Lewis acid-base reaction theory

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... 

Proton (H" ) transfer (Bronsted-Lowry theory) is a special type of Lewis acid-base reaction. 

The scientist G.N. Lewis put forward a more elaborate theory regarding acids and bases. According to him, a Lewis acid is a species that can accept a pair of electrons from another species. He defined Lewis base as any species that can donate a pair of electrons to another species. For a thorough understanding of this concept, let s look at a typical Lewis acid-base reaction. The reaction between ammonia (NH3) and boron trichloride (BCI3) is shown ... 

These reactions are Lewis acid-base reactions. The Lewis theory of acids and bases defines an acid as a substance capable of accepting a pair of electrons and a base as a substance that donates a pair of electrons. The terms acceptor and donor are sometimes used for acid and base, respectively. A Lewis acid-base reaction results in the formation of a coordinate bond, equation (9). 

On a final note, we shall attempt to show how FMO theory can be used to connect the seemingly diverse Lewis acid-base reactions and oxidation-reduction reactions. When the LUMO of the acid and HOMO of the base have similar energies and appropriate symmetries for nonzero overlap, they can combine to form a... 

Lewis acid-base theory (Jensen, 1980) is an outgrowth of the Lewis model of chemical bonds. A Lewis acid is a chemical species that can accept an electron pair. Lewis acids can be cations like Fe or Cu or they can be species with empty or partially empty valence orbitals such as CO2 or SO2. Lewis bases can donate an electron pair. Lewis bases are anions like OH" or S or they can be species with lone pairs such as HjO or NHj. The transfer of cations from a solid, such as szomolnokite (FeS04 H20), to form a hydrated ferrous ion in solution is a typical Lewis acid-base reaction. 

The reaction of boron trichloride with ammonia is a typical Lewis acid-base reaction. The Lewis theory is sufficiendy general that it covers all acid-base reactions that the other theories include, plus many additional reactions such as metal complex formation (see Chapter 25). 

In Chapter 21, the theory of this section will be applied to Lewis acid-base reactions. 

Lewis Acids and Bases— The Lewis acid-base theory views an electron-pair acceptor as a Lewis acid and an electron-pair donor as a Lewis base. The addition compound of a Lewis acid-base reaction is referred to as an adduct. The theory is most useful in situations that cannot be described by means of proton transfers, for example, in reactions involving gases and solids and in reactions between organic compounds (considered in Chapter 27). 

Thus an acid-base reaction involves the transfer of an oxide ion (compared with the transfer of a proton in the Bronsted theory) and the theory is particularly applicable in considering acid-base relationships in oxide, silicate and aluminosilicate glasses. However, we shall find that it is subsumed within the Lewis definition. 

This theory was advanced by G. N. Lewis (1916, 1923, 1938) as a more general concept. In his classic monograph of 1923 he considered and rejected both the protonic and solvent system theories as too restrictive. An acid-base reaction in the Lewis sense means the completion of the stable electronic configuration of the acceptor atom of the acid by an electron pair from the base. Thus ... 

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 . 

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). 

In the same year that Bronsted and Lowry proposed their definition of acids and bases, an American chemist named Gilbert Lewis proposed an alternative definition that not only encompassed Bronsted-Lowry theory but also accounted for acid-base reactions in which a hydrogen ion isn t exchanged. Lewis s definition relies on tracking lone pairs of electrons. Under his theory, a base is any substance that donates a pair of electrons to form a coordinate covalent bond with another substance, while an acid is a substance that accepts that electron pair in such a reaction. As we explain in Chapter 5, a coordinate covalent bond is a covalent bond in which both of the bonding electrons are donated by one of the atoms forming the bond. 

Using this theory, a number of organic reactions can be considered as acid-base reactions, because they do not have to occur in solution. Lewis acids are known as aprotic acids, compounds that react with bases by accepting pairs of electrons, not by donating protons. 

More recently, a quantitative scale for Lewis acidity based on fluoride ion affinities was calculated using ab initio calculations at the MP2/B2 level of theory.26 Due to its high basicity and small size, the fluoride ion reacts essentially with all Lewis acids thus the fluoride affinity (or reaction enthalpy) may be considered as a good measure for the strength of a Lewis acid. An abbreviated pF scale is given in Table 1.3. This scale was used recently by Christe and Dixon112 for estimating the stability of salts of complex fluoro anions and cations. The pF value represents the fluoride affinity in kcal mol 1 divided by 10. 

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. 

The Laplacian of the electron density plays a dominant role throughout the theory.191 In addition, Bader has shown that the topology of the Laplacian recovers the Lewis model of the electron pair, a model that is not evident in the topology of the electron density itself. The Laplacian of the density thus provides a physical valence-shell electron pair repulsion (VSEPR) basis for the model of molecular geometry and for the prediction of the reaction sites and their relative alignment in acid-base reactions. This work is closely tied to earlier studies by Bader of the electron pair density, demonstrating that the spatial localization of electrons is a result of a corresponding localization of the Fermi correlation hole. 

Lewis noted that the Brvj/nsted theory was limited because it focused exclusively on the transfer of a protofi (H+). He argued that a more general definition of acid-base reactions could be obtained by looking at what happens when an H+ ion combines with an OH" ion to form water. 

In combustion analysis of organic compounds, solid NaOH is used to absorb the CO2 from the combustion gases. Account for this reaction in terms of Lewis acid-base theory. 

Lewis acid-base theory— To describe reactions where no proton transfer occurs, - Lewis has proposed a more general acid-base concept where an acid is a molecule or an ion with an incomplete outer electron sphere, i.e., an acceptor of electron pairs. A base is a molecule or an ion having a free electron pair, i.e., an electron pair donor. A neutralization reaction is characterized by the formation of a coordination or covalent bond. The following reactions illustrate the Lewis-definition ... 

Donor number (or donicity), DN — is an empirical semiquantitative measure of nucleophilic properties (-> acid-base theories, subentry Lewis acid-base theory) of a solvent defined as the negative of the standard molar heat of reaction (expressed in kcalmol-1) of the solvent D with antimony pentachloride to give the 1 1 adduct, when both are in dilute solution in the inert diluent 1,2-dichloroethane, according to the reaction scheme ... 

According to the Lewis theory of acids and bases (to be discussed in detail in Chapter 5), an acid is an electron pair acceptor and a base is an electron pair donor. Accordingly, the following reactions are acid-base reactions because they represent processes in which electron pair donation and acceptance occurs ... 

Lewis examined four characteristics of acid-base reactions that are to be accounted for by any theory ... 

According to the Lewis acid-base theory, we can now consider reactions such as the following to be acid-base reactions ... 

Electron donation-acceptance reactions, which are considered to be Lewis acid-base interactions, also include the formation of coordination compounds, complex formation through hydrogen bonding, charge transfer complex formation, and so on. It should be apparent that the Lewis theory of acids and bases encompasses a great deal of both inorganic and organic chemistry. 

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