This comes from another thread on the same subject written by Basenoter hirch_duckfinder:
"Aldehydes are a group of chemicals, several of which are in common use in perfumery. They all smell different, some fruity, some floral. They occur naturally in florals (good rose absolutes and jasmine oils contain them) but are often added to emphasise the aspects of florals which smell "brilliant", "shiny", "sparkly". They also occur naturally in citrus oils and are often used with those or to blend those with florals. Chanel No.5 is famously very strong in aldehydes."