The Structural Theory of Organic Chemistry, first proposed by Archibald Scott CouperCouper, Archibald Scott (1831–1892) and (Friedrich) August KekuléKekulé, (Friedrich) August (1829–1896) in 1858, and then clarified and extended by Aleksandr MikhailovichButlerov, Aleksandr Mikhailovich Butlerov (1828–1886) in 1861, was a necessary prelude to the van’t Hoff and Le Bel proposals of the tetrahedral carbonTetrahedral carbon. At the time of Butlerov’s proposal of his theory, he took pains to make it clear that his proposals were related to chemical, not physical atoms, making it more acceptable to those chemists for whom the concept of physical atoms was anathema. On the basis of his theory, Butlerov and his student, Vladimir Vasil’evich MarkovnikovMarkovnikov, Vladimir Vasil’evich (1837–1904), predicted the numbers of isomers of organic compounds. The details about chemical bonding were to await the next century, but like his contemporaries, Butlerov used the concept of “affinities” in his explanations about isomerism in organic chemistry. As part of his theoretical considerations, Butlerov used a model to rationalize the number of equivalent valencies of the carbon atom, usingTetrahedron a tetrahedron—an object with four identical faces—as an analogy. However, he did not propose anything more with the analogy. The key development needed to allow the advance by was the realization by van’t Hoff and Le Bel that physical atoms existed and that their positions could be deduced, which was made easier by their realization that atoms that were chemically close to each other in the molecule might also be physically close to each other…