Problem 89 - Introductory Chemistry Atoms First

Question

Shown below are the structures for two isomers of hydroxy-benzaldehyde. Structure (a) is 2-hydroxy-benzaldehyde and structure (b) is 4 -hydroxy- benzaldehyde. The melting point for isomer (a), \(2{ }^{\circ} \mathrm{C}\), is significantly lower than the melting point of isomer (b), \(118^{\circ} \mathrm{C}\). Based on your knowledge of intermolecular forces, suggest a substantive reason for this difference in melting points. (Hint: Consider the intermolecular forces that must be overcome to melt each compound and postulate a reason for the difference in the strengths of those forces).

Step-by-Step Solution

Answer

The difference in melting points between 2-hydroxy-benzaldehyde and 4-hydroxy-benzaldehyde is largely due to differences in hydrogen bonding. 2-hydroxy-benzaldehyde has a weaker intramolecular hydrogen bond between the hydroxy group and the aldehyde group, requiring less energy to break and melt the compound, whereas 4-hydroxy-benzaldehyde has stronger intermolecular hydrogen bonds. Consequently, the melting point of 2-hydroxy-benzaldehyde is significantly lower than that of 4-hydroxy-benzaldehyde.

Step 1: Understanding different types of intermolecular forces

There are several types of intermolecular forces present in these compounds, including hydrogen bonding, dipole-dipole interactions, and dispersion forces. Step 2: Analyzing hydrogen bonding possibilities

Step 2: Analyzing the structures for hydrogen bonding possibilities

In 2-hydroxy-benzaldehyde, a hydrogen atom of the hydroxy group can form a hydrogen bond to the oxygen atom of the aldehyde group. In 4-hydroxy-benzaldehyde, however, the hydroxy group and the aldehyde group are too far apart to form an intramolecular hydrogen bond. Both compounds are also able to form intermolecular hydrogen bonds with neighboring molecules. Step 3: Comparing dispersion and dipole-dipole forces

Step 3: Investigating the role of dispersion and dipole-dipole forces

In both isomers, there are benzene-ring and aldehyde groups, which introduce similar dispersion and dipole-dipole forces. Therefore, the difference in melting points must be largely due to differences in hydrogen bonding. Step 4: Concluding the reason for the difference in melting points

Step 4: Comparing hydrogen bonding to explain the difference

The intramolecular hydrogen bond in 2-hydroxy-benzaldehyde is weaker than the intermolecular hydrogen bonds in 4-hydroxy-benzaldehyde. This means that less energy is required to break the intramolecular hydrogen bond in 2-hydroxy-benzaldehyde and melt the compound. Hence, the melting point of 2-hydroxy-benzaldehyde is significantly lower than that of 4-hydroxy-benzaldehyde.

Key Concepts

Hydrogen Bonding

Hydrogen bonding is a special type of intermolecular force that occurs when a hydrogen atom, covalently bonded to an electronegative atom like oxygen, nitrogen, or fluorine, interacts with another electronegative atom. This type of bonding is significant because it dramatically influences the properties of molecules, such as volatility, boiling, and melting points.

In 2-hydroxy-benzaldehyde, an intramolecular hydrogen bond forms because the hydroxyl group is close enough to the aldehyde group. This intramolecular bond is less energetically demanding to break compared to intermolecular hydrogen bonds. Hence, the energy required for melting is lower, resulting in a lower melting point.

Meanwhile, in 4-hydroxy-benzaldehyde, the hydroxyl and aldehyde groups are too far apart to form such a bond. Instead, it forms strong intermolecular hydrogen bonds with neighboring molecules. These bonds are more robust and require more energy to disrupt, raising the melting point of 4-hydroxy-benzaldehyde.

Isomers

Isomers are molecules that share the same molecular formula but differ in the arrangement of their atoms or the connectivity of those atoms. This can significantly affect the physical and chemical properties of the compounds.

2-hydroxy-benzaldehyde and 4-hydroxy-benzaldehyde are examples of positional isomers. Though they share the same formula (C₇H₆O₂), they differ by the position of the hydroxyl group on the benzene ring. In 2-hydroxy-benzaldehyde, the hydroxyl group is ortho to the aldehyde group, enabling intramolecular hydrogen bonding. In contrast, it's para to the aldehyde in 4-hydroxy-benzaldehyde, preventing such bonding but allowing for strong intermolecular hydrogen bonding.

This difference in hydrogen bonding possibilities due to their structural arrangement explains the varying melting points and other properties, highlighting the importance of isomerism in chemistry.

Melting Point

The melting point of a substance is the temperature at which a solid becomes a liquid. It is heavily influenced by the strength of intermolecular forces present within the substance.

In the case of our isomers, the melting point difference is a direct result of the hydrogen bonding patterns. 2-hydroxy-benzaldehyde melts at just 2°C due to its weaker intramolecular hydrogen bonding, making it easier to disrupt the structure and transition to a liquid phase. Conversely, 4-hydroxy-benzaldehyde requires a higher temperature of 118°C to melt because the strong intermolecular hydrogen bonds between molecules demand more energy to break.

This example clearly showcases how even small changes in molecular structure, such as the position of a functional group, can lead to significant differences in physical properties like melting point, due to variations in intermolecular forces.