Cinnamaldehyde
The compound that gives cinnamon and cassia their sweet, warm scent and a mild tingle.
Also: cinnamic aldehyde, cinnamon oil compound
Cinnamaldehyde makes up well over half of cassia bark oil and about half of Ceylon cinnamon bark oil. It smells sweet, warm and spicy, and it produces a mild tingling heat by activating the TRPA1 receptor, the same one that responds to mustard. It is the reason cinnamon candy and chewing gum taste as they do. Cinnamon leaf oil is rich in eugenol rather than cinnamaldehyde, so bark and leaf differ in flavour. Cassia contains more, and so tastes bolder. Cinnamaldehyde reacts with air over time, which is one reason old ground cinnamon smells flat.