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Chapter 13: Problem 30

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

Short Answer

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

Step by step solution

01

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.
02

Identify the Solvents

Water is a polar solvent capable of hydrogen bonding, whereas hexane is a nonpolar solvent that cannot form hydrogen bonds.
03

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.'
04

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.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

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鈥檛 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.

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Most popular questions from this chapter

Compounds like sodium stearate, called "surfactants" in general, can form structures known as micelles in water, once the solution concentration reaches the value known as the critical micelle concentration (cmc). Micelles contain dozens to hundreds of molecules. The cmc depends on the substance, the solvent, and the temperature. At and above the \(\mathrm{cmc}\), the properties of the solution vary drastically. (a) The turbidity (the amount of light scattering) of solutions increases dramatically at the \(\mathrm{cmc}\). Suggest an explanation. (b) The ionic conductivity of the solution dramatically changes at the cmc. Suggest an explanation. (c) Chemists have developed fluorescent dyes that glow brightly only when the dye molecules are in a hydrophobic environment. Predict how the intensity of such fluorescence would relate to the concentration of sodium stearate as the sodium stearate concentration approaches and then increases past the \(\mathrm{cmc}\)

At \(63.5^{\circ} \mathrm{C},\) the vapor pressure of \(\mathrm{H}_{2} \mathrm{O}\) is \(23.3 \mathrm{kPa},\) and that of ethanol \(\left(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\right)\) is \(53.3 \mathrm{kPa}\). A solution is made by mixing equal masses of \(\mathrm{H}_{2} \mathrm{O}\) and \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\). (a) What is the mole fraction of ethanol in the solution? (b) Assuming idealsolution behavior, what is the vapor pressure of the solution at \(63.5^{\circ} \mathrm{C} ?(\mathbf{c})\) What is the mole fraction of ethanol in the vapor above the solution?

The Baltic Sea has a salinity of \(1.0 \%\), that is, its water contains \(10 \mathrm{~g}\) of salt for every liter of solution. Assuming that the solute consists entirely of \(\mathrm{NaCl}\) (in fact, over \(90 \%\) of the salt is indeed \(\mathrm{NaCl}\), calculate the osmotic pressure of this seawater at \(15^{\circ} \mathrm{C}\).

Caffeine \(\left(\mathrm{C}_{8} \mathrm{H}_{10} \mathrm{~N}_{4} \mathrm{O}_{2}\right)\) is a stimulant found in coffee and tea. If a solution of caffeine in the solvent chloroform \(\left(\mathrm{CHCl}_{3}\right)\) has a concentration of \(0.0500 \mathrm{~m},\) calculate \((\mathbf{a})\) the percentage of caffeine by mass, (b) the mole fraction of caffeine in the solution.

Describe how you would prepare each of the following aqueous solutions: (a) \(1.50 \mathrm{~L}\) of \(0.110 \mathrm{M}\left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{4}\) solution, starting with solid \(\left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{4} ;\) (b) \(225 \mathrm{~g}\) of a solution that is \(0.65 \mathrm{~m}\) in \(\mathrm{Na}_{2} \mathrm{CO}_{3}\), starting with the solid solute; (c) 1.20 \(\mathrm{L}\) of a solution that is \(15.0 \% \mathrm{~Pb}\left(\mathrm{NO}_{3}\right)_{2}\) by mass (the density of the solution is \(1.16 \mathrm{~g} / \mathrm{mL}\) ), starting with solid solute; (d) a \(0.50 M\) solution of HCl that would just neutralize \(5.5 \mathrm{~g}\) of \(\mathrm{Ba}(\mathrm{OH})_{2}\) starting with \(6.0 \mathrm{MHCl}\).
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