Basic Thermodynamics

In all its reactions the lone pair of thiazole is less reactive than that of pyridine. Table 1-61 shows three sets of physicochemical data that illustrate this difference. These are (1) the thermodynamic basicity, which is three orders of magnitude lower for thiazole than for pyridine (2) the enthalpy of reaction with BF3 in nitrobenzene solution, which is 10% lower for thiazole than for pyridine and (3) the specific rate of quaterni-zation by methyl iodide in acetone at 40°C, which is about 50% lower for... 

Scales for bases that are too weak to study in aqueous solution employ other solvents but are related to the equilibrium in aqueous solution. These equilibrium constants provide a measure of thermodynamic basicity, but we also need to have some concept of kinetic basicity. For the reactions in Scheme 5.4, for example, it is important to be able to make generalizations about the rates of competing reactions. 

If the equilibrium were established rapidly, reduction of the free ketone as it formed would result in a substantial loss of product. Lithium enolates are more covalent in character than are those of sodium and potassium and consequently are the least basic of the group. This lower thermodynamic basicity appears to be paralleled by a lower kinetic basicity several workers have shown that lithium enolates are weaker bases in the kinetic sense than are those of sodium and potassium." As noted earlier, conjugated enones... 

I. M. Klotz and R. M. Rosenburg, Chemical Thermodynamics Basic Theory and Methods, W. A. Benjamin Inc. Menlo Park, California. 1972, p. 276. 

The reactions discussed in the following sections take place in aprotic solvents, and reference to known or estimated thermodynamic basicities will relate to DM SO unless otherwise noted, since DM SO is the polar aprotic solvent in which most thermodynamic acidities have been measured [55-58]. Values of pK determined in DM SO can usually be assumed to parallel values in DMF [59, 60], MeCN, and other polar aprotic solvents whereas pK values (and relative pK values) related to water and other hydroxylic solvents can be very different. 

Hindered phenolates have low nucle-ophilicity and may in aprotic solvent act as EGBs. The tetraethylammonium pheno-late of (35H), (pK 16.8) can be formed ex situ by direct reduction of (35H) [80, 81], similar to Scheme 20. Since the thermodynamic basicity is low, the applicabihty as an EGB is entirely dependent on the removal of deprotonated substrate by further reaction. 

The reasons for the reluctance of the diamines to undergo protonation is due to the inaccessibility of the basic sites. The high thermodynamic basicity is probably due to a combination of the formation of a strong intramolecular hydrogen bond and to unfavourable lone pair interactions in the diamines that cannot be relieved by solvation. 

Chemical Thermodynamics Basic Concepts and Methods, Seventh Edition. By Irving M. Klotz and Robert M. Rosenberg... 

Anokhina, M.S., Il in, M.M., Semenova, M.G., Belyakova, L.E., Polikarpov, Yu.N. (2005). Calorimetric investigation of the thermodynamic basics of the effect of malto-dextrins on the surface activity of legumin in the presence of small-molecule surfactants. Food Hydrocolloids, 19, 455 166. 

---