Hard acids, defined

Later on, Pearson [75] introduced the concept of hard and soft acid and bases (HSABs) hard acids (defined as small-sized, highly positively charged, and not easily polarizable electron acceptor) prefer to associate with hard bases (i.e., substances that hold their electrons tightly as a consequence of large electronegativities, low polarizabilities, and difficnlty of oxidation of their donor atoms) and soft acids prefer to associate with soft bases, giving thermodynamically more stable complexes. According to this theory, the proton is a hard acid, whereas metal cations may have different hardnesses. 

In conclusion, the C/E ratios for donors (acids) indicate whether hardness or softness is most important in interactions of a particular donor (acid), but softness or hardness so defined does not enable one to predict even the relative strength of interaction towards a soft or hard acid (base) because the magnitude of the C and E numbers are lost when the ratio is taken. 

We define the hard acids simply as those of lovi polarizability. They would be of small size and high positive charge. The soft acids would be those of high polarizability, large size, and low positive charge. As you can see, there are many other acids besides metal ions which can be classified in this way. 

Chemical stability of carbon over the entire pH range has led to considerable interest in the development of carbon-based stationary phases for RPC. Porous graphitised carbon with sufficient hardness, well-defined and stable pore structure without micropores, which ensures sufficient retention and fast mass transfer can be prepared by a complex approach consisting of impregnation of the silica gel with a mixture of phenol and formaldehyde followed by formation of phenol-formaldehyde resin in the pores of the silica gel, then thermal carbonisation and dissolution of the silica gel by hydrofluoric acid or a hot potassium hydroxide. solution [48. The retention and selectivity behaviour of carbon phases significantly differs from that of chemically bonded pha.ses for RPC. Carbon adsorbents have greater affinity for aromatic and polar substances so that compounds can be separated that are too hydrophilic for adequate retention on a Cix column. Fixed adsorption sites make these materials more selective for the separation of geometric isomers [49]. 

Concepts of acidity and basicity are, in practice, defined and evaluated by their utility. Since overly formd definitions can be restrictive the concepts of acidity evolve towaids more comprehensive definitions. For example the Lewis definition includes the Broensted definition simply regarding the proton as an electron acceptor. Because the interaction of Broensted acids and bases in solutions involves a common process, protic transfer, scales of acidity can be established, for example the Hammett [1] acidity function. For Lewis acid-base interaction there is no common process to provide a unique basis for comparisons of acid strength. Experimentally, the strength of a Lewis acid depends upon the particular Lewis base. The classification of acids and bases as hard or soft in the principle of hard and soft acids and bases (HSAB principle) clarifies the interactions of Lewis acids and bases [2a]. Strong interactions occur between hard acid and hard base, or between soft acid and soft base, hi the hard-hard interaction there is a considerable electrostatic contribution to bonding and in the soft-soft interaction there is a major covalent contribution to bonding. The use of density functional analysis has clarified the concepts of hardness and softness and an empirical ranking of Lewis acids, based on local hardness is, proposed [2c]. 

Acidity of Lewis Acids. In contrast to the quantitative scale of acid strength established for Brqnsted acids a similar scale for Lewis acids has not been developed. Perhaps the best known of the various approaches is the hard and soft acids and bases (HSAB) principle introduced by Pearson, defining hardness and softness as specific qualities of acids and bases (16). Hard acids have small acceptor atoms, their outer electrons are not easily excited, and bear considerable positive charge. Soft acids, in turn, have large acceptor atoms, the outer electrons are easily excited, and possess lower positive charge. A selection of typical hard and soft acids is given in Table 3. The HSAB concept predicts that hard acids prefer to associate with hard bases and soft acids with soft bases... 

In general, interactions (defined as simple coordinations between the acid and base, or as reactions such as substitutions) are most facile between hard acids and hard bases and between soft acids and soft bases (a trend called the principle of hard and soft acids and bases, HSAB). In other words, interactions between acids and bases of the same type are more facile than between a hard acid and a soft base, and vice versa. For example, an interaction between lithium and hydroxide is predicted to be more favorable than between lithium and iodide, while iodide will interact well with Cu (see Table 5.8). A well known example of this is the strong interaction between mercury metal and sulfur (leading to the old fashioned name mercaptan for a thiol). 

Besides the estimation of the strength of the -OH bonding at the active site in the zeolite by ab initio calculation of the energy levels, it is possible to extract information relating to the intrinsic acidity using concepts in the framework of hard and soft acids and bases. The fundamental parameter is the absolute hardness t), defined as the second derivative of the energy with respect to the number of particles... 

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