Question

Which of the following is moderately soluble both in water and in benzene \(\left[\mathrm{C}_{6} \mathrm{H}_{6}(1)\right]\), and why? (a) 1 -butanol, \(\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{2} \mathrm{CH}_{2} \mathrm{OH} ;\) (b) naphthalene, \(\mathrm{C}_{10} \mathrm{H}_{8} ;\) (c) hexane, \(\mathrm{C}_{6} \mathrm{H}_{14} ;\) (d) \(\mathrm{NaCl}(\mathrm{s})\)

Short answer

1-butanol is moderately soluble in both water and benzene due to the presence of both polar (hydroxyl group) and nonpolar (alkyl chain) characteristics.

Step-by-step solution

Understand the molecular structure

Evaluate each option and determine whether the substance is polar or nonpolar. Polar substances will have a significant difference in electronegativity between the atoms, while nonpolar substances will not. This step is important to determine which substances can dissolve in both solvents.

Evaluate Option (a)

1 -butanol, \(\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{2}\mathrm{CH}_{2} \mathrm{OH}\), is a polar molecule due to the presence of the -OH (hydroxyl) group, which contains a polar O-H bond. Therefore, it is soluble in water. But, the longer alkyl chain also makes it somewhat soluble in nonpolar solvents like benzene.

Evaluate Option (b)

Naphthalene, \(\mathrm{C}_{10}\mathrm{H}_{8}\), is a nonpolar molecule due to symmetrical arrangement of electron charge distribution in it. So, it's not soluble in water but soluble in benzene.

Evaluate Option (c)

Hexane, \(\mathrm{C}_{6} \mathrm{H}_{14}\), is a nonpolar substance because of the symmetrical arrangement of its atoms. Therefore, hexane does not dissolve in water, but it is soluble in benzene.

Evaluate Option (d)

Sodium Chloride, \(\mathrm{NaCl}(\mathrm{s})\), is an ionic compound and it's quite polar. As a result, it is soluble in polar solvent like water but not in a nonpolar solvent like benzene.

Key concepts

Polar and Nonpolar Molecules

When we talk about solubility, understanding polar and nonpolar molecules can be extremely helpful. Molecules can either have a uniform distribution of electrical charge or an uneven one.

Polar molecules have an uneven distribution of electrons. This means that one side of the molecule may be slightly positive while the other side is slightly negative. A common example of a polar molecule is water, with its slight positive and negative ends due to its bent shape and O-H bonds.

Nonpolar molecules, on the other hand, have an even distribution of electrical charge. These molecules do not have distinct poles. Naphthalene and hexane from our exercise are examples of nonpolar substances. Such molecules generally do not dissolve well in polar solvents like water but mix well with nonpolar solvents like benzene.

Understanding the polarity of a molecule can help predict its solubility behavior in different solvents.

Solvent Compatibility

Solvent compatibility refers to the ability of a substance to dissolve in a given solvent. This depends largely on the principle that "like dissolves like," meaning polar substances dissolve in polar solvents, and nonpolar substances dissolve in nonpolar solvents.

This principle helps explain why 1-butanol, with its polar -OH group, can dissolve in water. Water is a polar solvent and attracts other polar substances, making them dissolve.

In contrast, the nonpolar nature of naphthalene makes it compatible with nonpolar solvents like benzene. This compatibility enables naphthalene to dissolve in benzene while mostly repelling a polar solvent like water.

Recognizing which solvents are compatible with a given substance is key to anticipating whether a substance will dissolve or not.

Molecular Structure

The molecular structure of a substance greatly influences its solubility. Different parts of a molecule can be responsible for its ability to dissolve.

In 1-butanol, the -OH group plays a crucial role in establishing its polar character. This hydroxyl group is a common polar functional group in organic chemistry, making the molecule partially soluble in water.

However, the molecule also has a longer hydrophobic (water-repelling) alkyl chain. This makes it somewhat compatible with nonpolar solvents, allowing it to also have moderate solubility in benzene.

Understanding the significance of different molecular parts helps in predicting solubility across various solvents.

Chemical Polarity

Chemical polarity is related to the concept of polar and nonpolar molecules and is dictated by the difference in electronegativity between atoms in a molecule.

Electronegativity refers to the strength with which an atom attracts electrons. If two bonded atoms have a significant difference in electronegativity, the bond is polar.

Consider sodium chloride (NaCl): Na has low electronegativity, and Cl has high electronegativity. This large difference creates a polar ionic bond, allowing NaCl to dissolve in water, another polar substance.

Understanding chemical polarity helps explain why some compounds dissolve in certain solvents and not others. For instance, although hexane is nonpolar, its similar polarity to benzene allows it to dissolve readily in benzene.