Aldehyde

Aldehyde was discovered in 1835, and is categorized by the length of its carbon chain (the number of carbon atoms present in a molecule). The length of this chain varies between 1 and 12 carbon atoms.
It's only in the beginning of the 20th century that chemists come up with a stable fabrication method that allows them to create large-scale aldehyde molecules.
The aldehyde's diversity and olfactory characteristics made them the most used molecule in perfumery in the first half of the 20th century.  

Discovery
It's at the very beginning of the 20th century that chemists discover a method of creating aldehyde molecules. In 1902, Auguste Darzens synthesizes aliphatic aldehyde molecules, which are essentially aldehydes, with a relatively long carbon chain.
Auguste Darzens's method, however, does not work perfectly, leaving the quantity, as well as the quality of the molecules, entirely random and uncertain. It's only 15 years later, in 1918, that a new method was put in place and the industrial production of the molecule could begin.

Characteristics
The length of the carbon chain determines the fragrance of the molecule. For example, the molecules with 9 to 11 carbon atoms (C9-C11) smell like candle wax, while molecules with 2 atoms smell like green apples, and molecules with 10 atoms smell like oranges.

Use in perfumery
Simply put, aldehyde has a warm and heavy metallic scent that can be compared to the scent of a hot iron, which brings strength, crispness and volume to a fragrance.
Aldehyde is most often used with floral tones, and has enabled the creation of many exotic feminine perfumes including Chanel No5, created by Ernest Beaux in 1921 who combined a C-12 aldehyde molecule with other floral scents.

To continue the story