Question

Consider a series of carboxylic acids whose general formula is \(\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{n} \mathrm{COOH}\). How would you expect the solubility of these compounds in water and in hexane to change as \(n\) increases? Explain.

Short answer

As the value of \(n\) increases in the carboxylic acids series with the general formula \(\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{n} \mathrm{COOH}\), their solubility in water is expected to decrease while their solubility in hexane is expected to increase, due to the different polarities of the solvents and the increasing hydrocarbon chain length contributing to a more nonpolar overall molecular structure.

Step-by-step solution

1. Understanding the solubility of compounds in water and hexane

The solubility of compounds in water and hexane comes down to the balance of polarity. Polar molecules tend to dissolve better in polar solvents like water, while nonpolar molecules dissolve better in nonpolar solvents like hexane. The solubility depends on the ability of these solvents to form interactions with the solute molecules, such as hydrogen bonds or van der Waals forces. Carboxylic acids have both a polar functional group (the -COOH group) and a nonpolar part (the hydrocarbon chain). As \(n\) increases, the length of the hydrocarbon chain increases, making the molecule more nonpolar overall.

2. Predicting solubility in water

Since water is a polar solvent, the polar carboxylic acid group can form hydrogen bonds with water molecules. However, as the hydrocarbon chain length (represented by \(n\)) increases, the nonpolar part of the carboxylic acid molecule also increases. This growing nonpolar region will make it less favorable for carboxylic acids to dissolve in water because the stronger hydrophobic interactions between the hydrocarbon chains and the disruption of the hydrogen bonding network between water molecules will reduce solubility. Therefore, as \(n\) increases, we would expect the solubility of carboxylic acids in water to decrease.

3. Predicting solubility in hexane

Hexane is a nonpolar solvent and the carboxylic acids have a hydrocarbon chain that will preferentially dissolve in it due to its nonpolar nature. As \(n\) increases, the length of the hydrocarbon chain also increases, making the carboxylic acid molecule more nonpolar and increasing its compatibility with the hexane solvent. In this case, as \(n\) increases, the solubility of carboxylic acids in hexane is expected to increase.

Conclusion

In summary, as the value of \(n\) increases in the carboxylic acids series with the general formula \(\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{n} \mathrm{COOH}\), their solubility in water is expected to decrease while their solubility in hexane is expected to increase. This behavior results from the different polarities of the solvents and the increasing hydrocarbon chain length contributing to a more nonpolar overall molecular structure.