An unknown compound melted at 131-133°C. It is thought to be one of the following compounds(mp,°C): trans-cinnamic acid (133-134); benzamide(128-130); DL-malic acid(131-133); or benzoin(135-137). The mixture melting points of the unknown compound with each of the test compounds are listed below. What is the unknown compound? Briefly explain your reasoning.
-trans-cinnamic acid...........110-120
-benzamide.....................130-132
-DL-malic acid..................114-124
-benzoin.........................108-116
With mixed melting points you look for the following two criteria:
1. A sharp melting point (for example 110-120 is not anywhere near what I would call sharp)
2. A melting point that is VERY close to the unmixed unknown.
benzamide
To identify the unknown compound, we need to compare its melting point with each of the given test compounds.
The melting point of the unknown compound falls within the range of 131-133°C. Let's analyze it further:
- The melting point of trans-cinnamic acid is 133-134°C, which is slightly higher than the melting point range of the unknown compound. Therefore, we can eliminate it as a possibility.
- The melting point of benzamide is 128-130°C, which is slightly lower than the melting point range of the unknown compound. It is still a possibility.
- The melting point of DL-malic acid is 131-133°C, which exactly matches the melting point range of the unknown compound. It is also a possibility.
- The melting point of benzoin is 135-137°C, which is higher than the melting point range of the unknown compound. We can eliminate it as a possibility.
Based on the melting point ranges, both benzamide and DL-malic acid are possibilities. However, we can compare the unknown compound's melting point with that of the test compounds to make a more confident determination.
Comparing the melting points of the unknown compound with each test compound:
- The mixture melting point with trans-cinnamic acid is 110-120°C, significantly lower than the unknown compound's melting point. We can eliminate it as a possibility.
- The mixture melting point with benzamide is 130-132°C, which matches the melting point of benzamide itself. This strongly suggests that the unknown compound is benzamide.
- The mixture melting point with DL-malic acid is 114-124°C, which is significantly lower than the unknown compound's melting point. We can eliminate it as a possibility.
- The mixture melting point with benzoin is 108-116°C, significantly lower than the unknown compound's melting point. We can eliminate it as a possibility.
Therefore, based on the comparison of melting points, the unknown compound is most likely benzamide.
To determine the unknown compound, we need to analyze the mixture melting points with each of the test compounds. The unknown compound will have a similar melting point range when mixed with the correct test compound.
Looking at the mixture melting points provided:
- trans-cinnamic acid mixture: 110-120°C
- benzamide mixture: 130-132°C
- DL-malic acid mixture: 114-124°C
- benzoin mixture: 108-116°C
We need to find a test compound that, when mixed with the unknown compound, produces a melting point range that includes the melting point range of the unknown compound (131-133°C).
From the given data, only DL-malic acid has a mixture melting point range (114-124°C) that includes the melting point range of the unknown compound (131-133°C).
Therefore, the unknown compound is likely to be DL-malic acid.