Question

(a) Would you expect stearic acid, \(\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{16} \mathrm{COOH},\) to be more soluble in water or in carbon tetrachloride? (b) Which would you expect to be more soluble in water, cyclohexane or dioxane?

Short answer

(a) Stearic acid would be more soluble in carbon tetrachloride due to its predominantly non-polar nature. (b) Dioxane would be more soluble in water than cyclohexane due to its polar nature and ability to form hydrogen bonds with water molecules.

Step-by-step solution

(a) Determine the polarity and solubility of stearic acid in water and carbon tetrachloride

Stearic acid is chemically represented as \(\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{16} \mathrm{COOH}\). The long carbon chain makes the compound mostly non-polar, but the carboxylic acid group \(\mathrm{COOH}\) has a polar nature due to the presence of electronegative oxygen atoms. Even though the molecule has that polar moiety, the large non-polar part will dominate the solubility preference of the molecule. Therefore, since carbon tetrachloride (\(\mathrm{CCl}_4\)) is a non-polar solvent, stearic acid will be more soluble in carbon tetrachloride than in polar water.

(b) Determine the polarity and solubility of cyclohexane and dioxane in water

Cyclohexane is a non-polar hydrocarbon molecule that contains only C and H atoms. Hence, it tends to be more soluble in non-polar solvents. On the other hand, dioxane is a molecule with oxygen atoms making it moderately polar due to the oxygen atoms interacting with the hydrogen atoms of water via hydrogen bonding. Comparing the solubility of cyclohexane and dioxane in water, dioxane is expected to be more soluble in water than cyclohexane due to its polar nature, and ability to form hydrogen bonds with water molecules.

Key concepts

Stearic Acid Solubility

Stearic acid is an intriguing compound when it comes to solubility, and its behavior provides a valuable lesson about molecular structure and solubility principles. Chemically, it is represented as \( \mathrm{CH}_3\left(\mathrm{CH}_2\right)_{16} \mathrm{COOH} \), featuring both a polar carboxylic acid group \( \mathrm{COOH} \) and a long non-polar hydrocarbon chain. This dual character influences its solubility.

• The non-polar hydrocarbon tail is quite large, overshadowing the smaller polar head.
• As a result, stearic acid is more soluble in non-polar solvents.

This leads to higher solubility in substances like carbon tetrachloride, a non-polar solvent, compared to water, which is polar. Understanding this concept allows one to predict solubility based on molecular structure.

Polarity in Chemistry

Polarity is a fundamental concept in chemistry that describes the distribution of electrical charge over the atoms joined by the bond. This distribution leads to the molecule obtaining a dipole moment.

• Molecules with uneven distribution of electrons are polar, like water (\( \mathrm{H}_2\mathrm{O} \)).
• Non-polar molecules have an even distribution of charge, for instance, methane (\( \mathrm{CH}_4 \)).

The presence of polar groups, such as \( \mathrm{OH} \) or \( \mathrm{COOH} \), can affect overall molecular polarity. However, a long non-polar segment in a molecule, such as in stearic acid, can reduce the molecule's apparent polarity.

Understanding polarity helps predict solubility patterns: polar solutes dissolve better in polar solvents, and non-polar solutes in non-polar solvents.

Solvent-Solute Interactions

Solvent-solute interactions are essential to determining solubility. These interactions are guided by the principle "like dissolves like," meaning polar solvents dissolve polar solutes, and non-polar solvents dissolve non-polar solutes.

• Polar solvents, like water, can form hydrogen bonds or dipole-dipole interactions with polar solutes.
• Non-polar solvents interact with non-polar solutes through dispersion forces.

For example, dioxane's solubility in water is due to its polar nature and the hydrogen bonding capacity, whereas cyclohexane, like stearic acid, is mostly non-polar and prefers non-polar environments.

The strength and type of interaction between a solvent and a solute's molecules ultimately affect their solubility, further emphasizing the importance of understanding molecular structures and intermolecular forces.