Acid-base equilibria structural effects

In this solvent the reaction is catalyzed by small amounts of trimethyl-amine and especially pyridine (cf. 9). The same effect occurs in the reaction of iV -methylaniline with 2-iV -methylanilino-4,6-dichloro-s-triazine. In benzene solution, the amine hydrochloride is so insoluble that the reaction could be followed by recovery. of the salt. However, this precluded study mider Bitter and Zollinger s conditions of catalysis by strong mineral acids in the sense of Banks (acid-base pre-equilibrium in solution). Instead, a new catalytic effect was revealed when the influence of organic acids was tested. This was assumed to depend on the bifunctional character of these catalysts, which act as both a proton donor and an acceptor in the transition state. In striking agreement with this conclusion, a-pyridone is very reactive and o-nitrophenol is not. Furthermore, since neither y-pyridone nor -nitrophenol are active, the structure of the catalyst must meet the conformational requirements for a cyclic transition state. Probably a concerted process involving structure 10 in the rate-determining step... 

Addition of water to these N/H = 2 acid-base mixtures also shifts the proton transfer equilibrium. In the presence of four water molecules per acid-base pair, the ApA a is further reduced from 2.3 to 0.9. In analogy with the effect of the molecular structure, i.e., the fact that increasing the number of N-sites considerably changes the symmetiy of the bonds, the presence of water obviously shifts the proton transfer equitihiium in favor of the polar structure of O- HFN bmids. For dilute aqueous solutions, the behavior may most probably follow the prediction of Gilli et al. [17], i.e., ApA a = 0. 

The present chapter is concerned primarily with measured molecular structural effects on reactions 2 and 4 in the gas phase. These have been obtained only very recently from direct equilibrium-constant determinations. Work in this area is still in a very active state, so that this chapter serves as a preliminary progress report. Useful comparison can now be made of structural effects on equation 2 with the following related topics (1) proton-transfer equilibria in condensed phases (2) other Lewis acid-base equilibria in the gas phase (3) theoretical calculations of proton-transfer energetics (4) hydrogen-atom transfer equilibria between cation radicals and saturated cations (5) hydrogen-bonded complex formation, in hydrocarbon solvents and (6) gas-phase equilibria for attachment of neutral molecules to cations and anions. Each of these topics is considered at least briefly. 

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