Problem 30 - Chemistry The Central Science In Si Units

Question

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

Step-by-Step Solution

Answer

Alanine is expected to be more soluble in water than in hexane.

Step 1: Understand the Nature of Alanine

Alanine is an amino acid that contains both a carboxyl group (-COOH) and an amino group (-NH2). These groups are polar and can form hydrogen bonds.

Step 2: Identify the Solvents

Water is a polar solvent capable of hydrogen bonding, whereas hexane is a nonpolar solvent that cannot form hydrogen bonds.

Step 3: Determine Solubility Principles

Substances are more soluble in solvents with similar polarity. Polar substances dissolve better in polar solvents, and nonpolar substances dissolve better in nonpolar solvents. This is often summarized by the rule 'like dissolves like.'

Step 4: Apply Solubility Principles to Alanine

Since alanine is polar due to its carboxyl and amino groups, it is expected to be more soluble in a polar solvent like water than in a nonpolar solvent like hexane.

Key Concepts

Polar Solvents

Polar solvents have molecules with an uneven distribution of charge. This creates a partial positive side and a partial negative side within the molecule.
Water, for example, is a classic polar solvent. It is known for its ability to dissolve many substances.

• The polar nature of water allows it to engage in hydrogen bonding with other molecules.
• This makes it an excellent solvent for polar substances.

When thinking about the solubility of a compound like alanine, it's important to consider the solvent's polarity. Because polar solvents can arrange themselves around polar molecules, they effectively pull these molecules apart, allowing them to dissolve.

Nonpolar Solvents

Nonpolar solvents are characterized by molecules that do not have a significant charge separation. This means there's no distinct positive or negative side.
Hexane is a typical example of a nonpolar solvent. It lacks the charge that is useful in dissolving polar substances. Instead, nonpolar solvents are ideal for dissolving other nonpolar substances.

• Nonpolar molecules cluster together in nonpolar environments due to lack of attraction to polar molecules.
• In nonpolar solvents, this means that substances like oils or fats dissolve much more readily than polar substances.

So, when considering if alanine, a polar molecule, would dissolve in hexane, the answer is no. This is due to hexane lacking the polarity needed to break alanine's intermolecular forces.

Like Dissolves Like

The principle of 'like dissolves like' is a general rule in chemistry that describes how substances of similar polarity tend to dissolve in each other.
Polar solvents will dissolve polar solutes, and nonpolar solvents dissolve nonpolar solutes.

• This is because the intermolecular forces that govern the solubility of substances work best when the solvent and solute's polarity match.
• If you put a polar solute in a nonpolar solvent, they won’t interact in a way that allows the solute to dissolve.

This rule makes it easier to predict and understand why certain substances dissolve in particular solvents. When applied to alanine, this rule suggests it will dissolve well in water (polar) but not in hexane (nonpolar).

Hydrogen Bonding

Hydrogen bonding is a strong type of dipole-dipole attraction between molecules. It occurs when a hydrogen atom is bonded to a highly electronegative element like nitrogen, oxygen, or fluorine.
In water, hydrogen bonds are responsible for its high boiling point and solvent properties.

• Alanine has both a carboxyl group (-COOH) and an amino group (-NH2), which can form hydrogen bonds.
• This makes it possible for alanine to engage in hydrogen bonding with water molecules.

Since hydrogen bonds are one of the strongest types of intermolecular attractions, they significantly increase the solubility of polar molecules like alanine in polar solvents like water. This is why alanine is more soluble in water than in hexane.