EXAMPLE Predicting Relative Equilibrium Vapor Pressures and Boiling Point Temperatures:
Consider the following pairs of molecules. Which substance in each pair would you expect to have the higher equilibrium vapor pressure (at equal temperatures), and which do you think would have the higher normal boiling point?
a. The ketone 2-heptanone has a clovelike odor and is found in oil of cloves. It also contributes to the odor of bleu cheese. Pentanoic acid, or valeric acid, is also known for its odor and in fact is used for flavorings and perfumes.
b. Butane gas, C4H10, can be used as fuel to cook a steak. Part of the smell as the meat cooks is due to the formation of acrolein, CH2CHCHO.
a. The ketone 2-heptanone is a polar molecular compound with dipole-dipole attractions between the molecules. Pentanoic acid, like all carboxylic acids, has molecules mutually attracted by hydrogen bonds. For molecules of about the same size, hydrogen bonds are stronger than dipole-dipole attractions, and so the attractions are stronger between pentanoic acid molecules. Stronger attractions lead to a lower equilibrium vapor pressure and a higher normal boiling point temperature. With an external pressure of 1 atm, pentanoic acid boils at 185.4 C, and 2-heptanone boils at 151.5 C.
b. Butane is a hydrocarbon, so it is a nonpolar molecular compound with London forces between the molecules. The C-O bond in acrolein is polar, so acrolein is a polar molecular substance with dipole-dipole attractions between the molecules. For molecules of about the same size, dipole-dipole attractions are stronger than London forces, so we expect acrolein to have a lower equilibrium vapor pressure and a higher normal boiling point temperature than butane. At one atmosphere of pressure, acrolein boils at 52.5 C, and butane boils at 0.5 C.