Organic Chemistry - "Function", Part 20: Molecular Crystal Structure of Aspirin (Acetylsalicylic Acid)

Hello, 

This post is a discussion of the crystal structure of the molecular compound, Aspirin. Basic theory of understanding molecular structure of higher molar mass compounds and details of Aspirin's structure (Structure-Within-Structure (SWS)) are covered. 

 2-Dimensional Structure Elucidation 

 Three-dimensional crystal structures of low molar mass (LMM) compounds are relatively easy to model and understand. For example, the LMM compounds, water (ice) & Carbon Dioxide (dry ice) have molecular crystal structures that can be described as, hexagonal "tubular" and cubic, respectively. Ice water crystals consist of hexagonal tubes, assembled by stacks of 2-dimensional fused-ring-network sheets; each ring formed by strong hydrogen bonding across an array of six water molecules. Ice water has a prevalence of strong hydrogen bonding within, and between, fused-ring-network sheets. The ice crystal layers stack such that the hexagonal rings are perfectly in-line, which results in repeating arrays of fused hexagonally-shaped channels. Dry ice, on the other, consists of cubic-shaped units, fused together to create identical repeating geometrical structure in all three dimensions of space. Each carbon atom, of every carbon dioxide molecule, exists at the corners of the dry ice cubic-lattice structure. Unlike solid water, the predominant intermolecular type of bonding, within solid CO2, is via London Dispersion Forces, which is related to the very low freezing-point of dry ice. 

 I've just described the crystal geometries of two simple solid-state molecular compounds, although my descriptions may not seem so simple. By extension, discussions about, and representations of, organic compound molecular crystals are greatly more challenging and complex. This complex nature necessitates a focus on 2-dimensional structure within an organic molecular crystal, followed by a "building-up" of the 2-d geometry to describe, and model,the overall 3-dimensional space structure (Structure-Within-Structure: SWS). Part of the complexity of organic compound molecular crystal geometry is the existence of mixed modes (e.g. polar bonding & dispersion forces) of intermolecular bonding within varying components of the crystal structure. An example of this mixed mode bonding is seen when looking at the crystal geometry of aspirin, discussed in the next section below. 

 The Crystal Structure of Aspirin 

 Structure-Within-Structure (SWS) 

 The aspirin molecule has two high-polarity regions, namely the carboxyl and acetyl functional groups. It turns out, because of certain key assymetries, aspirin molecules form two dimer-type polar intermolecular bonding regions: 1) acetyl-acetyl and 2) carboxyl-carboxyl. These dimer-type attractions result in long chains of aspirin molecules (I'll call them "fibrels"). Aspirin fibrels line-up, in parallel fashion, due to favorable steric factors (low steric-hindrance) and dispersion-force attractions, to form 2-d layers. The 2-d layers come together, again due to steric factors and londen dispersion forces, to form the overall aspirin 3-dimensional structure. 

 Group-Symmetry-Theory (GST) 

 The details of GST are well-beyond the scope of this blog post, however basic GST is useful for describing, and representing, the crystal structure of aspirin. Fundamentally, GST seeks to identify centers-of-symmetry, or point-symmetries (PS). PS exist, in a crystal molecular structure, at the interface between components deemed "symmetrical", according to certain GST definitions. The PS, in turn, are used to describe the 2-d structure of a molecular crystal. The 3-D geometry is then an extension of the 2-d form. GST basics can be applied to aspirin, as shown in the chart below.

My next post will explain the difference in melting point of salicylic acid compared with acetylsalicylic acid. 

 Thank you for reading.