Suggest an explanation for the observations that ethanol, \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\), is completely miscible with water and that ethanethiol, \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{SH},\) is soluble only to the extent of \(1.5 \mathrm{g}\) per \(100 \mathrm{mL}\) of water.

Hydrogen bonding is a special type of dipole-dipole attraction that occurs between molecules, a powerful intermolecular force that significantly affects solubility and miscibility. It specifically involves a hydrogen atom that is bonded to an electronegative atom like oxygen, nitrogen, or fluorine. This hydrogen atom experiences an attraction to an unshared pair of electrons from an electronegative atom in a nearby molecule.

In the context of our exercise, ethanol (\( \text{C}_2\text{H}_5\text{OH} \) displays strong hydrogen bonding thanks to its hydroxyl \( -OH \) group. The attraction between the hydrogen atom of one ethanol molecule and the oxygen atom of another ethanol molecule OR an adjacent water molecule is strong enough to allow ethanol to mix thoroughly with water, creating a homogeneous solution - hence, full miscibility.

Intermolecular forces are the forces of attraction or repulsion which act between neighboring particles (atoms, molecules or ions). These forces are relatively weak compared to the strong intramolecular forces, such as covalent bonds, within a molecule. Intermolecular forces are responsible for various physical properties of substances, such as boiling and melting points, vapor pressure, and, in our case, solubility and miscibility.

There are several types of intermolecular forces, with van der Waals forces encompassing London dispersion forces, dipole-dipole interactions, and hydrogen bonds. In comparing ethanol and ethanethiol, the difference in their intermolecular attractions with water molecules is key to understanding why ethanol is completely miscible with water, while ethanethiol has limited solubility. The weaker van der Waals forces in ethanethiol cannot compete with the stronger hydrogen bonding of ethanol, which results in their different behaviors in water.

The polarity of a molecule is a measure of how evenly it shares electrons between the atoms that make up the molecule. If the atoms share electrons unequally, the molecule has a net dipole moment, making it polar. For substances to dissolve in each other, it is generally necessary that they have similar polarities (like dissolves like).

Ethanol and water both have strong polar characters due to their -OH groups, which allows them to interact through hydrogen bonds. In contrast, ethanethiol's -SH group does have some polarity but it's not as strong as the ethanol's polar -OH group. As a result, it's less effective in interacting with the polar water molecules, which clarifies why it has a much lower solubility in water compared to ethanol's complete miscibility.

Organic molecules contain functional groups, which are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. They are the sites of chemical reactivity. Functional groups like alcohols (\( -OH \) groups) and thiols (\( -SH \) groups) influence the polarity and intermolecular bonding capabilities of organic compounds.

Ethanol contains an alcohol functional group which is very polar due to the electronegativity difference between the oxygen and the hydrogen atom. This characteristic gives ethanol a high level of miscibility with water. On the other hand, ethanethiol contains a thiol group that is less polar, which results in weaker intermolecular attractions with water and thus less solubility. The contrast in the behavior of these two functional groups in response to water is a clear demonstration of the importance of functional groups in determining the physical and chemical properties of organic compounds.