Problem 48 - Chemistry and Chemical Reactivity

Question

Rationalize the observation that \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH}\) 1-propanol, has a boiling point of \(97.2^{\circ} \mathrm{C},\) whereas a compound with the same empirical formula, methyl ethyl ether \(\left(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OCH}_{3}\right),\) boils at \(7.4^{\circ} \mathrm{C}\)

Step-by-Step Solution

Answer

1-Propanol has a higher boiling point due to hydrogen bonding, absent in methyl ethyl ether.

Step 1: Identify the Compounds

First, identify the chemical structures involved. 1-Propanol is an alcohol with the chemical structure \( ext{CH}_3 ext{CH}_2 ext{CH}_2 ext{OH}\), and methyl ethyl ether is an ether with the chemical structure \( ext{CH}_3 ext{CH}_2 ext{OCH}_3\). Both have the same empirical formula, \( ext{C}_3 ext{H}_8 ext{O}\).

Step 2: Understand the Molecular Interactions

Recognize that the boiling point is heavily influenced by the type of intermolecular forces present in the compounds. 1-Propanol can form hydrogen bonds due to the presence of the hydroxyl (\( ext{OH}\)) group, increasing its boiling point. Methyl ethyl ether lacks an \( ext{OH}\) group and cannot form hydrogen bonds.

Step 3: Compare the Intermolecular Forces

Realize that hydrogen bonding is stronger than dipole-dipole and London dispersion forces found in molecules like ethers. The ability of 1-propanol to hydrogen bond leads to higher intermolecular attraction compared to the weaker forces in methyl ethyl ether.

Step 4: Conclude the Reason for Boiling Point Differences

Conclude that the significant boiling point difference between these two compounds, despite having the same empirical formula, arises from the presence of hydrogen bonding in 1-propanol, which is absent in methyl ethyl ether, leading to lower boiling point for the latter.

Key Concepts

1-propanol

1-Propanol is a type of alcohol, which belongs to a family of organic compounds that possess a hydroxyl group (-OH) attached to an alkyl chain. In the case of 1-propanol, its structure is characterized by a three-carbon alkyl chain with the formula CH extsubscript{3}CH extsubscript{2}CH extsubscript{2}OH.

• This hydroxyl group is critical for the formation of hydrogen bonds.
• Hydrogen bonds significantly increase the boiling point of alcohols like 1-propanol.

Because of these hydrogen bonds, 1-propanol exhibits a higher boiling point compared to compounds that do not have this capacity.
The boiling point of 1-propanol is fairly high at 97.2°C, reflecting the strong hydrogen bonding that occurs between its molecules.

methyl ethyl ether

Methyl ethyl ether, like 1-propanol, shares the same empirical formula, C extsubscript{3}H extsubscript{8}O. However, the molecular structure is quite different. In methyl ethyl ether, the oxygen atom is located between two carbon groups, forming the structure CH extsubscript{3}CH extsubscript{2}OCH extsubscript{3}.

• This positioning of oxygen allows for weaker dipole-dipole interactions compared to hydrogen bonding.
• As a result, London dispersion forces are the predominant force acting between ether molecules.

These intermolecular forces are much weaker than the hydrogen bonds found in 1-propanol, which is why methyl ethyl ether has a lower boiling point of 7.4°C.
The absence of a hydroxyl group means reduced opportunity for strong intermolecular attractions.

hydrogen bonding

Hydrogen bonding is a special type of dipole-dipole attraction that occurs when a hydrogen atom is covalently bonded to a highly electronegative atom, such as oxygen, nitrogen, or fluorine. This interaction is especially evident in molecules like alcohols.

• The hydroxyl group ( O-H) in 1-propanol allows for hydrogen bonding.
• These bonds create a strong attraction between molecules, substantially raising the boiling point.

Since hydrogen bonds are considerably stronger than other types of intermolecular forces, they play a major role in determining the physical properties of the compound, such as its boiling and melting points.

intermolecular forces

Intermolecular forces are forces of attraction that occur between molecules, contributing to many physical properties such as boiling points. The main types of intermolecular forces include:

• Hydrogen bonding: A particularly strong type of dipole-dipole force.
• Dipole-dipole interactions: Occur between polar molecules.
• London dispersion forces: Present in all molecules, but are the only forces in nonpolar compounds and tend to be weaker.

In 1-propanol, hydrogen bonding is the dominant force, which explains its high boiling point. In contrast, methyl ethyl ether lacks hydrogen bonding and contains weaker London dispersion and dipole-dipole forces, leading to its lower boiling point. Understanding these intermolecular forces helps to rationalize why even small changes in molecular structure can lead to significant differences in boiling points.