional bonds, which can either connect to hydrogen atoms or alkyl groups. This specific bonding pattern determines whether the molecule will be classified as an aldehyde or a ketone.

Aldehydes

Aldehydes are derivatives of the carbonyl group where the carbonyl carbon is bonded to one hydrogen atom and one alkyl group. The exception to this is formaldehyde, which is the only aldehyde that bears two hydrogen atoms attached to the carbonyl group. Because of this bonding arrangement, the carbonyl group in aldehydes is always found at the end of the molecule.

The term “aldehyde” has an interesting origin. It is derived from a Latin abbreviation. The discovery of aldehydes dates back to 1774 when Carl Wilhelm Scheele synthesized acetaldehyde. However, the complete understanding of its structure was not achieved until 1835, when Justus von Liebig determined its constitution and coined the term “aldehyde” for this chemical group (R-CHO). The name itself comes from the Latin phrase “alcohol dehydrogenatus,” which translates to “alcohol without hydrogen,” reflecting the chemical transformation that occurs when alcohol is oxidized to form an aldehyde.

Aldehydes play a crucial role in the fragrance industry, and by “fragrance industry,” I refer to all its facets, not just perfumery. It’s almost a cliché to mention Chanel No. 5 when discussing aldehydes, as this iconic fragrance, developed by Russian-born French perfumer Ernest Beaux in 1921 for Chanel, is often celebrated as a milestone in the popularization of synthetic chemistry in perfumery. However, the significance of Chanel No. 5 is as much a triumph of marketing as it is of chemistry. Aldehydes were used in perfumery well before Chanel No. 5, as seen in fragrances like Rêve D’Or (1889) and Après l’Ondée (1906), although they were not as prominently featured or as distinctive as they became in later years.

Classic Aldehydes

When we think about aldehydes in perfumery, certain classic ones immediately come to mind: Aldehyde C6, C7, C8, C9, C10, C11 Undecylic, C11 Undecylenic, C12 MNA, C12 Lauric, C14, C16, C18 and C20. This is perhaps because their industrial names begin directly with the word “aldehyde,” or because some of them gained fame for their inclusion in iconic fragrances like Chanel No. 5. Regardless of the reason, the impact of these molecules on the fragrance industry is undeniable. Due to their popularity and wide usage, let’s take a closer look at each of these classic aldehydes:

Aldehyde C6 (Hexanal) (CAS No: 66-25-1): The smallest member of this star-studded family, Aldehyde C6 is known for its smell reminiscent of rancid sunflower seed oil and freshly cut grass. In its pure form, its scent is quite sharp and almost unpleasant. However, when diluted, it becomes a powerful enhancer, boosting fresh floral and fruity notes in fragrance compositions.
Aldehyde C8 (Octanal) (CAS No: 124-13-0): As the carbon chain lengthens, the scent profile of the aldehyde softens. Octanal, with eight carbon atoms, carries a scent of lemon peel mixed with lemon seeds but in a very simplified, almost pixelated way (think of lemon peel as a high-resolution 4K image, and octanal as a 144p version). It’s less sharp and can bring a subtle freshness to perfume.
Aldehyde C9 (Nonanal) (CAS No: 124-19-6): With nine carbons, nonanal brings a scent closer to orange peel combined with a touch of rose, akin to a budget air freshener. Its scent is less sharp than octanal and more complex, allowing for diverse interpretations and more widespread use in fragrance formulations.
Aldehyde C10 (Decanal) (CAS No: 112-31-2): Moving further along the carbon chain, decanal shifts more towards floral notes while still retaining a distinctive synthetic orange scent. From this molecule onward, the descriptor “aldehydic” becomes more commonly used, indicating a specific olfactory character associated with aldehydes.
Aldehyde C11 (Undecylic) (CAS No: 112-44-7): Described as having an aldehydic citrus-floral scent, C11 is often reminiscent of the smell of liquid detergent with a hint of lemonade. This molecule encapsulates the clean, soapy fragrance that many associate with freshness and cleanliness.
Aldehyde C11 (Undecylenic) (CAS No: 112-45-8): Structurally different from its undecylic sibling due to the presence of a double bond on the opposite side of the carbonyl group, this aldehyde maintains the waxy, detergent-like scent but introduces a more pronounced rose note, distinguishing it from the more citrus-inclined undecylic.
Aldehyde C12 Lauric (CAS No: 112-54-9): This molecule is often considered the queen of detergent smells. The familiar scent of “clean laundry dried under the sun” can be attributed largely to aldehyde C12 Lauric. Despite attempts to associate this scent with something natural, the most accurate description remains the simulacrum of freshly laundered clothes, underscoring the term “aldehydic-clean laundry.”
Aldehyde C12 MNA (CAS No: 110-41-8): The main difference between C12 MNA and Lauric is structural: C12 MNA features a methyl group attached to the second carbon (from the carbonyl group), whereas Lauric has a straight chain. Known as methyl nonyl acetaldehyde, C12 MNA offers a more vibrant and complex scent, akin to clean laundry with sweet, fruity undertones.
Aldehyde C14, C16, C18, C20: Although these compounds are traditionally referred to as aldehydes, they are actually lactones. They will have to wait for their spotlight in our upcoming post dedicated to esters and lactones, where we can explore their creamy, coconut-like scent profiles.

Special Aldehydes

While classic aldehydes often steal the spotlight, there are many other aldehydes in perfumery that are just as significant. From a chemical perspective, there is no formal classification of “classic” versus “special” aldehydes since they all contain the carbonyl group. However, from an olfactory perspective, special aldehydes often have more complex and multifaceted scents that can sometimes be more easily associated with natural aromas. The aldehydes described here are not an exhaustive list of those used in perfumery, but they represent some of the most intriguing and exciting examples.

Benzaldehyde (CAS No: 100-52-7): One of the most famous aldehydes in both perfumery and chemistry, benzaldehyde consists of a carbonyl group attached to a benzene ring (phenyl group) on one side and a hydrogen atom on the other. Its scent is reminiscent of bitter almonds and is commonly used to mimic the aroma of cherry seeds, as in Tom Ford’s “Lost Cherry.” It’s a straightforward scent note, characterized by its pure bitterness.
Cinnamaldehyde (CAS No: 104-55-2): Officially known as (2E)-3-phenylprop-2-enal, this molecule is responsible for the characteristic scent and flavor of real cinnamon. You don’t need to imagine its smell because it’s a major component of cinnamon’s aroma. Structurally, it features a benzene ring with a carbonyl group separated by a two-carbon chain connected by a double bond, adding warmth and spiciness to fragrances.
Hexyl Cinnamaldehyde (CAS No: 101-86-0): Also known as Jasmonal H, this aldehyde is one of the most widely used in the fragrance industry due to its affordability and versatile scent profile. It has a sharp, dark floral scent similar to jasmine and lily-of-the-valley (muguet), making it a staple in shampoos, creams, and laundry softeners. However, it must be listed on ingredient labels due to its potential allergenic properties.
Citral (CAS No: 5392-40-5): A fascinating molecule with a distinct, direct scent reminiscent of citrus fruits. Even though it doesn’t perfectly replicate the natural scent of real citrus, citral has been used so extensively in products labeled as “citrus-scented” that our brains have come to associate its scent directly with citrus.
Citronellal (CAS No: 106-23-0): Known for its characteristic scent of citronella, lemongrass, verbena, and rose, citronellal carries a slight soapy note. It’s commonly found in insect repellents and is widely used in fragrances for its fresh, uplifting aroma.
Anisic Aldehyde (CAS No: 123-11-5): As the name suggests, this aldehyde has a scent similar to anise. Many people will recognize this fragrance from “baby powder” type scents and white floral laundry softeners. It provides a soft, sweet, and slightly spicy note that adds comfort and familiarity to various compositions.
Bourgeonal (CAS No: 18127-01-0): A captivating molecule with a distinctive white floral, watery scent. Bourgeonal is similar to the main white floral note found in CH 212 Women perfume, offering a light, airy, and slightly green fragrance profile.
Triplal (CAS No: 68039-49-6): One of my personal favorites, triplal has a very strong, grassy green smell. If you’ve ever encountered a sharp, intensely green aloe vera fragrance, it’s likely based on triplal. Its fresh, green character makes it ideal for evoking the scent of fresh-cut grass or leafy greens.
Trans-2,cis-6-Nonadienal (CAS No: 557-48-2): Also known as cucumber aldehyde, this molecule smells exactly like fresh cucumber. Even at a dilution of 0.01%, it replicates the crisp, watery, and green scent of cucumber, making it popular in fresh and clean fragrance compositions.
Trans-2-Hexenal (CAS No: 6728-26-3): An aldehyde with an extremely strong scent reminiscent of apples, fresh bananas, and other fruits. While some might find its pure form reminiscent of the smell of stink bugs rather than apples, it can add a wonderful apple-like note to a fragrance composition when used in the right dilution.

Aldehydic Note

As we’ve seen, aldehydes can exhibit a wide range of olfactory characters. So, what do “aldehydic note” smell like, and what kind of scents does it describe? Typically, when people refer to classic aldehydes scent, they are talking about the synthetic, fresh laundry-like, white floral scents associated with Aldehydes C11 and C12. These molecules are often linked to the clean, crisp, and slightly soapy smell that is reminiscent of freshly washed linens or the fresh-out-of-the-shower scent.

From a chemical perspective, it’s impossible to group all aldehydes under one olfactory descriptor, as each aldehyde can possess a vastly different scent profile. For example, the bitterness of benzaldehyde or the spicy warmth of cinnamaldehyde are distinct and easily recognizable. However, when perfumers encountered a scent that did not neatly fit into familiar categories—such as the aldehydes used in Chanel No. 5—they coined the term “aldehydic” to provide a general sense of what to expect. This term has since become a catch-all for describing the unique, abstract, and sometimes metallic character that many associate with classic aldehyde-rich fragrances.

In essence, “aldehydic” has become shorthand for a certain type of olfactory experience—one that suggests freshness, cleanliness, and an almost futuristic scent quality that transcends the simple replication of natural aromas.

Aldehydes in Perfumery

Aldehydes are widely used in various consumer products, including perfumes, bar soaps, shower gels, shampoos, creams, and other personal care items. Their unique scent profiles make them invaluable in adding freshness, complexity, and distinct olfactory notes to a wide range of formulations. However, the use of aldehydes in these products also raises considerations regarding their stability and potential reactions over time.

In fine fragrances, aldehydes are prized for their ability to add brightness and longevity. However, aldehydes can be sensitive to light, air, and heat, which can cause them to oxidize over time. Oxidation can lead to changes in the scent profile, often resulting in a less desirable, off-note smell. To mitigate these effects, perfumers often use stabilizers and antioxidants in the formulations to preserve the integrity of aldehydes. Packaging in opaque or dark glass bottles also helps to minimize exposure to light and extend the fragrance’s shelf life.

In liquid formulations like shower gels and shampoos, aldehydes are often used to provide a fresh, invigorating scent. These products typically have a lower pH than bar soaps, which can help stabilize aldehydes. However, aldehydes in aqueous environments are still prone to oxidation, especially if the product is exposed to air or light frequently. Stabilizers and encapsulation technologies can be employed to protect the aldehydes, ensuring the scent remains stable and consistent throughout the product’s lifespan.

Aldehydes are also found in a variety of other consumer goods, such as air fresheners, detergents, and household cleaners. In these applications, aldehydes help provide a sense of cleanliness and freshness. However, exposure to environmental factors like air and light can lead to oxidation, potentially altering the scent profile over time. For this reason, these products are often formulated with stabilizers or in airtight packaging to prolong shelf life.

Reactions Affecting Aldehydes in Products

Oxidation: Aldehydes are prone to oxidation, a reaction in which they combine with oxygen from the air. This can lead to the formation of carboxylic acids, which may not only change the scent profile but also produce off-notes or a more acidic smell. This process can be slowed down by using antioxidants and by minimizing exposure to light and air.
Schiff Base Formation: When aldehydes come into contact with primary amines (methyl anthranilate), they can react to form Schiff bases, compounds characterized by a carbon-nitrogen double bond (C=N). Schiff base formation can change the scent of a product significantly and may result in less desirable olfactory characteristics. This reaction is particularly relevant in products that contain amino-based compounds or proteins.
Decolorization: Some aldehydes can cause or undergo decolorization in products, affecting both the scent and appearance. In the presence of alkaline conditions (common in bar soaps), aldehydes can react and lead to changes in the color of the product, often turning it yellowish or brown. This reaction highlights the importance of careful formulation and the use of stabilizers to maintain the desired aesthetic.

Conclusion

While aldehydes can be classified relatively easily based on their chemical structures, doing so from an olfactory perspective is far more challenging. Aldehydes are often associated with a clean, laundry-like, white floral scent, which has led to the creation of the term “aldehydic.” However, as we’ve explored, aldehydes offer a wide range of scent nuances, from fresh and fruity to spicy and bitter. Attempting to categorize all aldehydic scents under a single descriptor does not do justice to their diversity.

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: