Comparatively Speaking: Molecular Notation of Compounds vs. Compositions

February 25, 2009 | Contact Author | By: Anthony J. O'Lenick, Jr., Siltech LLC
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Title: Comparatively Speaking: Molecular Notation of Compounds vs. Compositions
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In this look at representative chemical structures, Tony O'Lenick compares the structural notation of molecules present in sodium chloride with those present in sodium polymethacrylate. A previous feature compared the number of molecules in compounds vs. compositions, and a future feature will discuss differences in their distribution.

Accuracy of the Structure

NaCl (Figure 1) is a simple structure that needs no explanation. Sodium polymethacrylate (Figure 2), however, is a different matter. Look at the structure and determine what is missing—or at least, unclear.

Undergraduate chemists are told that all carbon molecules require four bonds around them. However, to which element are the carbon molecules in the parenthesis bonded? There is a bond outside the parenthesis that has no partner; (-) is not an element. Should it thus be concluded that the material is cyclic—i.e., that is connecting to the other (-)?

The terminal groups on the molecule, meaning those that are attached to the (-), relate to the initiator used to run the reaction and the type of post treatment used in making the polymer.



Figure 1. Sodium chloride

Sodium chloride is a compound.

There is a salient difference between polymers and compounds. Pure compounds like NaCl are single molecules.

Figure 2. Sodium polymethacrylate

Sodium polymethacrylate

Polymers are complex oligomeric mixtures.

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