Question

For each of the following pairs, predict which substance is more soluble in water. a. \(\mathrm{CH}_{3} \mathrm{NH}_{2}\) or \(\mathrm{NH}_{3}\) b. \(\mathrm{CH}_{3} \mathrm{CN}\) or \(\mathrm{CH}_{3} \mathrm{OCH}_{3}\) c. \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) or \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{3}\) d. \(\mathrm{CH}_{3} \mathrm{OH}\) or \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) e. \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CCH}_{2} \mathrm{OH}\) or \(\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{6} \mathrm{OH}\) f. \(\mathrm{CH}_{3} \mathrm{OCH}_{3}\) or \(\mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H}\)

Short answer

a. \(\mathrm{NH}_{3}\) is more soluble in water. b. \(\mathrm{CH}_{3} \mathrm{OCH}_{3}\) is more soluble in water. c. \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) is more soluble in water. d. \(\mathrm{CH}_{3} \mathrm{OH}\) is more soluble in water. e. \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CCH}_{2} \mathrm{OH}\) is more soluble in water. f. \(\mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H}\) is more soluble in water.

Step-by-step solution

Identify polar molecules

Both molecules have a polar N-H bond. \(\mathrm{NH}_{3}\) has hydrogen bonding, while \(\mathrm{CH}_{3} \mathrm{NH}_{2}\) also has it but with an additional nonpolar methyl group.

Predict the solubility

Both substances are polar and can hydrogen bond with water. However, \(\mathrm{CH}_{3} \mathrm{NH}_{2}\) has a nonpolar methyl group, which slightly reduces its solubility in water. Therefore, \(\mathrm{NH}_{3}\) is more soluble in water. b. \(\mathrm{CH}_{3} \mathrm{CN}\) or \(\mathrm{CH}_{3} \mathrm{OCH}_{3}\)

Identify polar molecules

\(\mathrm{CH}_{3} \mathrm{CN}\) has a polar carbon-nitrogen bond. On the other hand, \(\mathrm{CH}_{3} \mathrm{OCH}_{3}\) has a polar carbon-oxygen bond and can form stronger hydrogen bonds with water.

Predict the solubility

As \(\mathrm{CH}_{3} \mathrm{OCH}_{3}\) can form stronger hydrogen bonds with water, it will be more soluble in water than \(\mathrm{CH}_{3} \mathrm{CN}\). c. \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) or \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{3}\)

Identify polar molecules

\(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) has a polar oxygen-hydrogen bond and can form hydrogen bonds with water. \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{3}\) is a nonpolar molecule as it only contains nonpolar C-H and C-C bonds.

Predict the solubility

Since \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) can form hydrogen bonds with water while \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{3}\) is nonpolar, \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) will be more soluble in water. d. \(\mathrm{CH}_{3} \mathrm{OH}\) or \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\)

Identify polar molecules

Both molecules have a polar oxygen-hydrogen bond and can form hydrogen bonds with water. The difference is that \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) has an additional nonpolar methyl group.

Predict the solubility

Both molecules can hydrogen bond with water, but \(\mathrm{CH}_{3} \mathrm{OH}\) has less nonpolar character. Therefore, \(\mathrm{CH}_{3} \mathrm{OH}\) will be more soluble in water. e. \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CCH}_{2} \mathrm{OH}\) or \(\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{6} \mathrm{OH}\)

Identify polar molecules

Both molecules have polar oxygen-hydrogen bonds and can hydrogen bond with water. However, both substances also have large nonpolar hydrocarbon chains which decrease their solubility in water.

Predict the solubility

Since both molecules can hydrogen bond with water, the one with the shorter nonpolar hydrocarbon chain will be more soluble in water. Therefore, \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CCH}_{2} \mathrm{OH}\) is more soluble in water compared to \(\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{6} \mathrm{OH}\). f. \(\mathrm{CH}_{3} \mathrm{OCH}_{3}\) or \(\mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H}\)

Identify polar molecules

\(\mathrm{CH}_{3} \mathrm{OCH}_{3}\) has a polar carbon-oxygen bond and can form hydrogen bonds with water. \(\mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H}\) has a polar oxygen-hydrogen bond, and multiple polar carbon-oxygen bonds, which allows for stronger hydrogen bonding with water.

Predict the solubility

Since \(\mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H}\) can form stronger hydrogen bonds with water due to the presence of multiple polar bonds, it will be more soluble in water compared to \(\mathrm{CH}_{3} \mathrm{OCH}_{3}\).

Key concepts

Hydrogen Bonding

Hydrogen bonding is a type of strong intermolecular force that significantly influences the solubility of substances in water. It occurs when a hydrogen atom, which is covalently bonded to a highly electronegative atom like oxygen, nitrogen, or fluorine, interacts with another electronegative atom. This results in an attraction that is stronger than regular dipole-dipole interactions. This bonding is crucial for the solubility of molecules in water because water is a polar molecule itself and can participate in hydrogen bonding.

Consider molecules like ammonia (\(\mathrm{NH}_{3}\)) or methanol (\(\mathrm{CH}_{3}\mathrm{OH}\)); their ability to form hydrogen bonds makes them more soluble in water. The more opportunities a molecule has to form hydrogen bonds, the greater its solubility in water. For example, methanol has an -OH group that can form hydrogen bonds, enhancing its solubility.

When molecules cannot form hydrogen bonds or do so less effectively due to the presence of nonpolar groups, their solubility decreases. Therefore, understanding hydrogen bonding is key in predicting how well a substance will dissolve in water.

Polar Molecules

Polar molecules have an uneven distribution of electron density, resulting in a molecule that has a distinct positive and negative side or pole. This polarity due to a difference in electronegativity between the atoms results in a molecule with a dipole moment.

These molecules are exceptionally important in chemistry, especially concerning solubility in polar solvents such as water. Substances with significant polar characteristics, like \(\mathrm{CH}_{3}\mathrm{OH}\) or \(\mathrm{CH}_{3}\mathrm{CN}\), tend to be more soluble in water because 'like dissolves like.' Water, being a polar molecule, is good at dissolving other polar substances due to the attractive forces between oppositely charged ends of the molecules.

In contrast, when a polar molecule also contains significant nonpolar components, its overall solubility may be affected. The presence of longer nonpolar chains in substances can reduce the effect of the polar groups, thus decreasing solubility in water. Therefore, the architecture of the molecule significantly impacts its solubility due to the presence of polar and nonpolar parts.

Nonpolar Groups

Nonpolar groups in molecules consist of atoms that share electrons more equally, resulting in no significant charge separation within the molecule. Common nonpolar groups include hydrocarbon chains like -CH\(_3\) or -C\(\mathrm{H}_{2}\) chains. These groups do not engage effectively in interactions like hydrogen bonding with polar solvents.

When nonpolar groups are present in a molecule, they tend to decrease its solubility in polar solvents like water. This is because they cannot form favorable interactions with the polar water molecules. For example, \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{CH}_{3}\) (propane) is predominantly nonpolar, making it less soluble in water than an alcohol with the same number of carbon atoms.

The influence of nonpolar groups becomes more pronounced as their size increases; large hydrocarbon chains tend to overpower the effect of any small polar groups present in the molecule, reducing overall solubility in water. Understanding nonpolar groups' effect is critical in predicting and explaining the solubility behaviors of different compounds in water and similar polar solvents.