Book Reading Update - Fontana History of Chemistry Chap 7

In Chapter 7, the author gave a detailed historical summary of the transition from mere classification using either the "radicalist" and "typist" view to a focus on constitutional and structural theory using concepts of the tetravalency of carbon and its tetrahedral geometric projection in 3-dimensional space.  Frankland and Kekule both played a role in bringing chemists to this level of structural conceptualization with Frankland's observations of the tri- and penta-valency of elements such as N and P although much of the distillation and refinement that brought chemists to a comfortable acceptance of structural theory is attributed to Kekule.  Ironically, despite being the more challenging concept in structural chemistry, much of the hypotheses and ideas on structure were derived from what were known about the existence of compounds that have different properties although they have the same constitution, the group of special compounds we refer to as isomers.  A teravalent carbon atom explained the existence of observed optical activity and non-activity between tartaric acid and "racemic" acid, we studied and differentiated by Pasteur on the basis of crystal shapes visualized under a microscope.  It was then determined that asymmetric carbon atoms that have four different groups around it will have optical activity unless a plane of symmetry exists within the molecule.  Even the deduction of the hexagonal shape of benzene was facilitated by reconciling predicted and experimentally observed numbers of isomers.  The impact of Kekule's and others' work on structure can best be described by the conclusion offered by the author at the end of the chapter: “Just as Picasso later transformed art by allowing the viewer to see within and behind things, so Kekule had transformed chemistry.  Chemical properties arose from the internal structures of molecules, which could now be ‘seen’ and ‘read’ through the experienced optic of the analytical and synthetic chemist.  The future of chemistry, as well as industry, after 1865 was indeed, to lie in structural chemistry at the sign of this hexagon.  But it was also to lie in a closer familiarity with physics, for it was this that provided a closer understanding of the combining capacities of atoms.”