What do aldehydes smell like? The sparkle behind Chanel No. 5

Aldehydes are not a flower, a fruit or a wood. They are a family of molecules. Yet they are behind one of the most famous smells in the world: the opening of Chanel No. 5. Aldehydes give a perfume a fizzy, sparkling lift, like champagne bubbles or freshly laundered sheets. Many people know the smell without having a name for it. Once you learn it, you will notice it everywhere.

What do they smell like?

On their own, aldehydes can smell surprisingly odd. In a finished perfume, they usually come across as:

• Sparkle: a bright, fizzy, effervescent lift at the top of a scent
• Clean laundry: soapy, crisp, like fresh white sheets or ironing
• Waxy and powdery: like a candle, lipstick or face powder
• Metallic: a cool, slightly sharp edge, like cold air or snow
• Citrus peel: some aldehydes smell like orange or lemon zest

The overall effect is often called "aldehydic". Perfumes in this style can feel elegant, abstract and a little old-fashioned, in the best sense: think of glamour from the 1920s to the 1960s.

What are aldehydes?

An aldehyde is any molecule with a particular chemical group: a carbon atom bonded to an oxygen atom and a hydrogen atom. Many natural smells come from aldehydes. Vanillin (vanilla), cinnamaldehyde (cinnamon) and benzaldehyde (bitter almond) are all aldehydes.

But when perfume lovers talk about "aldehydes", they usually mean a smaller group: the aliphatic aldehydes, also called fatty aldehydes. These are simple chains of carbon atoms. Perfumers name them by how many carbon atoms they have, using C-numbers:

• C-8 (octanal): sharp, orange-peel-like
• C-9 (nonanal): waxy, with a rosy, citrusy edge
• C-10 (decanal): strong orange peel, waxy
• C-11 (undecylenic aldehyde): clean, fresh, soapy and sparkling
• C-12 (lauric aldehyde): waxy, soapy, with a soft floral, violet-like tone
• C-12 MNA (2-methylundecanal): herbal, warm and slightly ambery or incense-like. One of the most important aldehydes in classic perfume.

These descriptions are approximate. Aldehydes smell harsh and fatty when strong, and only show their pretty side when diluted.

Where they come from

Fatty aldehydes do occur in nature, in small amounts. They are found in citrus peels, in coriander leaves, in rose and in many other plants. They help give orange peel its zing and coriander its distinctive smell.

But in perfume, they are almost always made in the lab. Natural sources contain too little to extract usefully, and synthetic aldehydes are pure, consistent and affordable.

How they're made

Aldehydes are produced by chemical synthesis, often from simple starting materials such as fatty acids or petrochemical building blocks. For example, 2-methylundecanal (C-12 MNA) can be made from a ten-carbon molecule through a series of reactions.

Aldehydes are powerful, so perfumers use them in small amounts and usually diluted. They can also react with alcohol in a perfume over time, which is one reason aldehydic perfumes may change slightly as they age.

A short history of aldehydes

• Early 1900s: chemists began to make fatty aldehydes, and a few perfumers experimented with them. 2-methylundecanal is reported to have been used by perfumer Robert Bienaimé in Houbigant's Quelques Fleurs (1912).
• Ernest Beaux: a Russian-born French perfumer working in Moscow, used aldehydes in Rallet No. 1 (1914). After leaving Russia, he settled in the south of France.
• Chanel No. 5 (1921): Beaux created No. 5 for Coco Chanel. It used a large dose of aldehydes (C-10, C-11, C-12 and C-12 MNA) over rose, jasmine and other flowers. The result was a perfume that smelled abstract rather than like a single flower. It became the most famous perfume in the world.
• A legend: a popular story says an assistant added far too much aldehyde to No. 5 by mistake. It is a good tale, but it is not well supported, and Beaux had already worked with aldehydes before.
• The aldehydic era: for decades after No. 5, aldehydes were a mark of elegance. Lanvin's Arpège (1927), Yves Saint Laurent's Rive Gauche (1971) and Estée Lauder's White Linen (1978) are classics of the style.
• Decline and return: by the 1990s, aldehydes felt dated to many people. Since the 2000s, niche brands have brought them back in cleaner, lighter, "fresh laundry" forms.

Aldehydes in modern perfume

Today aldehydes are used in two ways:

• As a feature: in classic-style "floral aldehydic" perfumes and in modern "clean skin" or "laundry" scents
• As a hidden helper: a tiny touch gives lift and freshness to almost any perfume, from citrus colognes to woody scents

They are also everywhere in household products. Much of the "clean" smell of soaps and detergents comes from aldehydes.

Real vs synthetic aldehydes

There is no meaningful natural "aldehyde" material in perfume. Aldehydes are synthetic molecules, even though identical molecules exist in small amounts in nature. They are a good reminder that synthetic does not mean cheap or unnatural. Without them, many of the greatest perfumes in history could not exist.

What aldehydes pair well with

• Rose and jasmine: the classic floral aldehydic heart
• Iris and violet: for a powdery, lipstick-like feel
• Musk: for a clean, fresh-laundry effect
• Citrus: aldehydes make citrus sparkle even more
• Sandalwood and vanilla: give a soft, warm base under the fizz

Perfumes to try

• No. 5 by Chanel (1921): the reference. Try it in a modern version if the original feels too formal.
• Arpège by Lanvin (1927): a rich, glamorous floral aldehydic classic.
• White Linen by Estée Lauder (1978): crisp, clean and bright, like fresh sheets in the sun.
• Blanche by Byredo: a modern, minimal "clean laundry" aldehydic scent.
• Rive Gauche by Yves Saint Laurent (1971): cool, metallic and chic.

How to wear aldehydes

• Aldehydic perfumes can be strong at first. Spray lightly and give them a few minutes. The opening fizz settles quickly.
• Classic aldehydic perfumes suit formal occasions, evenings and cooler weather.
• Modern "clean" aldehydic scents work well every day, in the office and in warm weather.
• If you think you dislike "old lady perfume", try a modern aldehydic scent first. You may be surprised.

Who will love it

People who like elegant, polished, classic perfume. Fans of vintage style and old Hollywood glamour. Anyone who loves the smell of freshly washed laundry or clean skin. And anyone curious about the science of perfume, since aldehydes show how a single family of molecules can change the history of scent.