Octodecanal is a long-chain aldehyde, with the chemical formula C18H36O (also known as stearyl aldehyde). Octadecanal is used by several species of insect as a pheromone. Octodecanal is a long-chain aldehyde, with the chemical formula C18H36O (also known as stearyl aldehyde). Octadecanal is used by several species of insect as a pheromone.