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Difficulty: Very hardPure and Impure Substances and Criteria of Purity

A pharmaceutical analyst evaluated an organic solid compound ZZ suspected of containing a soluble non-volatile impurity. When heated slowly in a melting point apparatus, the sample began to melt at 141.2∘C141.2^\circ\text{C} and completely liquefied at 147.8∘C147.8^\circ\text{C} (literature melting point of pure Z=151.0∘CZ = 151.0^\circ\text{C}). A thin-layer chromatography (TLC) plate developed with a 12.0 cm12.0\text{ cm} solvent front produced a major spot at a distance of 4.8 cm4.8\text{ cm} from the baseline and a minor impurity spot at 9.6 cm9.6\text{ cm}. Which of the following correctly identifies the main component's RfR_f value and describes the melting characteristics of pure compound ZZ relative to this impure sample?

  1. The main component has an RfR_f value of 0.400.40, and pure compound ZZ melts sharply at 151.0∘C151.0^\circ\text{C} over a very narrow temperature range.Answer
  2. B
    The main component has an RfR_f value of 2.502.50, and pure compound ZZ melts at a lower temperature over the same broad range.
  3. C
    The main component has an RfR_f value of 0.400.40, and the presence of the soluble impurity elevates the melting point while narrowing the melting range.
  4. D
    The main component has an RfR_f value of 0.500.50 obtained by comparing the two spot distances, and pure compound ZZ melts over a broad temperature range.

Answer

The main component has an RfR_f value of 0.400.40, and pure compound ZZ melts sharply at 151.0∘C151.0^\circ\text{C} over a very narrow temperature range.
Calculating the retention factor gives Rf=4.8/12.0=0.40R_f = 4.8 / 12.0 = 0.40. Pure chemical substances exhibit sharp, well-defined physical constants. For a solid compound, chemical purity is confirmed when the substance melts sharply at its exact literature value (151.0∘C151.0^\circ\text{C}) over a narrow range of less than 1∘C1^\circ\text{C}. The crude sample's lowered melting point (141.2∘C141.2^\circ\text{C}) and wide range (6.6∘C6.6^\circ\text{C}) are classic indicators of impurity.

Step-by-Step Solution

1
Calculate the retention factor (RfR_f) of the main component spot from the TLC data.
Rf=Distance moved by main spotDistance moved by solvent front=4.8 cm12.0 cm=0.40R_f = \frac{\text{Distance moved by main spot}}{\text{Distance moved by solvent front}} = \frac{4.8\text{ cm}}{12.0\text{ cm}} = 0.40
RfR_f is defined as the ratio of solute migration distance to solvent front migration distance.
2
Analyze the impact of a soluble non-volatile impurity on the melting point of a solid compound.
The presence of impurities depresses the melting point (sample melts below 151.0∘C151.0^\circ\text{C}) and broadens the melting range (141.2∘C−147.8∘C141.2^\circ\text{C} - 147.8^\circ\text{C}, a span of 6.6∘C6.6^\circ\text{C}).
Foreign molecules disrupt the crystal lattice of the solid, requiring less kinetic energy to collapse the structure over a variable range of local impurity concentrations.
3
Determine the melting criteria for a completely pure sample of compound ZZ.
A pure sample melts sharply at its characteristic literature temperature (151.0∘C151.0^\circ\text{C}) within a range of 0.5∘C0.5^\circ\text{C} to 1.0∘C1.0^\circ\text{C}.
Chemical purity is characterized by fixed physical constants, including a sharp melting point and single chromatographic spot.

Key Concept

Criteria of purity for solids: sharp melting point at literature value and single spot on TLC with correct Rf.
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Topics

Pure and Impure Substances and Criteria of Purity