Problem 30 - Chemistry The Central Science

Question

Would you expect alanine (an amino acid) to be more soluble in water or in hexane?

Step-by-Step Solution

Answer

Alanine, an amino acid, would be more soluble in water than in hexane because it has both polar (NH2 and COOH) and nonpolar (CH3) groups. The polar groups can form hydrogen bonds with water molecules, a polar solvent, making it more soluble in water. On the other hand, hexane is a nonpolar solvent and unable to form hydrogen bonds with alanine's polar groups, resulting in less solubility in hexane.

Step 1: Identify the structure of alanine

First, we need to examine the structure of alanine, which is an amino acid. The structure of alanine is: \[CH_3 – CH(NH_2) – COOH\] The molecule has a hydrophobic aliphatic chain (CH3), an amino group (NH2), and a carboxyl group (COOH).

Step 2: Analyze the polar and nonpolar parts of alanine

Now that we know the structure of alanine, we'll analyze its polar and nonpolar parts. The amino group (NH2) and carboxyl group (COOH) are polar due to the presence of electronegative atoms, like nitrogen and oxygen, which create regions of partial positive and negative charges on the molecule. These regions make alanine able to form hydrogen bonds, making it more attracted to polar solvents. On the other hand, the CH3 part of alanine is a nonpolar hydrophobic group, which will be more attracted to nonpolar solvents.

Step 3: Consider the "like dissolves like" principle

The principle "like dissolves like" states that polar solvents generally dissolve polar solutes, and nonpolar solvents dissolve nonpolar solutes. Since alanine has both polar and nonpolar parts, its solubility in a particular solvent will depend on the stronger attraction between the solvent and the solute.

Step 4: Compare solubility of alanine in water and hexane

Water is a polar solvent and able to form hydrogen bonds. Since alanine has polar groups (NH2 and COOH) that can form hydrogen bonds with water molecules, alanine is likely more soluble in water. Hexane, on the other hand, is a nonpolar solvent. It will not be able to form hydrogen bonds with alanine. Although alanine has a nonpolar CH3 group, the presence of polar groups in the molecule will make it less likely to dissolve in hexane.

Step 5: Conclude which solvent alanine is more soluble in

Based on the analysis of the polar and nonpolar groups in alanine and the solvents' properties, we can conclude that alanine would be more soluble in water (a polar solvent) than in hexane (a nonpolar solvent). This is because the polar parts of alanine can form hydrogen bonds with water molecules, resulting in a stronger attraction between alanine and water.