Question

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

Short answer

Alanine is expected to be more soluble in water than in hexane due to its amphiphilic nature and the solubility principle "like dissolves like". The polar regions of alanine interact favorably with polar water molecules, while the nonpolar side chain has weaker interactions in water. In hexane, the nonpolar side chain interacts favorably, but the polar groups have weaker interactions, leading to overall lower solubility in hexane.

Step-by-step solution

Determine the polarity of alanine

To determine the solubility of alanine in water or hexane, we need to understand its polarity. Alanine is an amino acid with the molecular formula C3H7NO2. Its structure consists of an amino group (-NH2), a carboxyl group (-COOH), and a side chain methyl group (-CH3) attached to a central carbon atom. The amino group and carboxyl group are polar due to the presence of electronegative oxygen and nitrogen atoms. The side chain methyl group is nonpolar. This makes alanine an amphiphilic molecule, having both polar and nonpolar regions.

Understanding the solubility principle

The solubility of a substance is determined by the principle "like dissolves like". This means that polar substances tend to dissolve well in polar solvents, while nonpolar substances dissolve well in nonpolar solvents. Water is a polar solvent, with a partial negative charge on the oxygen atom and partial positive charges on the hydrogen atoms due to the difference in electronegativity. Hexane is a nonpolar solvent, with a structure consisting of carbon and hydrogen atoms with relatively equal electronegativity, resulting in a uniform distribution of electron density and no polar regions.

Comparing solubility in water and hexane

Alanine, being an amphiphilic molecule, has both polar and nonpolar regions. The polar regions of alanine are expected to interact well with the polar water molecules through hydrogen bonding and dipole-dipole interactions, while the nonpolar side chain will have weaker interactions in water due to hydrophobic effects. In hexane, the nonpolar side chain of alanine will interact favorably through London dispersion forces with the nonpolar hexane molecules, while the polar amino and carboxyl groups will have weaker interactions in the nonpolar hexane environment.

Conclusion

Based on the amphiphilic nature of alanine and the solubility principle "like dissolves like", alanine is expected to be more soluble in water than in hexane. This is because the polar regions of alanine will have stronger interactions with the polar water molecules, while the nonpolar side chain will have weaker interactions in water. In hexane, the nonpolar side chain will interact favorably, but the polar groups will have weaker interactions, leading to overall lower solubility in hexane.

Key concepts

Amino Acids

Amino acids are organic compounds that have both amino (-NH2) and carboxylic acid (-COOH) functional groups. These building blocks are crucial in forming proteins, which are essential for life processes. Each amino acid has a central carbon atom, also known as the alpha carbon, to which these functional groups are attached. Additionally, a variable R group is connected to this central carbon, influencing the properties of the amino acid. The specific characteristics of each R group determine whether the amino acid is polar, nonpolar, or exhibits other properties.
Understanding amino acids helps us predict the behavior of proteins in various environments. They can be amphiphilic, meaning they contain both polar and nonpolar parts, which allows them to interact with different surrounding molecules. Alanine, for instance, has a nonpolar methyl group as its R group, yet it still possesses the polar amino and carboxyl groups. This makes alanine versatile for interactions in biological systems.

Polar and Nonpolar Molecules

Molecules can be classified based on their polarity, which is determined by the distribution of electrons and the presence of electronegative atoms.
A polar molecule has regions with slight positive and negative charges due to differences in electronegativity. For example, in water (H2O), the oxygen atom is more electronegative than hydrogen, resulting in a partial negative charge on oxygen and a partial positive charge on hydrogen. This molecular characteristic makes polar molecules like water capable of forming hydrogen bonds and interacting with other polar substances.
Nonpolar molecules, on the other hand, do not have distinct charged regions as their electron distribution is more even. Molecules like hexane (C6H14) fall into this category due to the equal sharing of electrons between carbon and hydrogen atoms. This uniform electron distribution results in London dispersion forces being the predominant interaction, which are weaker than the hydrogen bonds in polar molecules.
Understanding the difference between polar and nonpolar molecules is key in predicting solubility and interaction behaviors in various solvents.

Solubility Principle

The solubility principle, often summarized as "like dissolves like," is fundamental in chemistry. This principle explains how substances dissolve based on their polar or nonpolar nature.
Polar substances such as salts and sugars are soluble in polar solvents like water because they can form interactions like hydrogen bonds or dipole-dipole forces with water molecules. These interactions allow the solvent to effectively surround and separate the solute molecules, leading to dissolution.

• Polar solvents dissolve polar solutes.
• Nonpolar solvents dissolve nonpolar solutes.

On the other hand, nonpolar substances such as oils dissolve well in nonpolar solvents like hexane. In this case, dispersion forces help dissolve the nonpolar solutes as they distribute evenly throughout the solvent.
Recognizing this principle helps predict how different molecules will behave in varying environments. For instance, an amino acid like alanine, with both polar and nonpolar characteristics, will have different solubility behaviors depending on the solvent. In polar solvents like water, the polar regions of alanine will interact well and lead to increased solubility. Conversely, in a nonpolar solvent like hexane, the nonpolar regions of alanine may interact more favorably, yet overall solubility would be reduced due to poor interaction of the polar regions with the nonpolar solvent.