- Look up and draw the structures for each of the following molecules: (a) glucose (b) naphthalene (c) dimethyl ether (d) alanine For each molecule, would you expect greater solubility in water or in hexane? For each case, indicate the kinds of intermolecular forces that occur between then solute and the solvent in which the molecule is most soluble.
- Look up and draw the structures for each of the following molecules:
(a) glucose (b) naphthalene (c) dimethyl ether (d) alanine
For each molecule, would you expect greater solubility in water or in hexane? For each case,
indicate the kinds of intermolecular forces that occur between then solute and the solvent in
which the molecule is most soluble.
The Correct Answer and Explanation is :
Let’s examine the molecular structures and solubility characteristics of the following compounds:
(a) Glucose
Glucose is a monosaccharide with the chemical formula C₆H₁₂O₆. Its structure consists of a six-membered ring with five hydroxyl groups (-OH) attached to the carbon atoms. These hydroxyl groups enable glucose to form hydrogen bonds with water molecules, making it highly soluble in water. In contrast, glucose’s extensive hydrogen bonding makes it poorly soluble in nonpolar solvents like hexane.
(b) Naphthalene
Naphthalene is an aromatic hydrocarbon with the formula C₁₀H₈. It features two fused benzene rings, resulting in a nonpolar, planar structure. Due to its nonpolarity, naphthalene is more soluble in nonpolar solvents like hexane, where London dispersion forces are the primary intermolecular interactions. Its solubility in water is minimal because water’s hydrogen bonding cannot effectively interact with naphthalene’s nonpolar structure.
(c) Dimethyl Ether
Dimethyl ether (DME) has the formula CH₃OCH₃ and consists of an oxygen atom bonded to two methyl groups. The oxygen atom imparts a slight polarity to the molecule, allowing for dipole-dipole interactions. However, DME’s nonpolar methyl groups reduce its overall polarity. This moderate polarity makes DME more soluble in nonpolar solvents like hexane, where London dispersion forces are the primary intermolecular interactions. Its solubility in water is limited due to the mismatch between its polarity and water’s hydrogen bonding capabilities.
(d) Alanine
Alanine is an amino acid with the formula C₃H₇NO₂. It contains both an amino group (-NH₂) and a carboxyl group (-COOH), enabling it to form hydrogen bonds with water molecules. Additionally, at physiological pH, alanine exists as a zwitterion, carrying both a positive and a negative charge, which enhances its solubility in water through ionic interactions. In contrast, alanine’s solubility in nonpolar solvents like hexane is minimal due to its polarity and the lack of compatible intermolecular forces.
Summary
- Glucose: Highly soluble in water due to extensive hydrogen bonding; poorly soluble in hexane.
- Naphthalene: Highly soluble in hexane due to London dispersion forces; poorly soluble in water.
- Dimethyl Ether: More soluble in hexane due to moderate polarity and London dispersion forces; limited solubility in water.
- Alanine: Highly soluble in water due to hydrogen bonding and ionic interactions; poorly soluble in hexane.
These solubility behaviors are governed by the principle that “like dissolves like,” where polar compounds are more soluble in polar solvents like water, and nonpolar compounds are more soluble in nonpolar solvents like hexane.