Problem 29 - 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

In hexane, the more soluble compounds are: (a) CCl4 (b) Benzene (c) Octanoic acid

Step 1: Hexane is a nonpolar compound, as it consists of carbon and hydrogen atoms bonded together in an alkane chain. Due to the equal distribution of electron density and lack of a significant difference in electronegativity between carbon and hydrogen atoms, there's no net dipole moment in the hexane molecule. The absence of a net dipole moment indicates that hexane is a nonpolar solvent. #Step 2: Compare the polarity of the first pair: CCl4 and CaCl2#

(a) Carbon tetrachloride (CCl4) is a nonpolar molecule in which the four chlorine atoms are symmetrically distributed around the central carbon atom. Because of its symmetry, the molecule has no net dipole moment. Calcium chloride (CaCl2), on the other hand, is an ionic compound, and thus is highly polar. Considering hexane is a nonpolar solvent, CCl4, the nonpolar compound, is more likely to be soluble in hexane. #Step 3: Compare the polarity of the second pair: benzene and glycerol#

Step 2: (b) Benzene (C6H6) has six carbon atoms in a ring structure, with alternating single and double bonds between the carbon atoms, and hydrogen atoms bonded to each carbon atom. Although the molecule contains \(\pi\) electrons in the ring, due to its symmetry, benzene has no net dipole moment, making it a nonpolar molecule. Glycerol (CH2(OH)CH(OH)CH2OH) is a polar molecule, as it has three hydroxyl groups capable of forming hydrogen bonds. As hexane is nonpolar, benzene, the nonpolar compound, is more likely to be soluble in hexane. #Step 4: Compare the polarity of the third pair: octanoic acid and acetic acid#

(c) Octanoic acid (CH3(CH2)6COOH) is a carboxylic acid with a long hydrocarbon chain and a polar carboxyl group (-COOH) at one end. The long nonpolar hydrocarbon chain makes the molecule mostly nonpolar, despite the presence of the carboxyl group at one end. Acetic acid (CH3COOH) is also a carboxylic acid but with a considerably shorter hydrocarbon chain. As a result, the polar carboxyl group has a greater influence on the overall polarity of the molecule, making acetic acid a polar compound. Since hexane is a nonpolar solvent, octanoic acid, the more nonpolar compound, is more likely to be soluble in hexane. In conclusion: (a) CCl4 is more soluble in hexane (b) Benzene is more soluble in hexane (c) Octanoic acid is more soluble in hexane

Key Concepts

Polarity of Molecules

Understanding the polarity of molecules is integral when predicting solubility patterns in chemistry. Molecules are composed of atoms held together by chemical bonds, and the distribution of electrons within these bonds determines the molecule's overall polarity. When atoms have differing electronegativities, or tendencies to attract electrons, it creates an uneven distribution of charge.

This can lead to the formation of dipoles, where parts of the molecule have slight positive or negative charges. Polar molecules have significant dipole moments, which means they have distinct ends with partial positive and negative charges. Water is a classic example of a polar molecule, with its oxygen end being slightly negative and its hydrogen ends slightly positive.

Understanding the concept of polarity is essential when determining solubility because polar molecules tend to dissolve well in polar solvents, while nonpolar molecules prefer nonpolar solvents. This is due to the principle of 'like dissolves like' where substances with similar properties (polar/nonpolar) are more likely to be compatible or soluble in each other.

Nonpolar and Polar Solvents

Solvents can be broadly classified into two categories: nonpolar and polar. The polarity of a solvent determines what type of substances it can dissolve. Nonpolar solvents, such as hexane, have molecules with a fairly even distribution of electric charge, so they don't have poles with a positive or negative charge.

This means that nonpolar solvents are more suitable for dissolving other nonpolar substances due to the lack of strong intermolecular forces, such as dipole-dipole interactions, that are present in polar solvents. On the other hand, polar solvents, like water, have molecules with considerable separation of charge, resulting in a positive pole and a negative pole.

Their molecules can interact strongly with ionic compounds and other polar molecules through dipole-dipole interactions and hydrogen bonding, making them effective at dissolving substances with significant polarity. As a result, when we consider solubility, it's important to match the polarity of the solute (substance being dissolved) with the appropriate solvent for optimal solubility.

Carbon-Hydrogen Bonds

Carbon-hydrogen (C-H) bonds are prevalent in organic chemistry and play a vital role in determining a molecule's polarity and reactivity. These bonds are formed between carbon atoms and hydrogen atoms and are generally considered nonpolar. This is due to the fact that the electronegativity difference between carbon and hydrogen is very small, leading to an even distribution of electron density across the bond.

Therefore, molecules that are made up primarily of carbon-hydrogen bonds, such as alkanes and aromatic hydrocarbons like benzene, tend to be nonpolar. These hydrocarbon compounds, without any significant polar functional groups, have a high solubility in nonpolar solvents but are poorly soluble in polar solvents.

The understanding of C-H bonds is crucial in predicting solubility and reactivity of organic substances. In the context of solubility, the presence of a large number of C-H bonds in a molecule is a strong indicator that it will be more soluble in nonpolar solvents like hexane.