Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Octodecanal is a long-chain aldehyde, with the chemical formula C17H35CHO (also known as stearyl aldehyde). Octadecanal is used by several species of insect as a pheromone. An aldehyde (pronounced / æld ha d/) is an organic compound containing a formyl group. This functional group, with the structure RC(=O)H, consists of a carbonyl centre bonded to hydrogen and an R group. The group without R is called the aldehyde group or formyl group. Aldehydes differ from ketones in that the carbonyl is placed at the end of a carbon skeleton rather than between two carbon atoms. Aldehydes are common in organic chemistry. Many fragrances are aldehydes.