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Chapter 7: Problem 89

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).

Short Answer

Expert verified
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 by step solution

01

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
02

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
03

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
04

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.

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