Organic Chemistry - "Function", Part 22: Melting Temperature Range and Enthalpy of Fusion

 Greetings,

This post covers the purity indication of melting temperature range and  Enthalpy of Fusion.

Purity and Melting Range

When solid organic compounds melt they do so over a certain temperature range.  A smaller melting range indicates a higher purity compound.  It's important to understand that 100% purity does not actually exist because there are always impurities present even for a highly-purified organic compound. When impurities exist at higher levels, the melting temperature range increases because impurities melt at different temperatures than the primary component of a "pure" compound. Furthermore, impurities disrupt the orderly crystal array resulting in a weakening of the structure and an overall decrease in energy of intermolecular forces: This causes the actual (observed) melting temperature to decrease because less heat energy is needed to "collapse" the crystal and form a liquid state.  An additional effect is the separation of main component crystalline regions (from each other) by pockets by impurity-inclusions, further weakening the crystal lattice structure.

The meltimg point range is routinely used as a quality control test for high-purity organic raw materials used for industrial manufacturing of pharmaceutical products.  The melting range test is typically part of a set of tests, which may include chromatographic analysis - e.g. HPLC (high pressure liquid chromatography) and TLC (thin layer chromatography).

Enthalpy of Fusion and Entropy Changes During Melting Using Ice as a Model

Water-ice can be used as a simple model to explain crystalline internal energy changes for even the most complex compounds because the energy effects during melting are generally the same.  The main effect during melting of a crystalline compound is that the free energy of the melting compound goes to zero.  This is because all of the heat energy (Enthalpy) going into the melting crystal is consumed by the increase in Entropy simultaniously occuring. The image, below, provides a visual explanation of energy changes during melting of a crystalline compound.

The chart, above, separates the melting process into stages of internal crystalline structural changes. This provides a simplified view of internal energy changes which is, actually, a model of the process.  Generally speaking, a model is a simplified explanation (theory) of a scientific law (a repeatable observation based on an experimentally-proven hypothesis). I consider the chart a "simplified view" because, in reality, the melting process does not occur as neat separate stages, but, rather, occurs as a continuous "fluid" event.

That's all for this post.  Thank you for reading.