Question Video: Identifying Which Compound Could Form Hydrogen Bonds within Its Molecules Chemistry

Video Transcript

Which of the following compounds could be able to form hydrogen bonds between its molecules?

The structural formula for the four molecules are shown. Since this question is asking us to identify the compound that forms a hydrogen bond between molecules, let us first discuss this concept.

Hydrogen bonds are dipole–dipole interactions that exist between covalently bonded hydrogen atoms and electron lone pairs on strongly electronegative elements. Let us take a look at a molecule in our everyday life that exhibits hydrogen bonding, the water molecule, and examine its dipole interactions that allow this molecule to be classified as polar.

The water molecule has a highly electronegative oxygen atom, which attracts most of the electron density of the two covalent OH bonds, which is labeled with a 𝛿−, since it is the partially negatively charged portion of the neutral molecule. The two hydrogen atoms in the water molecule are labeled as 𝛿+, since they are the partially positively charged region of the molecule, since the electron density is less.

When two water molecules interact with each other, the partial negative electrostatic charge of the oxygen atom on one water molecule is attracted to the partial positive electrostatic charge of the hydrogen atom on a different water molecule. This attraction between the difference in the electrostatic charges between the two water molecules is called the hydrogen bond, represented with the dotted line.

In this example with water, the strongly electronegative element is oxygen. But it could also be nitrogen or fluorine, since these are the three most electronegative elements within the periodic table.

Let us now apply our knowledge of the hydrogen bond to the molecules in our problem. All of the molecules in this question have at least one oxygen atom that is bonded to at least one carbon atom. The oxygen atom withdraws a significant amount of electron density from the carbon, making all of these molecules polar through the formation of a partially negative charge on the oxygen atom.

It is important to note that the ether in answer choice (B) may look nonpolar due to the symmetry of the Lewis structure. But keep in mind, it is a polar molecule since the oxygen atom has two lone pairs which repel one another, giving the molecule a bent shape.

When trying to identify a molecule that could form a hydrogen bond, we also need to look for a hydrogen atom attached to the oxygen atom. When looking back at the four molecules in this problem, the carboxylic acid also has a hydrogen atom attached to the same electronegative oxygen atom. This result in the oxygen atom withdrawing electron density from the hydrogen atom, resulting in the hydrogen atom having a permanent partial positive charge.

Let us clear some space in order to demonstrate this dipole–dipole interaction in molecule (C). In molecule (C), it is a partial positive charge on the hydrogen atom in one carboxylic acid molecule and the partial negative charge on an oxygen atom in another carboxylic acid molecule that allows a dipole–dipole interaction to form between the molecules.

We should also note that the hydrogen bond could also form between the two hydroxyl groups within molecule (C), as shown. As such, the compound that could form a hydrogen bond between its molecules in the molecule shown is answer choice (C).