What molecules give cinnamon, vanilla and almonds their smell?

Cinnamon's spice, vanilla's warmth and the scent of almonds each come mainly from one molecule, and all three belong to the same chemical family: the aldehydes, built around a carbon atom double-bonded to an oxygen and also holding a hydrogen. Cinnamon gets its flavor and odor from cinnamaldehyde, vanilla from vanillin, and bitter almond from benzaldehyde. Volatile aldehydes have pungent odors, and traces of many aldehydes in essential oils often give those oils their pleasant smell.

Vanilla's signature scent comes from vanillin, which makes up about 2 percent of a cured vanilla pod's dry weight and can even appear as a white frost on top-quality pods. It was first isolated as a fairly pure substance in 1858 by the chemist Théodore Nicolas Gobley, but people were using the flavor centuries earlier: since at least the early 1400s, the Aztecs flavored chocolate drinks called xocohotl with vanilla. Today, most vanillin sold worldwide is not extracted from vanilla pods at all but synthesized from petrochemical ingredients.

Cinnamaldehyde was first isolated from cinnamon oil in 1834 and first made in a lab twenty years later, in 1854. Benzaldehyde, with its smell of bitter almonds and cherry, was first extracted in 1803 by a French pharmacist investigating a far less pleasant topic: a poison. Almonds, apricot seeds and apple seeds contain amygdalin, which an enzyme can break apart into benzaldehyde, glucose and hydrogen cyanide, all in the same reaction.

Aldehydes aren't only about flavor. When a plant is attacked, it releases green leaf volatiles into the air, a group that includes simple six-carbon aldehydes such as hexanal, whose scent resembles freshly cut grass. Some of these chemicals even act as signals between plants, or between plants and insects.