Problem 70 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

Which of the following compound has the highest boiling point? (a) \(\mathrm{CH}_{3} \mathrm{OCH}_{3}\) (b) \(\mathrm{CH}_{3} \mathrm{COCH}_{3}\) (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) (d) \(\mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H}\)

Step-by-Step Solution

Answer

The compound with the highest boiling point is (d) \(\mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H}\).

Step 1: Identify Functional Groups

Examine each compound to determine the functional groups involved, as they play a crucial role in determining intermolecular forces and thus boiling points:- (a) \( \mathrm{CH}_{3} \mathrm{OCH}_{3} \) is an ether.- (b) \( \mathrm{CH}_{3} \mathrm{COCH}_{3} \) is a ketone.- (c) \( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH} \) is an alcohol.- (d) \( \mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H} \) is a carboxylic acid.

Step 2: Assess Intermolecular Forces

Determine the types and strengths of intermolecular forces for each compound: - Ethers exhibit dipole-dipole interactions and weak hydrogen bonding. - Ketones have dipole-dipole interactions, weaker than hydrogen bonds. - Alcohols can form hydrogen bonds, which are stronger than dipole-dipole interactions. - Carboxylic acids can form strong hydrogen bonds and likely dimerize, significantly increasing boiling point.

Step 3: Compare Boiling Points

Compare compounds based on the strength and type of their intermolecular forces:- Ethers, like \( \mathrm{CH}_{3} \mathrm{OCH}_{3} \), will have lower boiling points due to primarily weaker dipole interactions.- Ketones, such as \( \mathrm{CH}_{3} \mathrm{COCH}_{3} \), also rely on dipole interactions and have moderate boiling points.- Alcohols, \( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH} \), have stronger hydrogen bonds increasing boiling points.- Carboxylic acids, \( \mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H} \), can form two hydrogen bonds per molecule and dimerize, leading to the highest boiling point among these options.

Key Concepts

Functional Groups

In organic chemistry, molecules are often categorized by their functional groups. These groups involve specific atoms or bonds that impart distinct chemical properties to a molecule. Let’s break down each of the compounds in our exercise by their functional group to understand their impact on boiling points.

• The compound \( \mathrm{CH}_{3} \mathrm{OCH}_{3} \) is classified as an ether, characterized by an oxygen atom connected to two alkyl groups. This functional group brings about relatively weak intermolecular interactions.
• \( \mathrm{CH}_{3} \mathrm{COCH}_{3} \) is known as a ketone. Its functional group consists of a carbonyl group (a carbon double-bonded to oxygen) bonded between two carbon atoms. The polar nature of this group affects intermolecular forces somewhat more than ethers.
• The alcohol \( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH} \) contains a hydroxyl group (\( -\mathrm{OH} \)) attached to an alkane chain. This provides the ability to form hydrogen bonds more effectively than the other functional groups mentioned earlier.
• \( \mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H} \) is a carboxylic acid, characterized by a carboxyl group \((\mathrm{COOH})\), which includes both carbonyl and hydroxyl elements. This combination allows for exceptional hydrogen bonding capacity.

Each functional group contributes uniquely to the molecule's boiling point through its ability to engage in intermolecular forces.

Intermolecular Forces

The term intermolecular forces refers to the forces attracting molecules to each other. These forces directly affect physical properties like boiling points. Let’s explore the types of intermolecular forces relevant to our compounds.

• Dipole-dipole interactions are typical in polar molecules, like ketones. These involve the attraction between the positive end of one molecule and the negative end of another. Although stronger than dispersion forces, these are relatively weak compared to hydrogen bonds.
• Hydrogen bonding is a particularly strong type of dipole-dipole interaction. It occurs when a hydrogen atom is bonded to a highly electronegative atom, allowing the hydrogen to bond with lone pairs on other molecules. Alcohols and carboxylic acids can form these strong bonds, providing them with higher boiling points.
• Dispersion forces, the weakest intermolecular forces, arise due to temporary shifts in electron density in molecules or atoms. In our context, they play a minimal role compared to the influence of hydrogen bonds and dipole interactions.

Carboxylic acids and alcohols thus demonstrate higher boiling points due to their capability of forming hydrogen bonds. Carboxylic acids can even dimerize, forming additional pair interactions increasing their boiling points further.

Hydrogen Bonding

Hydrogen bonding is a critical aspect when analyzing boiling points of compounds with hydrogen-bonding functional groups. This phenomenon occurs extensively among molecules where hydrogen is bonded to oxygen, nitrogen, or fluorine.

Why is Hydrogen Bonding Important?

Hydrogen bonds are essential because they are significantly stronger than other types of dipole interactions. This leads to higher boiling points for molecules that can partake in hydrogen bonding.

• In alcohols, like \( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH} \), each molecule can form hydrogen bonds through its hydroxyl group. This causes the molecules to require more energy (and thus higher temperatures) to break apart when transitioning to the gaseous phase.
• Carboxylic acids, such as \( \mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H} \), not only participate in hydrogen bonding but often form dimers. These dimers effectively double the amount of hydrogen bonds per mole, raising the boiling point even higher than regular alcohols.

Understanding hydrogen bonding helps clarify why carboxylic acids and alcohols rank high in boiling points, demonstrating how specific molecular interactions can define physical properties.