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CH [Formula: see text] X (X = N, O, or Cl) hydrogen bonds formed intramolecularly in 2-methyl-4-(2-chloro-4,5-dimethoxyphenyl)thiazole (Ia), 2-amino-4-(2-chloro-4,5-dimethoxy phenyl)thiazole (Ib), 2-amino-4-(2,4,5-trimethoxyphenyl)thiazole (Ic), and 2-methyl-4-(2,4,5-trimethoxyphenyl)thiazole (Id) were studied by means of all-electron calculations performed with the B3LYP/6-311++G(d,p) method. Computed ground states, in the gas phase, show the presence of a single H-bond, CH [Formula: see text] Cl, in each Ia and Ib moiety, and two H-bonds, CH [Formula: see text] N and CH [Formula: see text] O, for each Ic and Id molecule. H [Formula: see text] Cl, H [Formula: see text] N, and H [Formula: see text] O distances are shorter than the sum of the X and H van der Waals radii. H-bond energies of ≅2.0 kcal/mol were estimated for Ia and Ib and ≅4.0 kcal/mol for Ic and Id. These results agree with those of the theory of atoms in molecules, since bond critical points were found for these H [Formula: see text] X bonds. Finally, the chemical shifts in the (1)H NMR were calculated by the GIAO method; in Ia and Ib they are merely due to the different topological positions of the H atoms. But in Ic and Id the shifts of H [Formula: see text] N and H [Formula: see text] O have signatures of H-bond formations.

The mechanism of osazone formation has been subject of many studies during a vast period of time.However, the regiochemistry related to osazones remained an unsolved problem until recently that we presented our theoretical findings that explain the factors that direct the regiochemistry of osazone formation. Now, we wish present to the teaching community this subject, as well as a discussion of an example of mixed osazone formation. This new topic includes the factors that conduce to the main product. This communication will fill a gap existing in the Organic Chemistry textbooks.

This Review is a brief account of our theoretical contributions in seven research communications in the field of reaction mechanisms. Some mechanisms were corrected as in the case of the Baeyer-Drewsen indigo synthesis. When two very different reaction mechanisms had been proposed, as in the Clemmensen Reduction, a unified theory was provided. In other cases there were no reaction mechanisms at all, as in the Baeyer-Emmerling synthesis of indigo and in the Froehde Reaction for opioids. This deficit has been solved. The reaction that controls fructosazone regiochemistry has been described, and an internal process in a mixed osazone formation has been explained. All the proposals are based on well known reactivities and we provide complete and coherent reaction series with commented steps.