Video Transcript
Which the following molecules is likely to have the lowest boiling point? (A) 1-Butanol, C4H10O; (B) butanoic acid, C4H8O2; (C) 1,4-butanediol, C4H10O2; or (D) methyl propanoate, C4H8O2.
Let’s start by discussing how the effects of intermolecular forces affect the boiling point of molecules. Intermolecular forces are the forces of attraction or repulsion between neighboring atoms, ions, or molecules. The general trend relating intermolecular forces to the boiling point of molecules is that as intermolecular forces of attraction increase, boiling point also tends to increase. In order to answer this question, we need to understand Van der Waals, dipole–dipole, and hydrogen bonding intermolecular forces.
First, let’s talk about Van der Waals forces, which are the electrostatic interactions between temporarily induced dipoles. Van der Waals forces occur in all atoms and molecules due to the random movement of electrons. Due to the relatively small and temporary dipoles that are induced, Van der Waals forces are also the weakest of the intermolecular forces.
Dipole–dipole forces are attractive forces between the positive end of one polar bond or molecule with the negative end of a different polar bond or molecule. Dipole–dipole intermolecular forces occur between molecules with polar bonds. And because of the stronger polarity of polar bonds, dipole–dipole interactions are stronger than Van der Waals forces.
Hydrogen bonding is a dipole–dipole interaction that exists between covalently bonded hydrogen atoms and electron lone pairs on strongly electronegative atoms. And hydrogen bonding most commonly occurs in molecules that contain alcohol, amine, carboxylic acid, or amide functional groups. Hydrogen bonding is the strongest of these three intermolecular forces. And this is due to the high polarity of a bond formed when a strongly electronegative atom forms a covalent bond with a hydrogen, one of the least electronegative atoms.
At this point, we should be able to use this information to analyze each of the answer choices. When evaluating the structure of answer choice (A), 1-butanol, we know that all molecules experience Van der Waals forces. We also know that due to the polarity of the carbon-oxygen and oxygen-hydrogen single bonds that there will be dipole–dipole interactions between molecules. And finally, since there is a nonbonding electron pair on the oxygen atom as well as a covalent bond between the strongly electronegative atom oxygen and hydrogen that there will also be hydrogen bonding between molecules.
Next, looking at the structure of butanoic acid, we know that all molecules experience Van der Waals forces. We know that the strongly electronegative oxygen atoms create polar bonds with carbon and hydrogen. And finally, the presence of an oxygen-hydrogen bond as well as multiple oxygens with free lone pairs of electrons, we know that hydrogen bonding will also occur in butanoic acid.
In answer choice (C), 1,4-butanediol, we know that there will be Van der Waals forces, dipole–dipole interactions due to the fact that there are multiple oxygen-hydrogen and oxygen-carbon bonds, and hydrogen bonding due to the presence of oxygen lone pair electrons and oxygen hydrogen bonds.
And finally, in methyl propanoate, we have Van der Waals forces and dipole–dipole intermolecular forces due to the oxygen-carbon bonds. But in methyl propanoate, all of the oxygens are bonded to carbon. But since there are no oxygen-hydrogen bonds, then hydrogen bonding cannot occur between methyl propanoate molecules.
As we’ve already discussed, hydrogen bonding is the strongest of these three intermolecular forces. And we also know that there is a trend that as intermolecular forces increase, boiling point tends to increase. And we can also say that the opposite is true. As intermolecular forces decrease, boiling point decreases. And at this point, we’re ready to answer the question.
Which of the following molecules is likely to have the lowest boiling point? And since molecules of methyl propanoate are incapable of hydrogen bonding with one another, we know that the correct answer is answer choice (D), because it has the weakest intermolecular forces and therefore likely the lowest boiling point.
