Problem 33 - Chemistry The Central Science

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

(a) \(\mathrm{CCl}_{4}\), (b) Benzene, and (c) Octanoic acid are more likely to be soluble in hexane due to their nonpolar or less polar nature. Hexane, being a nonpolar solvent, follows the principle of "like dissolves like".

Step 1: Determine the polarity of hexane and the solute compounds

Hexane is a nonpolar solvent. Its formula is \(\mathrm{C}_{6} \mathrm{H}_{14}\). Now, let's analyze the polarity of all the solute compounds given: (a) 1. Carbon Tetrachloride (\(\mathrm{CCl}_{4}\)) is a nonpolar molecule. It has a tetrahedral geometry, and the equal distribution of the chlorine atoms cancels out any dipole moments. 2. Calcium Chloride (\(\mathrm{CaCl}_{2}\)) is an ionic compound, which is highly polar. (b) 1. Benzene (\(\mathrm{C}_{6} \mathrm{H}_{6}\)) is a nonpolar molecule with a planar hexagonal structure. The electron cloud in the ring of carbons is delocalized and spread evenly across the whole molecule. 2. Glycerol (\(\mathrm{CH}_{2}(\mathrm{OH}) \mathrm{CH}(\mathrm{OH}) \mathrm{CH}_{2} \mathrm{OH}\)) is a polar compound due to the presence of three hydroxyl groups that contribute to hydrogen bonding. (c) 1. Octanoic acid (\(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{COOH}\)) is a fatty acid and has both a nonpolar hydrocarbon chain and a polar carboxyl group. However, its long hydrocarbon chain reduces the overall polarity. 2. Acetic acid (\(\mathrm{CH}_{3} \mathrm{COOH}\)) is a highly polar molecule with a carboxyl group.

Step 2: Predict the solubility in hexane based on polarity

Now, let's select the compound from each pair that is more likely to be soluble in hexane: (a) \(\mathrm{CCl}_{4}\) is likely to be more soluble in hexane since it is nonpolar, and hexane is nonpolar as well. (b) Benzene is likely to be more soluble in hexane as it is nonpolar and has similar properties to hexane, whereas glycerol is highly polar due to the presence of hydroxyl groups. (c) Octanoic acid is expected to be more soluble in hexane since it has a long nonpolar hydrocarbon chain, even though it also has a polar carboxyl group. Acetic acid has a higher polarity due to the carboxyl group, thus being less soluble in hexane. To summarize, the more likely soluble compounds in hexane are: (a) \(\mathrm{CCl}_{4}\), (b) Benzene, and (c) Octanoic acid.

Key Concepts

Polarity

Polarity is a fundamental concept in chemistry that describes the distribution of electric charge around atoms, molecules, or chemical groups. Polarity arises from the differences in electronegativity between bonded atoms, which creates partial positive and negative charges, often leading to dipole moments. These dipole moments determine whether a molecule is polar or nonpolar.

• A polar molecule has an uneven distribution of electrons, resulting in distinct positive and negative poles. Water is a classic example of a polar molecule.
• Nonpolar molecules, on the other hand, have an even distribution of electrons, and their dipole moments cancel out, leading to no overall charge imbalance.

Understanding polarity is crucial because it determines how molecules interact with each other, including their solubility in different solvents. The general rule "like dissolves like" helps predict solubility: polar solutes tend to dissolve in polar solvents, and nonpolar solutes in nonpolar solvents.

Nonpolar Solvents

Nonpolar solvents are liquids that possess an even electron distribution and lack significant charge separation. These solvents do not have the ability to form hydrogen bonds, which makes them distinct from polar solvents.

• Common nonpolar solvents include hexane, benzene, and carbon tetrachloride. These solvents are composed of molecules with symmetrical electron distribution.
• Nonpolar solvents are excellent for dissolving nonpolar substances like oils, fats, and other hydrocarbons.

Their insolubility with water, a polar solvent, means nonpolar solvents are often used in situations where water-based solutions might not be suitable. For example, hexane is used in extractions because it efficiently dissolves nonpolar compounds without mixing with water.

Hexane Solubility

Hexane is a classic example of a nonpolar solvent, which plays a significant role in solubility analysis. Hexane's chemical formula is \(\mathrm{C}_{6}\mathrm{H}_{14}\), consisting entirely of carbon (12 electrons) and hydrogen (28 electrons) atoms arranged in a linear chain, making it extremely nonpolar.
The solubility characteristics of hexane are aligned with the "like dissolves like" principle.

• Because hexane is nonpolar, it is ideal for dissolving other nonpolar substances, such as oils and certain organic molecules.
• This means compounds like carbon tetrachloride (\(\mathrm{CCl}_{4}\)) and benzene \((\mathrm{C}_{6}\mathrm{H}_{6})\), which have nonpolar characteristics, dissolve well in hexane.
• In contrast, highly polar substances like calcium chloride (\(\mathrm{CaCl}_{2}\)) do not dissolve in hexane.

By understanding the interaction between solute and solvent polarity, one can predict solubility patterns, making hexane a useful solvent in chemical reactions and extractions where water or other polar solvents might not suffice.