Problem 31 - Chemistry The Central Science In Si Units

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?

Step-by-Step Solution

Answer

(a) Stearic acid is more soluble in carbon tetrachloride. (b) Dioxane is more soluble in water than cyclohexane.

Step 1: Understanding Solubility of Stearic Acid

Stearic acid is a long-chain fatty acid with a hydrophobic tail ( \(\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{16}\) ) and a carboxylic acid hydrophilic head (\( \mathrm{COOH} \)). Water is polar and dissolves polar substances or those that can form hydrogen bonds, while carbon tetrachloride (\( \mathrm{CCl}_4 \)) is nonpolar and dissolves nonpolar substances. Given that most of stearic acid is nonpolar, we expect stearic acid to be more soluble in \( \mathrm{CCl}_4 \) than in water.

Step 2: Analyzing Solubility of Cyclohexane and Dioxane

Cyclohexane is a nonpolar hydrocarbon while dioxane is an ether that contains two oxygen atoms, making it polar and capable of forming hydrogen bonds with water. Water, being polar, will dissolve substances that are also polar or can engage in hydrogen bonding. Therefore, dioxane, with its oxygen atoms, is more likely to dissolve in water compared to the nonpolar cyclohexane.

Key Concepts

Stearic Acid

Stearic acid is an interesting molecule because it has both polar and nonpolar components. Its long hydrocarbon chain, known as a hydrophobic tail, consists of 18 carbon atoms in a linear arrangement. This makes the majority of stearic acid nonpolar. However, its chemical structure also ends with a carboxylic acid group (\( \text{COOH} \)), which is polar. This is often referred to as the hydrophilic head.

Polarity in molecules influences their solubility in different solvents. Polar molecules tend to dissolve well in polar solvents while nonpolar molecules will dissolve in nonpolar solvents. For stearic acid, its nonpolar tail makes it largely insoluble in water but more soluble in nonpolar solvents like carbon tetrachloride (\( \text{CCl}_4 \)). Understanding the dual nature of stearic acid can help in predicting its behavior in various chemical environments.

Polar and Nonpolar Solvents

Solubility relies heavily on the interaction between solute and solvent molecules. Polar solvents, such as water, have molecules with an uneven distribution of electrical charge, which allows them to dissolve other polar substances or molecules capable of forming hydrogen bonds.

Nonpolar solvents, on the other hand, have molecules with a more balanced charge distribution. These solvents include compounds like carbon tetrachloride (\( \text{CCl}_4 \)) and cyclohexane, which dissolve nonpolar substances due to similar intermolecular forces. The rule of thumb "like dissolves like" applies here, meaning that solutes dissolve best in solvents that have a similar polarity.

Hydrogen Bonds

Hydrogen bonds are a special type of dipole-dipole attraction that occurs between molecules. These bonds happen when a hydrogen atom, covalently bonded to a highly electronegative atom like oxygen or nitrogen, experiences an attraction to another electronegative atom from a different molecule.

In water, hydrogen bonds are responsible for many of its unique properties, including its solvent capabilities. Substances that can form hydrogen bonds with water, such as those with polar nitrogen or oxygen atoms, tend to dissolve well in water. This is why dioxane, which has oxygen atoms capable of forming hydrogen bonds, is more soluble in water compared to nonpolar substances like cyclohexane.

Cyclohexane

Cyclohexane is a simple hydrocarbon with a structure reminiscent of a hexagon. It belongs to the class of nonpolar solvents because its molecular structure leads to uniform charge distribution among its atoms. This lack of polarity means it does not mix well with polar substances like water.

Cyclohexane mainly dissolves substances that are also nonpolar, adhering to the principle that "like dissolves like." This makes it a useful solvent in certain chemical reactions and processes where polar interference must be minimized.

Dioxane

Dioxane is a polar solvent derived from ether, recognizable by its two oxygen atoms embedded within its cyclic structure. This arrangement permits dioxane to form hydrogen bonds, facilitating its interaction with water.

As a result, dioxane is much more soluble in water than nonpolar solvents like cyclohexane. It is often used in laboratory settings for reactions where its ability to dissolve a wide range of polar substances is beneficial. Dioxane’s polar nature and water compatibility make it suitable for applications needing a solvent that can efficiently manage polar compounds. Understanding these properties helps chemists choose the correct solvent for their specific experimental needs.