Explain why ethanol \(\left(\mathrm{C}_{2} \mathrm{H}_{3} \mathrm{OH}\right)\) is not soluble in cyclohexane \(\left(\mathrm{C}_{6} \mathrm{H}_{12}\right)\)

When we talk about the polarity of molecules, we're referring to how the molecule distributes its electrical charge. Mechanical differences between atoms in a molecule cause this uneven distribution. Let's take the case of ethanol \(\mathrm{C}_2\mathrm{H}_5\mathrm{OH}\). The O-H bond within ethanol is a giveaway of its polarity. Oxygen is more electronegative than hydrogen, meaning it pulls electrons toward itself more strongly, creating a partially negative charge at the oxygen end and a positive charge at the hydrogen end.

Now you might wonder why this matters; it has massive implications for how molecules interact with each other. These partial charges lead to attraction and repulsion forces between different molecules, which can dictate the substance's state (solid, liquid, or gas), melting and boiling points, and solubility—quite important for understanding why some things mix well and others don't.

Diving deeper into the interactions between molecules, we encounter various intermolecular forces. These are the attractions and repulsions between molecules (not to be confused with intramolecular forces, which hold a single molecule together). The type and strength of intermolecular forces at play can profoundly impact the physical properties of a substance.

For instance, ethanol exhibits strong intermolecular forces called hydrogen bonds—thanks to its polar nature. Hydrogen bonds occur when a hydrogen atom, already covalently bonded to a highly electronegative atom like oxygen, is also attracted to another electronegative atom nearby. This is much stronger than the weaker Van der Waals forces that you'd find in non-polar molecules like cyclohexane \(\mathrm{C}_6\mathrm{H}_12\), where temporary dipoles induce a fleeting attraction. The stark difference in the type of intermolecular forces between polar and non-polar molecules highlights why they don't mix—like trying to connect the wrong ends of two magnets.

The 'like dissolves like' principle is straightforward but crucial in understanding solubility. It states that solvents can typically dissolve solutes that have a similar polarity to them. It's not just a rule of thumb; it's a reflection of the compatibility between molecules.

In the case of ethanol (polar) and cyclohexane (non-polar), their differing polarities mean they're not compatible. The polar ethanol molecules cling to each other because of strong hydrogen bonds, while the non-polar cyclohexane molecules don't have the right kind of attraction to break these bonds. This results in ethanol not being soluble in cyclohexane—a classic example of the principle in action. So, next time you're trying to dissolve something, remember to match polar with polar or non-polar with non-polar, and you'll likely be successful.