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 hydrocarbon chain length (\(n\)) of carboxylic acids with the general formula \(\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{n} \mathrm{COOH}\) increases, their solubility in water decreases due to the increased nonpolar nature of the molecule. In contrast, their solubility in hexane, a nonpolar solvent, increases due to the growing nonpolar characteristics of the compounds.

Step-by-step solution

Understand the chemical structure of carboxylic acids

Carboxylic acids contain a polar carboxyl (COOH) group. They have a hydrocarbon chain, which is nonpolar, with a length proportional to the integer \(n\). As the hydrocarbon chain length increases, the nonpolar portion of the molecule becomes larger.

Identify the molecular structure of solvents

Water (H2O) is a polar solvent that can form hydrogen bonds. It is important to note that polar solvents will dissolve polar solutes more readily. Hexane (C6H14), on the other hand, is a nonpolar solvent that cannot form hydrogen bonds. Nonpolar solvents are more likely to dissolve nonpolar solutes.

Analyze solubility in water

As the value of \(n\) increases in the carboxylic acid molecule, the nonpolar portion of the molecule (hydrocarbon chain) becomes larger, and the molecule becomes less polar overall. Since water is a polar solvent, it has more trouble dissolving carboxylic acids with a longer nonpolar hydrocarbon chain. Therefore, the solubility of carboxylic acids in water decreases as \(n\) increases.

Analyze solubility in hexane

As the hydrocarbon chain grows (an increase in \(n\)), the nonpolar portion of the carboxylic acid molecule also becomes larger. Hexane, a nonpolar solvent, will dissolve compounds with more significant nonpolar characteristics. Consequently, the solubility of carboxylic acids in hexane increases as \(n\) increases.

Summarize the findings

In conclusion, as the hydrocarbon chain length (value of \(n\)) of the carboxylic acids increases, the solubility in water decreases due to the increased nonpolar nature of the molecule. In contrast, the solubility in hexane, a nonpolar solvent, increases due to the growing nonpolar characteristics of the compounds.