Structure from Synthesis

From WikidChem

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Slide 16

In 1867 Kekule modeled the creation of mesitylene, a benzene ring, from three acetones. This reaction had been done experimentally before. Three water molecules are lost from the synthesis of the three acetone molecules, resulting in a benzene ring with alternating double and single bonds. Thus the structure of a benzene ring, and the position of the three methly substituents is predicted based on how it is synthesized.

This model is somewhat analogous to James Dewar's brass strip kit, with brass strips representing the bonds.

(Nice try, but actually it is a little more complicated than this, as we will see later, because the C-H bond does not provide a sufficiently high HOMO. You will want to come back and fix this us at that time. - JMM) This can be analyzed from a HOMO/LUMO perspective. The C=O pi* is a low LUMO, due to poor overlap and poor energy match. The C-H bond is not particularly high, but the pi* is low enough to compensate. When the C-H's electrons are put in pi*, the C=O bond is broken (it's antibond orbital is filled).

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Slide 17

When Kekule created a 3D model of the benzene ring to show constitution in his lectures, his carbons are actually tetrahedral rather than a planar square. This might suggest that he had begun thinking about the three dimensional structures of molecules rather than simply their constitutions (the nature and sequence of bonds, what's bonded to what), but surprisingly this seems not to have been the case.