Question

Solubility of Ethanol in Water Ethane \(\left(\mathrm{CH}_{3} \mathrm{CH}_{3}\right)\) and ethanol \(\left(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\right)\) differ in their molecular makeup by only one atom, yet ethanol is much more soluble in water than ethane. Describe the features of ethanol that make it more water soluble than ethane.

Short answer

Ethanol is more soluble in water than ethane due to its polar hydroxyl group, which allows for hydrogen bonding.

Step-by-step solution

Understand Molecular Structure

Ethanol \((\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH})\) is comprised of an ethyl group \((\mathrm{CH}_{3} \mathrm{CH}_{2}-)\) bound to a hydroxyl group \((-\mathrm{OH})\). Ethane \((\mathrm{CH}_{3} \mathrm{CH}_{3})\) has no polar groups, consisting only of carbon-hydrogen bonds.

Identify Functional Groups

Ethanol contains a hydroxyl group \((-\mathrm{OH})\), which is a polar functional group capable of forming hydrogen bonds with water molecules. Ethane lacks any such polar group, being a nonpolar hydrocarbon.

Analyze Intermolecular Forces

The polar hydroxyl group in ethanol allows it to engage in hydrogen bonding with water, which is a strong intermolecular force. In contrast, ethane can only participate in weaker van der Waals interactions.

Evaluate Solubility Factors

Water is a polar solvent that interacts favorably with other polar molecules. This makes ethanol more soluble in water due to its ability to form strong hydrogen bonds compared to the weak dispersion forces that ethane would experience.

Key concepts

Hydrogen Bonding

Hydrogen bonding is a particularly influential type of intermolecular force that occurs when a hydrogen atom is directly bonded to a highly electronegative atom such as oxygen, nitrogen, or fluorine. In the case of ethanol, the hydrogen atom of the hydroxyl group (-OH) can form hydrogen bonds with the oxygen atoms of water molecules.
This is because the oxygen atom in the hydroxyl group is significantly more electronegative than hydrogen, causing the hydrogen to be partially positively charged. This charge difference causes a strong attraction with the partial negative charge of the oxygen atom in water.

• Hydrogen bonds are stronger than other van der Waals forces such as dipole-dipole interactions or London dispersion forces.
• They contribute significantly to the solubility of polar substances in water.

Due to hydrogen bonding, ethanol creates a network of interactions with water, greatly enhancing its solubility compared to nonpolar molecules like ethane.

Polar Functional Groups

Polar functional groups are sections of molecules that have a significant charge difference, resulting in increased reactivity and interactions with polar substances like water. The hydroxyl group (-OH) in ethanol is a classic example of a polar functional group.
This group includes highly electronegative atoms like oxygen, which draws electrons towards itself, creating a dipole. Ethanol is more water-soluble than ethane due to this group.

• The hydroxyl group is highly polar because of the oxygen atom's high electronegativity.
• It enables ethanol to participate in hydrogen bonding, enhancing ethanol's affinity for water.

Without a polar functional group, as evident in ethane, molecules are less capable of forming hydrogen bonds or interacting effectively with polar solvents, resulting in lower solubility in water.

Intermolecular Forces

Intermolecular forces are the forces of attraction or repulsion which act between neighboring particles. These forces are critical in determining the solubility of substances in solvents like water.
Ethanol's intermolecular forces are stronger due to its ability to form hydrogen bonds, while ethane, being nonpolar, cannot form such bonds and instead relies on weaker forces known as van der Waals forces.

• Hydrogen bonds in ethanol are a type of dipole-dipole interaction, which is generally stronger than other van der Waals forces.
• Van der Waals forces in ethane include London dispersion forces, which are temporary and weak.

Ethanol's capability to form strong hydrogen bonds with water results in higher solubility, illustrating the role of intermolecular forces in determining how well substances dissolve in polar solvents like water.