Lewis acids Gutmann acceptor number

Figure 18 Uptake ( /oS) of Lewis acids by PMMA and ArMCU versus the acidic character of the solutes. The %S is the solute per repeat unit molar ratio in the case of PMMA and the solute per nitrogen atom in the case of ArMCU. The solutes were characterized by AN, the Gutmann acceptor number CCI4, 2.3 l-2,dichloroethane (DCE), 6.4 dichloromethane (DCM) (CH2CI2), 13.5 trichloromethane (TCM), (chloroform) 18.7 hexafluoroisopropanol (HFIP), 66.3 and tri-fluoroacetic acid (TFAA), 111.

The chemistry of Lewis acid-base adducts (electron-pair donor-acceptor complexes) has stimulated the development of measures of the Lewis basicity of solvents. Jensen and Persson have reviewed these. Gutmann defined the donor number (DN) as the negative of the enthalpy change (in kcal moL ) for the interaction of an electron-pair donor with SbCls in a dilute solution in dichloroethane. DN has been widely used to correlate complexing data, but side reactions can lead to inaccurate DN values for some solvents. Maria and Gal measured the enthalpy change of this reaction... 

Many physical chemists have embraced the concepts of donicity (donor numbers, DN) and acceptor numbers (AN) as developed by Gutmann and his co-workers [12], The DN is measured by the heat of reaction of the donor solvent and antimony pentachloride in a 1 1 ratio as a dilute solution in 1,2-dichloro-ethane. It is taken to be a measure of the strength of the Lewis base. The AN is measured as the relative shift of the 31P NMR peak in triethylphosphine oxide dissolved in the sample solvent. Hexane is given the value of zero on the scale, and antimony pentachloride is given the value of 100. The AN is taken to be a measure of the strength of the Lewis acid. The applications of the concepts have... 

Solvents can be classified as EPD or EPA according to their chemical constitution and reaction partners [65]. However, not all solvents come under this classification since e.g. aliphatic hydrocarbons possess neither EPD nor EPA properties. An EPD solvent preferably solvates electron-pair acceptor molecules or ions. The reverse is true for EPA solvents. In this respect, most solute/solvent interactions can be classified as generalized Lewis acid/base reactions. A dipolar solvent molecule will always have an electron-rich or basic site, and an electron-poor or acidic site. Gutmann introduced so-called donor numbers, DN, and acceptor numbers, AN, as quantitative measures of the donor and acceptor strengths [65] cf. Section 2.2.6 and Tables 2-3 and 2-4. Due to their coordinating ability, electron-pair donor and acceptor solvents are, in general, good ionizers cf. Section 2.6. 

A detailed discussion and comparison of all these and further solvent softness scales can be found in references [173, 238, 239]. For other Lewis acid/base parameters of EPD and EPA solvents, derived from calorimetric measurements [e.g. Gutmann s donor and acceptor numbers), see reference [65] and Section 2.2.6. 

Gutimnn s Acceptor Number (AN). Mayer, Gutmann, and Gerger (112) have used infinite dilution P-nmr shifts of triethylphosphine oxide (52) as the basis for what they describe as Acceptor Number (AN), a quantitative empirical parameter for the electrophilic properties of solvents (the conversion factor is —5"" = AN/2.349). For protic solvents AN is intended to serve as a measure of HBD acidity for nonprotic solvents it is seemingly intended as a measure of Lewis-type acidity. Compared with a values which range from 33.5 to 41.3 for the aliphatic alcohols, AN values of representative non-HBD solvents are THF, 8.0 ethyl acetate, 10.8 DMSO, 19.3. Thus, the latter solvents are... 

Solvatation, solvolysis and ionic dissociation phenomena, in both aqueous and nonaqueous solutions are subsumed by the Lewis definitions. In addition to the previous discussion of the dual polarity character of Lewis acids and bases, it should be noted that many of them are amphoteric, by definition. Donor number, DN, was developed in order to correlate the behavior of a solute in a variety of donor solvents with a given basicity or donicity. A relative measurement of the basicity of a solvent D is given by the enthalpy of its reaction with an arbitrarily chosen reference acid (SbCls in the Gutmann s scale). Latter Mayer introduced an acceptor number, AN, as the relative P NMR shift induced by triethylphosphine, and relative to acidic strength (AN=0 for hexane and 100 for SbCls). In 1989, Riddle and Fowkes modify these AN numbers, to express them, AN ", in correct enthalpic unit (kcaLmol). Table 10.2.3 gathers electron acceptor number AN and AN " and electron donor number DN for amphoteric solvents. 

Much attention has been given to correlating A// against properties of the interacting species. Two of the most common are the Gutmann [96] donor number, DN, or acceptor number, AN, scales and the Drago [97] E C formulation. The donor number, DN, quantifies the Lewis basicity of a variety of solvents and was defined for a given base B as the exothermic heat of its reaction with the reference acid, antimony pentachloride, in a 10 - M solution in a neutral solvent (1,2-dichloroethane). The units are (kJ/mol). Specifically ... 

However, the strength of Lewis acid-base interaction can be expressed in energy terms, such as the exothermic molar heat, —for the equilibrium (III) of adduct formation. The enthalpy term is preferred because entropy effects accompanying the formation of coordinative bonds are difficult to determine. Various models have been proposed for the theoretical estimation of the enthalpy term based on molecular properties of reactants and are reviewed in Ref 5. The most significant developments have been the hard and soft acid-base principle of Pearson [6], the E C equation of Drago and Wayland [7], the donor and acceptor numbers of Gutmann [8], and the perturbation theory of Hudson and Klopman [9]. 

More recently, it has been shown, in particular by Fowkes and co-workers [2,6,7], that electron acceptor and donor interactions, according to the generalized Lewis acid-base concept, could be a major type of interfacial forces between two materials. This approach is able to take into account hydrogen bonds which are often involved in adhesive joints. Inverse gas chromatography at infinite dilution for example is a well adapted technique [8-10] for determining the acid-base characteristics of fibres and matrices. Retention data of probes of known properties, in particular their electron acceptor (AN) and donor (DN) numbers according to Gutmann s semi-empirical scale [11], allow the determination of acid-base parameters, and Kj), of fibre and matrix surfaces. It becomes then possible to define a "specific interactions parameter" A at the fibre-matrix interface, as the cross-product of the coefficients and Kq of both materials [10,11] ... 

The Gutmann approach was to allocate a donor number (DN) for bases, and an acceptor number (AN) for acids, corresponding to the definitions first proposed by Lewis. The DN was determined by measuring in a calorimeter the heat of reaction between the base under investigation and antimony pentachloride in a dilute 1,2 dichloroethane solution. The AN was determined from the P NMR shift of triethylphosphine oxide (TEPO) when dissolved in the acid under investigation. The number allocated was determined from a linear scale in which -hexane was designated as zero and antimony pentachloride was designated as 100. 

Several individuals have worked to quantify these trends, making scales of Lewis acidity and basicity. Gutmann has created a series of donor numbers (DN) and acceptor numbers (AN) for various solvents, while Drago and Wayland have assigned parameters E and C, which measure electrostatic interactions and covalent bonding potential, respectively. Lastly, Pearson treats each Lewis acid and base with two parameters, relating what is called the strength of the acid/base and the softness/hardness of the acid/base. In fact, the HSAB trend discussed above is primarily a concept developed by Pearson. 

The enthalpy of formation of a Lewis acid-base adduct can also be estimated by using Gutmann s [ 114] donor and acceptor numbers, DJV and AJV, where DiV characterizes the basicity and A/V the acidity of Lewis species. The enthalpy is given (approximately) by the simple relationship... 

Gutmann et al introduced the Acceptor Number (AN) as a measure of the solvent s Lewis acidity based on solvent-dependent NMR chemical shift of triethylphosphane oxide (Et3P=0). The dimensionless Acceptor Numbers also correlate linearly with (30) values according to the following equation ... 

The organic molecules or "probes used to investigate the dispersive surface energies of the fiber surfaces were a series of n-alkanes. The probes used to study the non dispersive forces were chosen based on their acidic or basic character as determined by Gutmann (8). Gutmann has practically defined basicity as the donor number, DN, or electron-donor capability in the Lewis sense. The donor scale is based on the value of the molar enthalpy for the reaction of the electron donor with a reference acceptor, SbCl. ... 

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