Problem 98 - Chemistry The Molecular Science

Question

There are three isomers with the formula \(\mathrm{C}_{6} \mathrm{H}_{6} \mathrm{O}_{2} .\) Each isomer contains a benzene ring to which two \(-\) OH groups are attached. (a) Write the Lewis structures for the three isomers. (b) Taking their molecular structure and the likelihood of hydrogen bonding into account, list them in order of increasing melting point.

Step-by-Step Solution

Answer

From lowest to highest melting point: Catechol < Resorcinol < Hydroquinone.

Step 1: Understand the Problem

We need to draw Lewis structures for three isomers of the molecular formula \(\mathrm{C}_6\mathrm{H}_6\mathrm{O}_2\). These isomers should contain a benzene ring with two hydroxyl groups \((-\mathrm{OH})\) attached. Then, we need to rank them in terms of increasing melting point based on their potential for hydrogen bonding and molecular structure.

Step 2: Draw the Three Lewis Structures

For benzene derivatives like these, the possible positions for two hydroxyl groups are ortho, meta, and para on the benzene ring. This gives us the following structures: 1. **Ortho-dihydroxybenzene (Catechol)**: Both \(-\mathrm{OH}\) groups are adjacent.2. **Meta-dihydroxybenzene (Resorcinol)**: One \(-\mathrm{OH}\) group is separated by one carbon from the other.3. **Para-dihydroxybenzene (Hydroquinone)**: The \(-\mathrm{OH}\) groups are on opposite ends of the benzene ring.

Step 3: Analyze Potential for Hydrogen Bonding

The ability to form hydrogen bonds affects the melting points. - **Catechol**: The adjacent \(-\mathrm{OH}\) groups can form intramolecular hydrogen bonds, which limit intermolecular hydrogen bonding and may decrease melting point.- **Resorcinol**: It does not favor intramolecular hydrogen bonding as strongly as catechol, enhancing potential for intermolecular bonds.- **Hydroquinone**: With \(-\mathrm{OH}\) groups on opposite sides, it maximizes intermolecular hydrogen bonding, increasing melting point.

Step 4: Rank the Isomers by Melting Point

Considering hydrogen bonding potential: 1. **Catechol** has the lowest melting point due to intramolecular hydrogen bonding. 2. **Resorcinol** follows as it allows more intermolecular hydrogen bonding than catechol. 3. **Hydroquinone** has the highest melting point due to optimal intermolecular hydrogen bonding configuration.

Key Concepts

Lewis structures

Lewis structures are diagrams that represent the atoms and bonds between them in a molecule. They help us visualize how valence electrons are shared or transferred among atoms. For the isomers of the molecular formula \(C_6H_6O_2\), each structure contains a benzene ring with two hydroxyl \((-\mathrm{OH})\) groups attached. Different positions of these groups lead to different isomers.
The three positions for the \((-\mathrm{OH})\) groups on the benzene ring are ortho, meta, and para.

• **Ortho-dihydroxybenzene (Catechol)**: Here, both \((-\mathrm{OH})\) groups are adjacent to each other on the benzene ring. This adjacency is represented by placing both groups next to one another in the Lewis diagram.
• **Meta-dihydroxybenzene (Resorcinol)**: One \((-\mathrm{OH})\) group is separated by a single carbon atom from the other. Thus, the groups are not directly next to each other.
• **Para-dihydroxybenzene (Hydroquinone)**: The \((-\mathrm{OH})\) groups are placed on opposite sides of the benzene ring, resulting in maximum separation.

Drawing accurate Lewis structures is essential because they allow for proper visualization of potential interactions between molecules, like hydrogen bonding.

Hydrogen bonding

Hydrogen bonding is a type of attractive interaction that occurs when a hydrogen atom is bonded to a highly electronegative atom, like oxygen, and is also attracted to another electronegative atom. This type of interaction plays a key role in the physical properties of molecules, such as melting point and boiling point.
In the case of our C6H6O2 isomers, the placement of the \(-\mathrm{OH}\) groups significantly affects how these molecules can hydrogen bond with each other:

• **Catechol**: Since the \(-\mathrm{OH}\) groups are adjacent, intramolecular hydrogen bonding can occur. This means the \(-\mathrm{OH}\) groups mainly bond within a single molecule, thus reducing the potential for hydrogen bonds between different molecules.
• **Resorcinol**: With the \(-\mathrm{OH}\) groups further apart, resorcinol doesn’t favor intramolecular hydrogen bonding strongly, allowing more hydrogen bonding between molecules, enhancing its intermolecular attractions.
• **Hydroquinone**: The optimal placement on opposite sides maximizes intermolecular hydrogen bonding. This configuration allows more molecules to interact strongly through hydrogen bonds.

Understanding these bonding interactions helps us predict how the isomers might behave under different conditions, including their melting points.

Melting points

Melting points of substances provide insight into the strength of intermolecular forces holding molecules together in a solid state. Isomers of the same formula can have varying melting points due to how they interact with each other.
For the C6H6O2 isomers:

• **Catechol** has the lowest melting point. Its intramolecular hydrogen bonding limits interactions between different molecules, resulting in a weaker solid lattice.
• **Resorcinol** has a moderate melting point. It allows for more intermolecular hydrogen bonds to form, providing a more robust structure than catechol.
• **Hydroquinone** presents the highest melting point. The arrays of \(-OH\) groups on opposite sides allow extensive intermolecular hydrogen bonding, resulting in a very strong and stable lattice.

This ranking highlights how subtle differences in molecular structure significantly influence physical properties like melting points.

Benzene derivatives

Benzene derivatives are compounds that include a benzene ring with various groups attached, which alter the chemical and physical properties of the original benzene molecule. In our study of C6H6O2 isomers, these derivatives include catechol, resorcinol, and hydroquinone. Each has unique characteristics based on the placement of the hydroxyl groups.

• **Benzene** itself is a highly stable, aromatic hydrocarbon with a unique ring structure characterized by alternating double bonds that delocalize electron density. This structure imparts special stability, referred to as "aromatic stability."
• **Catechol, resorcinol, and hydroquinone**: All maintain this aromaticity but differ due to the location of \(-\mathrm{OH}\) groups, influencing properties like reactivity and hydrogen bonding potential.
• These specific placements make them useful in different applications, such as antioxidants or photoprotective agents, due to their ability to engage in various chemical reactions.

Understanding benzene derivatives is crucial, as they form the basis for many key chemicals in synthetic and industrial chemistry.