Structural isomers occur when molecules from the same molecular formula have bonded together in a different order. This excludes arrangements that are different simply because the molecules are rotating. There are endless ways that a molecule can twist itself. Three categories of constitutional isomers are positional, skeletal, and functional isomers. There are 15 isomers with the formula C4H8O. They are butanal, butanone, cyclobutanol, epoxy butan, and methoxycyclopropane. Also included are allyl methyl ether, ethenyl ethyl ether, epoxybutane, epoxy methylpropane, and methyl propenol.