Aldehydes are organic compounds present in many natural materials (eg. orange rind, rose, cinnamon bark). Various aldehydes can also be synthesized artificially. There is hardly a fragrance without some type of aldehyde in it; however, it is the vividness of aliphatic aldehydes (a specific subgroup of the aldehydes family) that gives Chanel No 5, Lanvin Arpège, Yves Saint Laurent Rive Gauche and other floral-aldehydic fragrances their characteristic impressionist sparkle.
Their scent ranges from metallic, starchy and citrusy to green, fatty and waxy (for instance, aldehyde C-11 commonly found in rose and cilantro smells like metal and dirty hair to me, but in tiny quantities it adds an impressive lift and freshness to fragrances.) Aldehydes were used previously by Robert Bienaimé in Houbigant Quelques Fleurs (1912) and by Henri Alméras in Rosine Le Fruit Défendu (1914), Chanel No 5 became famous for its unprecedented overdose of several different aldehydes (a total of almost 1%.)