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) \(\mathrm{I}>\tilde{\mathrm{II}}>\overline{\mathrm{III}}\) (b) \(\mathrm{III}>\mathrm{I}>\mathrm{II}\) (c) \(\mathrm{I}>\mathrm{III}>\mathrm{II}\) (d) \(\mathrm{III}>\mathrm{II}>\mathrm{I}\)

Step-by-Step Solution

Answer

The correct order is (d) \( \mathrm{III} > \mathrm{II} > \mathrm{I} \).

Step 1: Identify Functional Groups

The three compounds mentioned are: (1) butanol, (2) butanal, and (3) butanoic acid. Identify the functional groups in these compounds: butanol has an alcohol group (-OH), butanal has an aldehyde group (-CHO), and butanoic acid has a carboxylic acid group (-COOH).

Step 2: Determine Intermolecular Forces

Determine the types of intermolecular forces for each compound: Alcohols (butanol) exhibit hydrogen bonding due to the -OH group, aldehydes (butanal) exhibit dipole-dipole interactions, and carboxylic acids (butanoic acid) exhibit both hydrogen bonding and dimerization through their -COOH groups, providing them with the strongest intermolecular forces.

Step 3: Arrange by Strength of Intermolecular Forces

Arrange the compounds by the strength of their intermolecular forces. Carboxylic acids (butanoic acid) are strongest because of the dimerization potential of the -COOH group, followed by alcohols (butanol) due to hydrogen bonding, and lastly aldehydes (butanal) with dipole-dipole interaction.

Step 4: Identify Boiling Point Order

Higher boiling points are associated with stronger intermolecular forces. Therefore, the order of boiling points from highest to lowest is: butanoic acid (III), followed by butanol (I), and then butanal (II).

Step 5: Select Correct Answer

The correct answer is option (d) which states: \( \mathrm{III} > \mathrm{II} > \mathrm{I} \), corresponding to the order of boiling points: butanoic acid (III), butanol (I), and butanal (II).

Key Concepts

Intermolecular Forces

Intermolecular forces are attractions that occur between molecules, affecting properties like boiling points. These forces are crucial because they influence how molecules interact and stick together. There are several types of intermolecular forces:

• Dispersion Forces: These are weak forces that exist in all molecules but are particularly significant in nonpolar molecules. They arise due to temporary fluctuations in electron distribution within molecules.
• Dipole-Dipole Forces: Found in polar molecules, these forces occur when the positive end of a polar molecule is attracted to the negative end of another polar molecule.
• Hydrogen Bonds: A special type of dipole-dipole interaction. It occurs in molecules where hydrogen is bonded to a highly electronegative atom, such as oxygen, nitrogen, or fluorine.

The strength of these forces often dictates the boiling point of compounds. Molecules with stronger forces, like hydrogen bonds, tend to have higher boiling points.

Functional Groups

Functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. They play a pivotal role in determining the chemical behavior and boiling points of compounds. Here are the functional groups relevant to our compounds:

• Alcohol (-OH): This group is found in butanol. It allows for hydrogen bonding due to the presence of the hydrogen atom bonded to oxygen. This bonding significantly increases the boiling point.
• Aldehyde (-CHO): Present in butanal, this group engages in polar interactions. The dipole-dipole forces here are weaker than hydrogen bonds, resulting in a lower boiling point.
• Carboxylic Acid (-COOH): Found in butanoic acid, this group can form hydrogen bonds and can also dimerize, forming a pair of molecules interconnected through hydrogen bonding. This gives carboxylic acids the highest boiling points among the three groups.

Identifying functional groups helps predict reactivity and physical properties like boiling points.

Hydrogen Bonding

Hydrogen bonding is a critical type of intermolecular force resulting from the attraction between a hydrogen atom, which is covalently bonded to a highly electronegative element such as oxygen or nitrogen, and another electronegative atom. This type of bond is much stronger than regular dipole interactions. Let's explore why:

• Hydrogen and Electronegative Atoms: When hydrogen bonds with elements like oxygen, it allows the molecule to create strong attractions with other similar molecules. This greatly impacts boiling points and solubility.
• Role in Boiling Points: Molecules with the capability of forming hydrogen bonds typically have higher boiling points compared to those only engaging in weaker forces like dipole-dipole interactions or dispersion forces.
• Comparison Among Models: In the context of the given compounds, butanoic acid, with its two oxygen atoms capable of forming bonds, and its self-associating dimer structure, has a significantly higher boiling point than butanol, which already exceeds the boiling point of molecules like butanal that don't utilize hydrogen bonding.

Understanding hydrogen bonding unveils much about the physical behavior of substances, including why some boil at higher temperatures than others.