Question

Boiling Point of Alcohols and Diols a. Arrange these compounds in order of expected boiling point.$$ \begin{gathered} \mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{OH} \\ \mathrm{HO}-\mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2}-\mathrm{OH} \\ \mathrm{CH}_{3}-\mathrm{OH} \\ \mathrm{HO}-\mathrm{CH}_{2} \mathrm{CH}_{2}-\mathrm{OH} \end{gathered} $$ b. What factors are important in predicting the boiling points of these compounds?

Short answer

Order: Ethylene glycol > 1,3-Propanediol > Ethanol > Methanol. Factors: Hydrogen bonding and molecular weight.

Step-by-step solution

Understand Boiling Points

Boiling points are influenced by molecular weight, intermolecular forces, and molecular structure. In the case of alcohols and diols, hydrogen bonding is a significant factor due to the presence of hydroxyl (-OH) groups.

Analyze Molecular Structure

List the compounds and analyze their structures:1. \( \text{CH}_3-\text{CH}_2-\text{OH} \) (ethanol)2. \( \text{HO}-\text{CH}_2 \text{CH}_2 \text{CH}_2-\text{OH} \) (1,3-propanediol)3. \( \text{CH}_3-\text{OH} \) (methanol)4. \( \text{HO}-\text{CH}_2 \text{CH}_2-\text{OH} \) (ethylene glycol)Diols, with two -OH groups, can form more hydrogen bonds than alcohols with one -OH group, increasing boiling points.

Arrange Based on Molecular Weight and Hydrogen Bonding

Consider both the molecular weight and the ability to form hydrogen bonds: - Ethylene glycol (two -OH groups and moderately high molecular weight) - 1,3-Propanediol (two -OH groups and highest molecular weight) - Ethanol (one -OH group and moderate molecular weight) - Methanol (one -OH group and lowest molecular weight) Order from highest to lowest boiling point: Ethylene glycol, 1,3-Propanediol, Ethanol, Methanol.

Factors Influencing Boiling Points

The boiling points rely on intermolecular forces, primarily hydrogen bonding due to -OH groups, and molecular weight, which affects how strongly molecules interact. Diols have more hydrogen bonding capabilities than single alcohols, raising their boiling points.

Key concepts

Molecular Structure and Boiling Point

The boiling point of a substance is not an arbitrary number. It is closely tied to the molecular structure of the compound. The molecular structure determines how strongly molecules attract each other. This attraction is necessary to overcome if we want to change the substance from liquid to gas. Let's take alcohols as an example. The molecular structure of alcohols includes a functional group called the hydroxyl group (-OH). This group plays a major role in influencing boiling points.

• Alcohols with larger molecular structures usually have higher boiling points. This is because the molecules have a greater surface area, which enhances dispersion forces between them.
• When comparing similar structures, an increase in molecular weight often leads to a higher boiling point.

The arrangement and composition of these molecular structures directly affect how much energy is required to break intermolecular forces and boil the substance.

Hydrogen Bonding in Alcohols

Alcohols have the unique ability to form hydrogen bonds due to their hydroxyl (-OH) groups. Hydrogen bonding is an especially strong type of dipole-dipole interaction, which significantly affects the boiling points of compounds. In the case of alcohols:

• Each OH group allows an alcohol molecule to form hydrogen bonds with other molecules. These bonds are stronger than ordinary van der Waals forces but weaker than covalent bonds.
• The presence of more OH groups means more hydrogen bonds can form. This results in a higher boiling point because more energy is required to break all the hydrogen bonds.

For instance, ethylene glycol with two OH groups can form more hydrogen bonds compared to ethanol, which has only one. Hence, ethylene glycol has a higher boiling point. Hydrogen bonding increases molecular cohesion, thereby raising the energy required to enter the gaseous state.

Factors Affecting Boiling Points

Several factors contribute to the boiling point of alcohols and similar organic compounds. Understanding these factors can help predict and explain boiling points more accurately. Here are the key influencers:

• Intermolecular Forces: Alcohols with stronger intermolecular forces, particularly hydrogen bonding, will have higher boiling points.
• Molecular Weight: As molecular weight increases, so does the boiling point. Heavier molecules require more energy to transition into the vapor phase.
• Presence of Polar Groups: Polar groups like OH enhance the ability to form hydrogen bonds, affecting boiling points significantly. Diols, having two OH groups, can both accept and donate hydrogen bonds more effectively than alcohols with a single OH group.

It is crucial to consider these factors in combination, rather than isolation. For example, while 1,3-propanediol has a higher molecular weight than ethylene glycol, ethylene glycol's ability to form hydrogen bonds effectively gives it a boiling point advantage.