Problem 146 - A Complete Resource Book in Chemistry for JEE Main

Question

Identify the correct order of boiling points of the following compounds: \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH}\) (I) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CHO}\) (II) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{COOH}\) (III) (a) \(\mathrm{I}>\mathrm{II}>\mathrm{III}\) (b) III > I > II (c) \(\mathrm{I}>\mathrm{III}>\mathrm{II}\) (d) \(\mathrm{III}>\mathrm{II}>\mathrm{I}\)

Step-by-Step Solution

Answer

The correct order is III > I > II.

Step 1: Identify Functional Groups

First, let's identify the functional groups present in each compound. Compound (I) has an -OH group, making it an alcohol. Compound (II) has a -CHO group, which is an aldehyde. Compound (III) has a -COOH group, identifying it as a carboxylic acid. Functional groups impact boiling points due to hydrogen bonding and molecular structure.

Step 2: Compare Hydrogen Bonding

Boiling points are often influenced by the ability to form hydrogen bonds. Carboxylic acids (III) can form two hydrogen bonds due to their -COOH group, while alcohols (I) can form one hydrogen bond due to their -OH group. Aldehydes (II) have weaker dipole-dipole interactions, as they do not effectively form hydrogen bonds. Thus, III will have the highest boiling point due to strong hydrogen bonding.

Step 3: Assess Molecular Structure and Interactions

Among alcohols and aldehydes, alcohols have a higher boiling point because they can form hydrogen bonds, while aldehydes primarily exhibit dipole-dipole interactions. Therefore, I will have a higher boiling point than II.

Step 4: Arrange Boiling Points

Based on the ability to form hydrogen bonds and molecular interactions, the correct order of boiling points from highest to lowest is: Compound III (carboxylic acid) > Compound I (alcohol) > Compound II (aldehyde).

Key Concepts

Hydrogen Bonding

Hydrogen bonding is a type of molecular interaction that can significantly affect boiling points. It occurs when a hydrogen atom is attracted to a highly electronegative atom, such as oxygen, nitrogen, or fluorine, in another molecule. In simpler terms, it's like forming a temporary bond that holds molecules closer together. This attraction can be strong and significantly affect the physical properties, like boiling points, of a substance.
In the context of boiling points:

• Compounds with hydrogen bonding, like alcohols and carboxylic acids, generally have higher boiling points than those without this feature.
• Carboxylic acids, with their -COOH group, can form two hydrogen bonds, leading to higher boiling points.
• Alcohols can form one hydrogen bond due to their -OH group, which also raises the boiling point compared to compounds that lack such capabilities.
• Aldehydes, while having dipole-dipole interactions, do not form hydrogen bonds as effectively, leading to lower boiling points.

Functional Groups

Functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules.
Understanding functional groups is crucial since they can greatly influence a compound's boiling point and physical properties.

• Alcohols (like compound I with its -OH group) can form hydrogen bonds, increasing their boiling point over ordinary hydrocarbons.
• Aldehydes (compound II with a -CHO group) primarily exhibit dipole-dipole interactions and lack the ability to form strong hydrogen bonds.
• Carboxylic acids (compound III with a -COOH group) have the unique ability to form two strong hydrogen bonds, making them very influential in raising the molecule's boiling point.

These functional groups determine how molecules interact with each other, which in turn affects boiling and melting points.

Molecular Interactions

Molecular interactions encompass various forces, such as Van der Waals forces, dipole-dipole interactions, and hydrogen bonding, that dictate how molecules interact. These interactions directly influence boiling points.
Their strength determines how much energy is required to separate molecules for a phase change from liquid to gas.

• Hydrogen bonding is a strong interaction, elevating boiling points as seen with alcohols and carboxylic acids.
• Dipole-dipole interactions, present in aldehydes, are weaker than hydrogen bonding, leading to lower boiling points.
• London dispersion forces, a subset of Van der Waals forces, are generally weaker than both hydrogen bonds and dipole-dipole forces, unless the molecules are exceptionally large.

Thus, the stronger the molecular interaction, the higher the boiling point, due to the greater energy requirement to break these interactions and convert the liquid into gas.