Problem 33 - Chemistry: The Central Science in SI Units, Global Edition

Question

Which of the following in each pair is likely to be more soluble in hexane, \(\mathrm{C}_{6} \mathrm{H}_{14}:\) (a) \(\mathrm{CCl}_{4}\) or \(\mathrm{CaCl}_{2}\), (b) benzene \(\left(\mathrm{C}_{6} \mathrm{H}_{6}\right)\) or glycerol, \(\mathrm{CH}_{2}(\mathrm{OH}) \mathrm{CH}(\mathrm{OH}) \mathrm{CH}_{2} \mathrm{OH},\) (c) octanoic acid, \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{COOH},\) or acetic acid, \(\mathrm{CH}_{3} \mathrm{COOH}\) ? Explain your answer in each case.

Step-by-Step Solution

Answer

The compounds more soluble in hexane are: (a) \(\mathrm{CCl}_4\), (b) benzene, and (c) octanoic acid. This is because hexane is a nonpolar solvent, and these compounds are overall less polar than their counterparts in each pair, following the principle "like dissolves like".

Step 1: Pair (a): \(\mathrm{CCl}_4\) or \(\mathrm{CaCl}_2\)

In this pair, we have \(\mathrm{CCl}_4\) (carbon tetrachloride), which is a nonpolar molecule, and \(\mathrm{CaCl}_2\) (calcium chloride), which is an ionic compound and thus highly polar. Based on the principle "like dissolves like", the more soluble compound in hexane is the nonpolar \(\mathrm{CCl}_4\).

Step 2: Pair (b): Benzene or Glycerol

In this pair, we have benzene (\(\mathrm{C}_6\mathrm{H}_6\)), which is a nonpolar aromatic compound, and glycerol (\(\mathrm{CH}_2(\mathrm{OH}) \mathrm{CH}(\mathrm{OH})\mathrm{CH}_2 \mathrm{OH}\)), which is a polar molecule due to the presence of multiple hydroxyl groups. Based on the principle "like dissolves like", the more soluble compound in hexane is the nonpolar benzene.

Step 3: Pair (c): Octanoic Acid or Acetic Acid

In this pair, we have octanoic acid (\(\mathrm{CH}_3\mathrm{CH}_2\mathrm{CH}_2\mathrm{CH}_2\mathrm{CH}_2\mathrm{CH}_2\mathrm{CH}_2\mathrm{COOH}\)), a linear nonpolar hydrocarbon chain with a polar carboxylic acid group at the end, and acetic acid (\(\mathrm{CH}_3\mathrm{COOH}\)), a smaller molecule that also has a polar carboxylic acid group. Although both molecules have polar carboxylic acid groups, the longer nonpolar hydrocarbon chain in octanoic acid makes it overall less polar than acetic acid. Therefore, octanoic acid is more likely to dissolve in hexane than acetic acid, based on the principle "like dissolves like".

Key Concepts

like dissolves like principle

The "like dissolves like" principle is a fundamental concept in chemistry that helps explain why certain substances dissolve in specific solvents. According to this principle, solubility is determined by the similarity in polarity between the solute and the solvent.

Substances tend to dissolve best in solvents that have a similar type of bonding and molecular structure. Essentially, nonpolar solutes dissolve well in nonpolar solvents, while polar solutes dissolve well in polar solvents. For example, nonpolar organic compounds like hexane are more inclined to dissolve other nonpolar substances due to their lack of strong intermolecular forces, such as hydrogen bonds.

This principle is particularly useful in organic chemistry to predict solubility. By comparing the polarities, we can make educated guesses about which solvents will dissolve which solutes. It helps simplify the understanding of complex molecular interactions by focusing on this matching principle.

nonpolar vs polar molecules

A clear understanding of nonpolar and polar molecules is crucial for predicting solubility using the "like dissolves like" concept.

**Nonpolar Molecules**: These molecules have an even distribution of electrical charge. They typically don't mix with water and other polar solvents. Nonpolar molecules include hydrocarbons like hexane and carbon tetrachloride ( CCl_4 ). Their structure is balanced, with no distinct positive or negative ends.

**Polar Molecules**: In contrast, polar molecules have areas of partial positive and negative charges, caused by a difference in electronegativity between atoms in a bond. This imbalance leads to the formation of dipoles. Water, ethanol, and glycerol are common polar molecules. They usually dissolve in polar solvents due to attractions between molecules.

Distinguishing between these types is key to understanding how substances interact in different environments, such as why calcium chloride ( CaCl_2 ) is soluble in water but not in hexane.

organic chemistry solubility

In organic chemistry, solubility plays a significant role in reactions and product manipulation. Understanding solubility in various solvents can be beneficial for experimentation and process improvements.

**Influence of Molecular Structure**: The solubility of organic compounds is influenced by their molecular structure. For example, long hydrocarbon chains are typically nonpolar, contributing to the solubility of compounds like octanoic acid in nonpolar solvents, despite having some polar functional groups.

**Solubility and Functional Groups**: Functional groups like -OH or COOH make a molecule polar, often increasing solubility in polar solvents. However, as seen in octanoic vs. acetic acid, the length of the nonpolar hydrocarbon chain can outweigh the polar influence of the COOH group, explaining why octanoic acid dissolves more readily in hexane compared to acetic acid.

Understanding these concepts helps chemists manipulate reactions by choosing appropriate solvents, improving yields, and conducting efficient separations.