Video Transcript
The following table describes the
chemical formula of five alcohols. Which of these alcohols should be
the least soluble in water?
Alcohols are organic molecules that
contain at least one hydroxy group. This question presents five alcohol
molecules that each contain a single hydroxy group. We need to identify the molecule
that should be the least soluble in water.
Substances tend to be soluble in
one another if both substances exhibit similar intermolecular forces. Water molecules are polar due to
the large electronegativity difference between the oxygen and hydrogen atoms. The partial positively charged
hydrogen atom of one water molecule experiences a strong electrostatic attraction to
the lone pair electrons of an oxygen atom from another water molecule. This strong electrostatic
attraction is called hydrogen bonding. Similar to water, the hydroxy group
of an alcohol is polar due to the large electronegativity difference between the
oxygen and hydrogen atoms. As such, small alcohol molecules,
such as methanol, exhibit strong hydrogen bonds with one another.
As methanol and water molecules
exhibit similar intermolecular forces, they can form strong hydrogen bonds with one
another and the two substances are soluble in each other. All alcohol molecules contain polar
hydroxy groups. But not all alcohol molecules are
as soluble in water as methanol, and some alcohols are considered insoluble in
water.
Let’s consider the alcohol
pentan-1-ol. The hydroxy group is polar and
could exhibit hydrogen bonding with water molecules. But the long hydrocarbon chain is
nonpolar and cannot hydrogen bond. Because the long hydrocarbon chain
cannot form hydrogen bonds with water, it is less energetically favorable for the
alcohol to dissolve. So pentan-1-ol is less soluble in
water than methanol. In general, the solubility of an
alcohol in water tends to decrease as the length of the carbon chain increases. So the alcohol that should be the
least soluble in water is the alcohol that has the longest carbon chain.
Of the answer choices, the alcohol
with the longest carbon chain is pentan-1-ol. So the alcohol that should be the
least soluble in water is pentan-1-ol.
