In perfumery, aldehydes are a class of organic compounds that are used to create a wide range of fragrances. Aldehydes are characterised by a characteristic structure, which consists of a carbon atom double-bonded to an oxygen atom and a hydrogen atom. This structure is responsible for the distinctive and sometimes pungent aroma of aldehydes, which can range from floral and sweet to citrusy and fresh.

Aldehydes have been used in perfumery since the early 20th century, when they were first introduced by the famous perfumer Ernest Beaux. One of the most famous aldehyde-based fragrances is Chanel No. 5, which contains a high concentration of the aldehyde known as citral. Aldehydes are also commonly used in other perfumes and fragrances to create a fresh and modern scent, as well as to enhance the overall odour of the fragrance.

In perfumery, aldehydes are often combined with other ingredients such as floral, woody, and spicy notes to create a more complex and balanced fragrance. The specific composition and concentration of aldehydes in a fragrance can be adjusted to create different variations of the scent, as well as to influence the overall longevity and strength of the fragrance. 

Depending on the aldehyde it may smell like fresh citrus, soap and even coconut.