Thermodynamic basics

Although useful, activity coefficients calculated hy the MSM can only he applied under restricted conditions. One limitation is that empirically determined 7 values hold only for the temperature, pressure and ionic strength of electrolyte solutions used in the laboratory measurements. A second complication for a mixed electrolyte like seawater is that 7 values do not account for ion pairing. This term refers to a specific direct association of two ions that can occur in addition to general electrostatic interactions. An example of an ion pairing reaction that occurs in seawater is 

The effects of this specific reaction would not he represented in MSM calculations. In fact, it is assumed that no ion pairs are formed at 1= 0.7 in the experiments with individual electrolytes used to calculate activity coefficients in the MSM. This is true mainly for salts of potassium and chlorine, which are the ones primarily used in the MSM calculations. Because any ions that join to form ion pairs in a mixed electrolyte solution are effectively removed from solution with respect to reaction potential, an accurate determination of the extent of ion pairing in addition to the free ion activity coefficient must he made to determine the total activity coefficient in seawater. 

The percentage of the total concentration of a specific ion in seawater that is involved in ion pairs can he determined from thermodynamic equilibrium information and will be demonstrated later in the chapter. For now, suffice it to say that the individual ion concentration is determined by multipl5dng the total ion concentration by a fraction between 0 and 1.0 representing the total ion that is free of ion pairs. The product, (%freej/100) Mj, can then be multiplied by the free ion activity coefficient to determine the activity. In general form, the total correction equation is 

After chemical concentrations are converted to activities, the latter can be used to predict the probabilities and extents of specific chemical reactions based on the concept of jree energy, the energy available in a chemical system to do work. This thermodynamically based method is founded on energetic relations that can be established for a chemical species or reaction system. The fundamental energetic property of a given chemical species is its standard free energy of 

The usual unit of AGj is kilojoules per mole (kj mol ). Tables of standard free energies of formation are collected in books with compilations of thermod5mamic data (see, for example, Stumm and Morgan, 1996 Morel and Herring, 1993 Handbook of Chemistry and Physics, 1970). 

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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. 

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