Video Transcript
Which of the following organic
molecules would you predict to have the highest boiling point?
In this question, we need to
determine which organic molecule has the highest boiling point. Molecules with a higher boiling
point will have stronger intermolecular forces. The strongest intermolecular force,
hydrogen bonding, will form between molecules with at least one hydrogen bonded to a
highly electronegative atom, such as fluorine, nitrogen, or oxygen. The electronegative atom carries a
partial negative charge, which we can show using the 𝛿 minus symbol, because it
withdraws most of the electron density from its bond with the hydrogen atom. The hydrogen atom has a low
electron density and a partial positive charge, which we can represent using a 𝛿
plus symbol.
Hydrogen bonds are dipole–dipole
interactions that exist between covalently bonded hydrogen atoms and electron lone
pairs on strongly electronegative elements of adjacent molecules. Hydrogen bonds are strong, and we
need a lot of energy to overcome them and separate the molecules. Thus, molecules engaging in these
types of interactions generally have high boiling points.
If we look at our answer choices,
not all listed molecules have a highly electronegative element bonded to a hydrogen
atom. It is likely that the organic
molecule with the highest boiling point will be able to engage in hydrogen bonds
since they are the strongest type of intermolecular force. For example, answer choice (C) has
an electronegative oxygen atom bonded to only carbon atoms and no hydrogen atoms and
would not engage in hydrogen bonds. Answer choice (C) would likely not
have the highest boiling point of these five molecules. Answer choices (A), (B), and (E)
can readily form hydrogen bonds as they have a strongly electronegative element
bonded to hydrogen.
However, the hydrogen bonding from
molecules (A) and (B) would be stronger since the oxygen atoms forming hydrogen
bonds are more electronegative than nitrogen. The intermolecular forces of
molecule (E) would not be as strong, and thus its boiling point would not be as
high. It is worth mentioning that
molecules (D) and (E) have similar boiling points, the reasoning for which is beyond
the scope of this video.
Molecule (B) has one group on the
molecule that can form hydrogen bonds as it contains an OH group. The molecule in answer choice (A)
contains two OH groups. Each molecule (A) can engage in
hydrogen bonding with adjacent molecules in two groups, while molecule (B)
cannot. We would need much more energy to
overcome the attractions between molecules of answer choice (A).
Therefore, we’ve determined the
organic molecule that will be predicted to have the highest boiling point. The correct answer is answer choice
(A).
