In prop-2-en-1-ol, the strongest intermolecular forces acting between the molecules are hydrogen bonds. In propanal, the strongest intermolecular forces acting between molecules would be permanent dipole-dipole forces. Between butane molecules, there would just be van der Waals forces. Since hydrogen bonds are stronger than permanent dipole-dipole forces, which are in turn stronger than van der waals forces, it follows that the order of boiling points from highest to lowest is: prop-2-en-1-ol, propanal and butane.