Bases Lewis theory

The Hard-Soft-Add-Base (HSAB) theory was developed by Pearson in 1963. According to this theory, Lewis acids and Lewis bases are divided into two groups on one hand hard acids and bases, which are usually small, weakly polarizable species with highly localised charges, and on the other hand soft acids and bases which are large, polarizable species with delocalised charges. A selection of Lewis acids, ordered according to their hardness in aqueous solution is presented in Table 1.3. 

A proton (H+) is an electron pair acceptor. It is therefore a Lewis acid because it can attach to ( accept") a lone pair of electrons on a Lewis base. In other words, a Bronsted acid is a supplier of one particular Lewis acid, a proton. The Lewis theory is more general than the Bronsted-Lowry theory. For instance, metal atoms and ions can act as Lewis acids, as in the formation of Ni(CO)4 from nickel atoms (the Lewis acid) and carbon monoxide (the Lewis base), but they are not Bronsted acids. Likewise, a Bronsted base is a special kind of Lewis base, one that can use a lone pair of electrons to form a coordinate covalent bond to a proton. For instance, an oxide ion is a Lewis base. It forms a coordinate covalent bond to a proton, a Lewis acid, by supplying both the electrons for the bond ... 

A note on good practice The entities that are regarded as acids and bases are different in each theory. In the Lewis theory, the proton is an acid in the Bronsted theory, the species that supplies the proton is the acid. In both the Lewis and Bronsted theories, the species that accepts a proton is a base in the Arrhenius theory, the species that supplies the proton acceptor is the base (Fig. 10.61. 

Lewis, G. N., 60, 64, 398 Lewis acid, 398, 473, 671 Lewis base, 398, 473 Lewis structure, 65 writing, 67 Lewis symbol, 60 Lewis theory, limitations of, 115... 

Heisenberg, Z. Physik, 33, 879 (1925). Bom and Jordan, ibid., 34, 858 (1925). Born, Heisenberg and Jordan, ibid., 35, 557 (1926) etc. Heisenberg based his theory on the thesis that it should not contain reference to quantities which are in principle unobservable. It is of interest that G. N. Lewis, in 1923, suggested this idea, writing. "we should look for no effects which depend upon the momentary position of any electron in its orbit ("Valence and the Structure of Atoms and Molecules, Chemical Catalog Co., New York, 1923, p. 52). 

Two acid-base theories are used in organic chemistry today the Br0nsted theory and the Lewis theory. These theories are quite compatible and are used for different purposes. ... 

At about the same time that Brpnsted proposed his acid-base theoiy, Lewis put forth a broader theory. A base in the Lewis theory is the same as in the Brpnsted one. 

Since Arrhenius, definitions have extended the scope of what we mean by acids and bases. These theories include the proton transfer definition of Bronsted-Lowry (Bronsted, 1923 Lowry, 1923a,b), the solvent system concept (Day Selbin, 1969), the Lux-Flood theory for oxide melts, the electron pair donor and acceptor definition of Lewis (1923, 1938) and the broad theory of Usanovich (1939). These theories are described in more detail below. 

Although Lewis and Bronsted bases comprise the same species, the same is not true of their acids. Lewis acids include bare metal cations, while Bronsted-Lowry acids do not. Also, Bell (1973) and Day Selbin (1969) have pointed out that Bronsted or protonic acids fit awkwardly into the Lewis definition. Protonic acids cannot accept an electron pair as is required in the Lewis definition, and a typical Lewis protonic add appears to be an adduct between a base and the add (Luder, 1940 Kolthoff, 1944). Thus, a protonic acid can only be regarded as a Lewis add in the sense that its reaction with a base involves the transient formation of an unstable hydrogen bond adduct. For this reason, advocates of the Lewis theory have sometimes termed protonic adds secondary acids (Bell, 1973). This is an unfortunate term for the traditional adds. 

It is better than the Lewis theory for describing acid-base cements, for it avoids the awkwardness that the Lewis definition has with protonic acids. However, as Day Selbin (1969) have observed, the generality of the theory is such that it includes nearly all chemical reactions, so that acid-base reactions could simply be termed chemical reactions . 

From this discussion it can be seen that there is no ideal acid-base theory for AB cements and a pragmatic approach has to be adopted. Since the matrix is a salt, an AB cement can be defined quite simply as the product of the reaction of a powder and liquid component to yield a salt-like gel. The Bronsted-Lowry theory suffices to define all the bases and the protonic acids, and the Lewis theory to define the aprotic acids. The subject of acid-base balance in aluminosilicate glasses is covered by the Lux-Flood theory. 

C) The Bronsted-Lowry or proton theory interprets the acid-base reaction as a mere proton exchange between the acid (proton donor) and the base (proton acceptor) however, the Lewis theory or electron theory interprets the reaction as a donation and acceptance of a lone pair of electrons, where the... 

Comparison of Bronsted reaction 4.48 with Lewis reaction 4.49 shows that the Lewis theory is more generally applicable, but its interpretation is different in terms of the definition of acids and complexes. In fact, the Lewis theory is valid for all acid-base reactions (cf., eqns. 4.39 and 4.40). 

Acid-base behavior according to the Lewis theory has many of the same aspects as does acid-base theory according to the Bronsted-Lowry theory. 

As we have seen, the Lewis theory of acid-base interactions based on electron pair donation and acceptance applies to many types of species. As a result, the electronic theory of acids and bases pervades the whole of chemistry. Because the formation of metal complexes represents one type of Lewis acid-base interaction, it was in that area that evidence of the principle that species of similar electronic character interact best was first noted. As early as the 1950s, Ahrland, Chatt, and Davies had classified metals as belonging to class A if they formed more stable complexes with the first element in the periodic group or to class B if they formed more stable complexes with the heavier elements in that group. This means that metals are classified as A or B based on the electronic character of the donor atom they prefer to bond to. The donor strength of the ligands is determined by the stability of the complexes they form with metals. This behavior is summarized in the following table. 

Based on Lewis theory, one would expect C2 to have the greater bond energy due to the formation of quadruple bond, CasC vs. Li—Li. The molecular orbital diagrams and bond orders are as follows. 

According to Lewis theory, water can act as a Lewis base (electron pair donor). Water contains an unshared pair of electrons that is utilized in accepting a proton to form the hydronium ion. 

At the microscopic level, acids are defined as proton (H ) donors (Bronsted-Lowry theory) or electron-pair acceptors (Lewis theory). Bases are defined as proton (H+) acceptors (Bronsted-Lowry theory) or electron-pair donors (Lewis theory). Consider the gas-phase reaction between hydrogen chloride and ammonia ... 

LEWIS ACID LEWIS BASE BR0NSTED THEORY Acid anhydrides,... 

According to the Lewis theory, alkaline earth metal hydroxides are weaker bases than their oxides, the order of the strength of the basic sites being Ba(OH)2> SrO(OH)2 > Ca(OH)2 > Mg(OH)2. The hydroxides have been used recently as solid catalysts for organic transformations, such as the conjugate addition of methanol to a, S-unsaturated carbonyl compounds (12), cyanoethylation of alcohols (163,164), and transesterification reactions (166,167,171,172) which are described above. The extensive work of Sinisterra et al. (282) on the number and nature of sites and on the catalytic activity of the most basic alkali metal hydroxide, Ba(OH)2, is emphasized. It was found that commercial barium hydroxide octahydrate can be converted into... 

Lewis Theory of Acids and Bases. According to Lewis, acids are electron-pair acceptors (EPA) and bases electron-pair donors (EPD) connected through the equilibrium (fig 3.2). 

Problem 13.53 How does the Lewis theory of acids and bases explain the functions of (o) ZnCl, in the Lucas reagent (b) ether as a solvent in the Grignard reagent M... 

The Lewis theory of acids and bases defines an acid as an electron-parr acceptor, and a base as an electron-parr donor. Thus, a proton is only one of a large number of species that may function as a Lewis acid. Any molecule or ion may be an acid if it has an empty orbital to accept a parr of electrons (see Chapter 2 for orbital and Lewis theory). Any molecule or ion with a pair of electrons to donate can be a base. 

At about the same time that Bronsted proposed his acid-base theory, Lewis put forth a broader theory, A base in the Lewis theory is the same as in the Brpnsted one, namely, a compound with an available pair of electrons, either unshared or in a tt orbital. A Lewis acid, however, is any species with a vacant orbital.1115 In a Lewis acid-base reaction the unshared pair of the base forms a covalent bond with the vacant orbital of the acid, as represented by the general equation... 

Chlorhexidine is a strong base (Lewis acid-base theory) because it reacts with acids to form salts of the RX2 type, and it is practically insoluble in water (<0.008% wt/vol at 20°C). The water solubility of the different salts varies widely as demonstrated in Table 2.13. Chlorhexidine is moderately surface-active (a net+chare over its surface) and forms micelles (molecular aggregates form colloidal particles) in solution the critical micellar concentration of the acetate is 0.01% wt/vol at 25°C (Heard and Ashworth 1969). Aqueous solutions of chlorhexidine are most stable within the pH range of 5-8, and above pH 8.0 chlorhexidine is precipitated because conditions for a base (>pH 7) reaction are present. 

Lewis theory of acids and bases a Lewis acid is a compound capable of accepting an electron pair, and a Lewis base is capable of donating an electron pair. 

Qualitative and quantitative aspects of the Lewis theory of acids and bases, and practical applications of Lewis acids, are discussed in a series of monographs [1,4-6,30-46] and reviews [47-49], The following aspects are taken into account (a) electronic configuration of acceptors (A = M MX are generally metal and boron salts), (b) nature of anions (usually halides), (c) peculiarities of thin structure of donors (B are generally the compounds containing N, P, As, Sb O, S, Se, Te F, Cl, Br, I atoms) their electronic structure, spatial accessibility, and mutual position of donor centers. Moreover, the nature of X, order of binding of A and B in formation of adducts of type AB , nature of solvents, and evaluation of AH or AG of the processes (1.1)—(1.5) [31,48] should also be considered. 

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