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).
