Lewis acids, classification

The bonds formed between germanium and elements of Group VA can be divided into two categories those in which the element is bonded via a covalent bond and others in which a Lewis acid-Lewis base interaction between the two elements results in coordinate bond formation. The same classification of bond types is also appropriate when discussing organogermanium derivatives of Groups VIA and VII A. 

Tthe reactions between LiOH and Li2COi with different manganese oxides can be viewed as acid-base interactions. According to Lewis classification, lithium compounds are bases whereas manganese oxides are acids of different strength. Acid-base properties of oxides significantly depend on the oxidation degree of cation. The acidity increases in the raw... 

Rather than introduce new words into an already confusing terminology, Bell (64) is of the opinion that the words acid and base should be confined to proton acids, and the Sidgwick classification of molecules as electron donor and electron acceptor (which is essentially equivalent to the Lewis classification of acids and bases) be employed, together with the categories of nucleophilic and electrophilic reagents as defined by Ingold (65). 

The surfaces of zeolites ean thus display Brpnsted or Lewis acid sites or a combination of the two, depending on how the zeolite is prepared. Brpnsted acid sites are converted into Lewis acid sites as the temperature is increased above approximately 500°C and water is driven off The overall catalytic activity of zeolites as solid acids depends on the number and the properties of the acid site. The strength of the acid sites is directly related to the framework composition of the zeolite. Zeolites with low Si/Al ratios can have higher concentrations of catalytic sites, and zeolites with high Si/Al ratios show strong acidity. Their classifications are shown in Table 2. An H-Y or H-ZSM-5 zeolite with a low density of acid groups is like an ideal solution of dispersed noninteracting protons in a solid matrix. 

The compounds considered as bases according to the Lewis classification are practically the same as those by the Brpnsted definition Species, which are able to add a proton, possess a pair of electrons and, naturally, they react with electron-pair acceptors. In contrast, there are substantial differences when acids are considered. First of all, Brpnsted acids are able to split off proton, whereas Lewis acids do not necessarily contain a hydrogen atom. Acid-base reactions, correspondingly, are much simpler in the Br0nsted sense, since they involve proton transfer, that is, the transfer of a nucleus without electrons. Furthermore, the reaction of any Brpnsted acid with a base B produces the same species, the conjugate acid BH+ irrespective of the acid. That is why the behavior of Brpnsted acids is similar toward all bases and indicators, and their catalytic effect is the same in specific acid catalysis. The products of the reaction of Lewis acids with a base, in turn, are different with each acid-base complex having specific properties. The behavior of Lewis acids toward bases and indicators, and their catalytic effect, therefore, may be substantially different. It is quite interesting to point out that an acid is a proton donor in the Brpnsted sense, but the proton itself is the acid in the Lewis picture due to its vacant orbital. 

An even more general theory of acids and bases was given by the American chemist G. N. Lewis in 1923. In this theory, an acid is an electron acceptor and a base is an electron donor. This is a more general theory than the Br0nsted-Lowry theory, because it allows the acid-base classification to be applied to reactions in which neither H (aq) nor OH (aq) play a role, or even to reactions in which there is no solvent. For example, the following are acid-base reactions in the Lewis theory... 

Several ways how (asymmetric) organocatalysis can be classified have been reported [14]. One general strategy is a classification according to Lewis acids/Lewis bases and Brpnsted acids/Brpnsted bases with the general simphfied activation modes depicted in Scheme 6.16. 

Analogous to the classification of Lewis acids and bases in hard and soft species, Ahrland et al. have su ested a division of donors and acceptors into classes a and 6. See Ahrland, S. Chatt, J. Davies, N.R. Quart. Rev. 1958, 77, 265... 

Reactions of another class are catalyzed by Pd(II) compounds which act as Lewis acids, and are treated in Chapter 5 and partly in Chapter 4. From the above-mentioned explanation, the reactions catalyzed by Pd(0) and Pd(II) are clearly different mechanistically. In this book the stoichiometric and catalytic reactions are classified further according to reacting substrates. However, this classification has some problems, viz. it leads to separate treatment of some unit reactions in different chapters. The carbonylation of alkenes is an example. Oxidative carbonylation of alkenes is treated in Chapter 3 and hydrocar-bonylation in Chapter 4. 

What Do We Need to Know Already This chapter draws on many of the principles introduced in the preceding chapters. In particular, it makes use of the electron configurations of atoms and ions (Sections 1.13 and 2.1) and the classification of species as Lewis acids and bases (Section 10.2). Molecular orbital theory (Sections 3.8 through 3.12) plays an important role in Section 16.12. 

The classification of Lewis acids and bases relevant to AB cements is shown below. 

Yatsimirskii considered that the hard and soft classification was too general and proposed instead a more detailed approach. He classified Lewis acids and bases into six groups, based on the nature of the adduct bonding. 

Another feature of the metal ions that are typically involved in cementitious bonding in AB cements is that most of them fall into the category of hard in Pearson s Hard and Soft Acids and Bases scheme (Pearson, 1963). The underlying principle of this classification is that bases may be divided into two categories, namely those that are polarizable or soft, and those that are non-polarizable or hard. Lewis acids too may be essentially divided into hard and soft, depending on polarizability. From these classifications emerges the useful generalization that hard acids prefer to associate with hayd bases and soft acids prefer to associate with soft bases (see Section 2.3.7). 

Present understanding of Lewis acidity and basicity is based mainly on the A and B type classification of metal ions of Ahrland, Chatt,... 

Arranged in order of increasing /A, or ionicity on the M-L band. Also shown are ionic radii and HSAB classification (5) of the Lewis acids. 

From our previous treatment of the Arrhenius, Bransted and Lewis acid-base theories, the importance of the choice between the divergent solvent types clearly appeared if we now confine ourselves to solvents to which the proton theory in general is applicable, this leads to a classification of eight classes as already proposed by Bronsted35,36 (Table 4.3). 

The concept of hard and soft acids and bases ( HSAB ) should also be mentioned here. This is not a new theory of acids and bases but represents a useful classification of Lewis acids and bases from the point of view of their reactivity, as introduced by R. G. Pearson. 

Early attempts to fathom organic reactions were based on their classification into ionic (heterolytic) or free-radical (homolytic) types.1 These were later subclassified in terms of either electrophilic or nucleophilic reactivity of both ionic and paramagnetic intermediates - but none of these classifications carries with it any quantitative mechanistic information. Alternatively, organic reactions have been described in terms of acids and bases in the restricted Bronsted sense, or more generally in terms of Lewis acids and bases to generate cations and anions. However, organic cations are subject to one-electron reduction (and anions to oxidation) to produce radicals, i.e.,... 

Table 16.2 Hard-Soft Classification of Metals (Lewis acids) and Ligands (Lewis bases). ...

This concept was introduced qualitatively in the late 1950s and early 1960s by Pearson, in the framework of his classification of Lewis acids and bases, leading to the introduction of the hard and soft acids and bases (HSAB) principle [19-21]. This principle states that hard acids prefer to bond to hard bases and soft acids to soft bases. In many contributions, the factor of 1/2 is omitted. The inverse of the hardness was introduced as the softness S=l/rj [22]. A third quantity, which can be expressed as a derivative with respect to the number of electrons is the Fukui function, was introduced by Parr and Yang [23,24] ... 

The solvent classifications used here are (1) solvents possessing both Lewis acid and Lewis base properties and a dielectric constant (D) > 25 (2) solvents possessing both Lewis acid and Lewis base properties and D < 25 (3) solvents possessing only Lewis acid properties and D> 25 (4) same as(3) but D < 25 (5) solvents possessing only Lewis base properties and D> 25 (6) same as(5)but D < 25 (7) solvents possessing negligible Lewis acid or base properties and D > 25 and (8) same as (7) but D < 25. 

A qualitative classification of hardness and softness hcis been presented (2). This dassification divides the Lewis acids in Table 3 as follows ... 

The ratio, C/E, gives a quantitative order of relative hardness or "softness for the various Lewis acids and agrees fairly well with the qualitative classification of Pearson (2). The adds which do not follow the qualitative classification are BF3 and SO2. As mentioned above, the parameters for BF3 were determined from data limited to oxygen donors. The qualitative ordering of SO 2 is incorrect, Emd as will be shown shortly when strong interactions are compared with weak ones, the procedures... 

The rapid evolution and the multifarious activity in the field of phosphorus chemistry is well illustrated by the proposal (Dupart)332 of a new systematic classification of free and coordinated phosphorus compounds to complete the systems proposed by Wolf333 and by Perkins et a/.334. In this new system, a code number NFE(NPL)D could be used to describe the compound, in which NFE would be the number of free electrons, N the number of valence shell electrons, L the number of ligands and D the number of electron doublets donated or accepted, which would be negative if the P atom acted as a donor (a Lewis base), or positive if P acted as an acceptor (a Lewis acid). 

Table 3.20 Pearson s Classification of Lewis Acids and Bases...

In recent years the analogy between the Friedel-Crafts acylation reaction and various nitrating systems, particularly those in which Lewis acids act as catalysts, has been stressed,3 but this classification adds nothing new in principle. 

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