Organic Chemistry, "Function" - Part 21: Internal Properties of the Salicylic Acid Crystalline Structure and their Connection to Melting Point

 Greetings,

This post explains the internal properties of crystalline salicylic acid and their connection to a higher melting point; compared to acetylsalicylic acid (aspirin).

The higher melting temperature of salicylic acid (higher than that of acetyl-salicylic acid, aka aspirin) is a function of the tightly packed molecular arrangement within its crystal lattice structure.  This results in an entropy-of-fusion (the energy associated with "orderliness") quite a lot lower than that of aspirin.  The entropy-of-fusion is the non-work component of the free energy equation: It's the thermodynamic property independent of work.  This is, as opposed to, Enthalpy-of- fusion (aka "heat of fusion") which is always a work component: Enthalpy defines the work that can be done by or the work that can be done on an object or by/on a chemical reaction, before subtracting the energy due to entropy.

The lower entropy-of-fusion of salicylic acid (compared with that of aspirin) means that more energy (aka heat) must be absorbed to produce the required degree of disorder for melting to occur.  Aspirin has a higher heat of fusion, which can indicate the heat needed to melt a crystalline compound; however, in the case of salicylic acid its low entropy value "offsets" the enthalpy effect, resulting in a higher melting point.

Salicylic acid consists of molecules which assume a flat geometry due to the strong intramolecular ("within the molecule") bonding between the hydroxyl-hydrogen and the double-bonded oxygen of the adjacent acetyl group.  This flat geometry "behavior" allows a more ordered array of molecules when salicylic acid crystallizes.  The figure, below, provides a visual depiction of the entropy effect.

My next post will provide the mathematical explanation of the entropy effect.

As always, thank you for reading!