Second-order reactions, classes reversible

This paper describes novel approaches to the exploitation of both furan monomers and a specific facet of furan reactivity in order to synthesize either conjugated oligomers incorporating the heterocycle in their backbone, or polymeric structures which can be crosslinked and returned to linear structures through the reversible chemistry of the Diels-Alder reaction. The first family of compounds showed interesting features in terms of conductivity, luminescence, mesogenic character and photoactivity. The second class of materials owes its interest to the possibility of recycling otherwise intractable polymers, e.g. tires, thanks to a simple thermal process. 

The second class of metallic solutes is represented by the less electropositive metals. Here, the situation is the reverse of that discussed above. Sodium amalgam is widely used in industry and in the laboratory and is a good example of this class. Upon addition of mercury to liquid sodium, the reactivity of the sodium toward aqueous solutions is vastly reduced, and reaction with hydrogen is slower by an order of magnitude than that for pure sodium. This fact is important in the operation of the Solvay cell for the industrial production of sodium hydroxide by electrolysis of brine, in which sodium amalgam forms one of the electrodes. In such amalgams, valency electrons from the conduction band of liquid sodium, which would normally be responsible for its chemical reactivity, are partially localized on the mercury atoms, thus inhibiting the reactivity of sodium. 

Two schemes have been proposed which systematize the available stability constant data. In addition to data contained in the compilations given at the end of this chapter, more qualitative evidence, based for example on the results of displacement reactions, has been included in arriving at the generalizations. Historically, the first scheme is that due to Chatt and Ahrland who pointed out that electron acceptors may be placed in one of three classes. Class-a metals, the most numerous, form more stable complexes with ligands in which the coordinating atom is a first-row element (N, O, F) than with those of an analogous ligand in which the donor is a second-row element (P, S, Cl). Class-b has the relative stabilities reversed. It is not difficult to extend the stability relationships to include heavier donor atoms. Class-a behaviour is, then, typified by a stability order... 

---