Question

For cach compound, would you expect greater solubility in water or in hexane? Indicate the kinds of intermolecular forces that occur between the solute and the solvent in which the molecule is most soluble. a. glucose b. naphthalene c. dimethyl ether d. alanine (an amino acid)

Short answer

a. Glucose is more soluble in water (hydrogen bonding). b. Naphthalene is more soluble in hexane (London Dispersion Forces). c. Dimethyl ether is more soluble in hexane (London Dispersion Forces). d. Alanine is more soluble in water (hydrogen bonding, ionic interactions).

Step-by-step solution

- Analyze Solubility of Glucose

Glucose is a polar molecule and has multiple -OH (hydroxyl) groups that can form hydrogen bonds with water. Water is a polar solvent, which makes it capable of dissolving other polar molecules through dipole-dipole interactions and hydrogen bonding.

- Analyze Solubility of Naphthalene

Naphthalene is a nonpolar molecule, consisting of fused benzene rings with no polar groups. Hexane is a nonpolar solvent, thus it can dissolve nonpolar substances like naphthalene through London Dispersion Forces (van der Waals forces).

- Analyze Solubility of Dimethyl Ether

Dimethyl ether, although it has an ether group, is relatively nonpolar and has only a slight dipole moment. Its solubility in water is limited and it is more soluble in hexane because hexane can better accommodate its nonpolar character through London Dispersion Forces.

- Analyze Solubility of Alanine

Alanine is an amino acid with both a carboxyl group (-COOH) and an amino group (-NH2), making it capable of forming hydrogen bonds with water. Additionally, amino acids are zwitterions at physiological pH, having both positive and negative charges, which increases their solubility in polar solvents like water.

Key concepts

Intermolecular Forces

Understanding the behavior of molecules in various environments starts with an appreciation for intermolecular forces. These are the forces between molecules, which influence the physical properties of substances, including melting and boiling points, vapor pressure, and, notably, solubility.

There are several types of intermolecular forces that one might encounter. London Dispersion Forces, the weakest among them, occur in all molecules. They result from temporary shifts in electron density, which create instantaneous dipoles that induce dipoles in neighboring molecules. This force is dominant in nonpolar molecules like naphthalene, allowing them to dissolve in nonpolar solvents such as hexane.

Dipole-dipole interactions occur when polar molecules, with permanent dipoles due to uneven distribution of electrons, interact with each other. Molecules with strong dipole-dipole interactions, such as glucose with multiple hydroxyl groups (-OH), are typically more soluble in polar solvents like water.

Hydrogen Bonding

Among the star players in intermolecular attractions is hydrogen bonding, a special type of dipole-dipole interaction that arises when a hydrogen atom is bound to a highly electronegative atom such as oxygen, nitrogen, or fluorine. This atom then interacts with an electronegative atom from another molecule.

Consider glucose and alanine, both of which exhibit hydrogen bonding. Hydrogen bonding substantially increases their solubility in water, which itself can forming hydrogen bonds. For glucose, the -OH hydroxyl groups create numerous opportunities for hydrogen bonding. In the case of alanine, both the amino group (-NH2) and the carboxyl group (-COOH) can participate in hydrogen bonding. This solubility not only applies to alanine but to amino acids in general, which have significant roles in biological systems because of their ability to interact with the aqueous environment.

Polar and Nonpolar Molecules

The concept of molecule polarity is central to understanding solubility tendencies. Polar molecules, such as glucose and alanine, have an uneven distribution of electron density. This creates a dipole moment, with a partial positive charge on one side and a partial negative charge on the other. Due to this polarity, such molecules tend to be soluble in polar solvents like water because of the electrostatic interactions that align with the polarity of the water.

On the contrary, nonpolar molecules like naphthalene and dimethyl ether lack significant differences in electron density across their structures, resulting in no permanent dipole moment. Their solubility in nonpolar solvents like hexane is attributed to the ability of the solvent to engage in London Dispersion Forces, which are suitable for dissolving molecules without pronounced charges or polarity. Solubility depends on the 'like dissolves like' principle, where polar solutes are soluble in polar solvents, and nonpolar solutes in nonpolar solvents.