The hot  in hot peppers is due to capsaicin (C18h27nO3), a colorless, odorless oil-like compound found in the fruit of a plant that is a close relative of the tomato. Capsaicin is primarily found in the membrane that holds the seeds.

Although most people think that the hottest part of a chilli is its seeds, in fact it is the white spongy layer you find inside, called the placenta.