Problem 34 - Chemistry: The Central Science in SI Units, Global Edition

Question

Which of the following in each pair is likely to be more soluble in water: (a) cyclohexane \(\left(\mathrm{C}_{6} \mathrm{H}_{12}\right)\) or glucose \(\left(\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6}\right)\), (b) propionic acid \(\left(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH}\right)\) or sodium propionate \(\left(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COONa}\right),(\mathbf{c}) \mathrm{HCl}\) or ethyl chloride \(\left(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Cl}\right) ?\) Explain in each case.

Step-by-Step Solution

Answer

(a) Glucose, (b) Sodium propionate, (c) HCl

Step 1: Determine the polarity of each compound

First, let's assess the polarity of each compound in the pairs. Polar molecules have regions of partial positive and negative charge due to the uneven distribution of electron density in the molecule, while nonpolar molecules have an even distribution of electron density. (a) cyclohexane (C6H12) - nonpolar; glucose (C6H12O6) - polar (has hydroxyl groups) (b) propionic acid (CH3CH2COOH) - polar (has carboxylic acid group); sodium propionate (CH3CH2COONa) - ionic (has an ionic bond between sodium and carboxylate group) (c) HCl - polar (due to the difference in electronegativity between hydrogen and chlorine); ethyl chloride (CH3CH2Cl) - polar but less polar than HCl (large alkyl group with chlorine)

Step 2: Analyze the interactions between each compound and water molecules

Now, let's analyze the possible interactions between each compound and water, a polar solvent: (a) cyclohexane - weak van der Waals forces with water; glucose - hydrogen bonding with water (b) propionic acid - hydrogen bonding with water; sodium propionate - ion-dipole interactions with water (c) HCl - strong ion-dipole interactions with water due to dissociation into ions; ethyl chloride - weaker dipole-dipole interactions with water

Step 3: Determine which compound is more soluble in water for each pair

Based on polarity and interactions with water, we can determine which compound is more soluble in water: (a) Glucose is more soluble in water than cyclohexane, because it is polar and can form hydrogen bonds with water molecules. (b) Sodium propionate is more soluble in water than propionic acid, because it is ionic and can form strong ion-dipole interactions with water molecules. (c) HCl is more soluble in water than ethyl chloride, because it can form strong ion-dipole interactions with water molecules when completely dissociated, while ethyl chloride can only form weaker dipole-dipole interactions.

Key Concepts

Polarity and Solubility in Water

Polarity is a key factor in determining a molecule's solubility in water. Polar molecules have regions with slight positive and negative charges due to differences in electron distribution. This allows them to interact better with water, which is also a polar solvent. For instance, glucose is more soluble in water than cyclohexane. This is because glucose is a polar molecule with hydroxyl (-OH) groups that can form attractions with water. Conversely, cyclohexane is nonpolar, lacking the partial charges needed to interact with water effectively.

Hydrogen Bonding

Hydrogen bonding is a special kind of attraction that occurs when hydrogen is bonded to a highly electronegative element like oxygen, nitrogen, or fluorine. This interaction is crucial for solubility in water.

• In glucose, the abundance of hydroxyl groups allows it to form hydrogen bonds with water, increasing its solubility.
• Propionic acid can also form hydrogen bonds due to its carboxyl group, making it more soluble than compounds without such groups.

Hydrogen bonds are surprisingly strong for dipole interactions and significantly impact a substance's ability to dissolve in water.

Ion-Dipole Interactions

Ion-dipole interactions occur between ions and polar molecules. These are even stronger than hydrogen bonds and are crucial for the solubility of ionic compounds.

• Sodium propionate, an ionic compound, is more soluble in water due to these interactions. The sodium ions interact strongly with water's polar molecules.
• HCl, though covalent in structure, dissociates into ions in water. These ions engage in ion-dipole interactions, improving its solubility.

The strength of ion-dipole interactions often leads to better solubility compared to compounds that can only engage in dipole or hydrogen bonding.