What Are Lewis Acids and Bases

Lewis acid Any molecule or Ion that can form a new covalent bond by accepting a pair of electrons. 

Note that, although we speak of a Lewis base as donating a pair of electrons, the term is not fully accurate. Donating in this case does not imply that the electron pair under consideration is removed completely from the valence shell of the base. Rather, donating means that the electron pair is shared with another atom to form a covalent bond. 

As we will see in the chapters that follow, a great many organic reactions can be interpreted as Lewis acid-base reactions. Perhaps the most important (but not the only) Lewis acid is the proton. Isolated protons, of course, do not exist in solution rather, a proton attaches itself to the strongest available Lewis base. When HCl is dissolved in water, for example, the strongest available Lewis base is an HgO molecule, and the following proton-transfer reaction takes place  

When HCl is dissolved in methanol, the strongest available Lewis base is a CH3OH molecule, and the following proton-transfer reaction takes place. An oxonium ion is an ion that contains an oxygen atom with three bonds and bears a positive charge. 

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In Chapter 6 we survey what has been accomplished and indicate directions for future research. Furthermore, we critically review the influence of water on Lewis acid - Lewis base interactions. This influence has severe implications for catalysis, in particular when hard Lewis acids and bases are involved. We conclude that claims of Lewis-acid catalysis should be accompanied by evidence for a direct interaction between catalyst and substrate. 

In reactions 13.46 and 13.49, which reactants are Lewis acids and which are Lewis bases Give an explanation for your answer. What is the general name for the products ... 

What are hard and soft Lewis acids and bases ... 

Eq. 5.30 is a general relationship for the interactions of electrophiles and nucleophiles, and is not restricted to definitions and discussions of hard and soft acids and bases. It tells us that the relative nucleophilicity of several Lewis bases will depend upon which electrophile is used, because the c s and yS values will change for each different electrophile. Similarly, the relative electrophilicities of several Lewis acids will depend upon what nucleophile is used. We will see exactly such results when we explore quantitative scales for various nucleophiles and electrophiles, where the scales are highly dependent upon the particular reaction that is chosen to analyze relative reactivities (see Chapter 8). Eq. 5.30 nicely explains the reactivity trends for soft acids and bases. It predicts that the Eoveriap will be best for Lewis acids and bases that have electrophilic and nuclephilic orbitals of roughly the same energy, which is the cases for the soft acids and bases of Table 5.8. 

In summary, the concepts of electrophilies and nucleophiles are very similar to those of Lewis acids and bases. A more thorough discussion of what makes good electrophiles and nucleophiles is left to Chapter 8. Until then, it is instructive to simply realize that trends of preferential reactivity fall into classes defined as hard and soft species, where nucleophiles and electrophiles within these individual classes prefer to react. The reactivity of the soft species is primarily due to better overlap of the orbitals, while for the hard species the electrostatic attraction dominates. 

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. 

The hydration of metal ions is a process that belongs to what we call the acid-base reaction in chemistry. The acid-base reaction involves no electron transfer between separate partners but a localized electron rearrangement to make up or break down a hybrid molecular orbital between an acid particle and a base particle, that is, the formation or the dissolution of a bonding molecular orbital due to the interaction between the frontier donor orbital of a particle Lewis base) and the frontier acceptor orbital of another particle Lewis acid). In order for an acid-base reaction to occur between a base particle and an acid particle, the electronic energy levels of the frontier orbitals for both acid and base particles are required to be close enough to each other for the orbital hybridization to prevail [3]. 

The properties and reactions of acids and bases are central to our study of organic chemistry. We need to consider exactly what is meant by the terms add and base. Most people would agree that H2SO4 is an acid and NaOH is a base. Is BF3 an acid or a base Is ethylene (H2C=CH2) an acid or a base To answer these questions, we need to understand the three different definitions of acids and bases the Arrhenius definition, the Brpnsted-Lowry definition, and the Lewis definition. 

What are the factors that determine the acid-base properties of solid surfaces such as metal oxides On the basis of the discussion thus far it seems appropriate to relate the appearance of Lewis acidity and disappearance of Bronsted acidity to the increase in the degree of dehydroxylation. Indeed, the interconversion of Lewis and Bronsted acid sites has been demonstrated for some oxides, such as ZnO or supported Mo03 Cr203, or WO3, by IR studies of pyridine or ammonia adsorption [59]. But which factors determine the strength of acid sites ... 

In the brief guidelines given above for what makes a good nucleophile and electrophile, we touched on the energy and accessibility of the electrophilic and nucleophilic orbitals. This brings us to another related concept, that of "hard" and "soft" acids and bases. In this definition, the acids and bases are best viewed as being of the Lewis type. Here we examine the "hardness" and "softness" of the acid and base to predict reactivity. In this analysis, the character of a nucleophile or electrophile is most often correlated with the polarizability of the species hard reactants are non-polarizable, whereas soft reactants are polarizable. The... 

On the other hand the (FTl2P)2 dimer represents the situation of the equivalent phosphorus atoms, both acting simultaneously as the Lewis acid and as the Lewis base what corresponds to two equivalent n(P) Opp orbital-orbital interactions (each characterized by the energy of 29.5 kcal/mol). Figure 15.12 presents the molecular graph of this complex. The authors described also P...P interactions in other complexes [55] for example for the FH2P...P(OH)H2 complex there are two important orbital-orbital interactions n(P) Opp and n(P) —Op-OH however the... 

What are the main characteristics of the molecules and ions that exhibit acid and base behavior In this chapter, we examine three different definitions the Arrhenius definition, the Br0nsted-Lowry definition, and the Lewis definition. Why three definitions, and which one is correct As Huheey noted in the quotation that opens this chapter, no single definition is correct. Rather, each definition is useful in a given instance. We discuss the Lewis definition of acids and bases in Section 15.11 here we discuss the other two. 

Summary of the Relationship between Diastereoselectivity and the Transition Structure. In this section we considered simple diastereoselection in aldol reactions of ketone enolates. Numerous observations on the reactions of enolates of ketones and related compounds are consistent with the general concept of a chairlike TS.35 These reactions show a consistent E - anti Z - syn relationship. Noncyclic TSs have more variable diastereoselectivity. The prediction or interpretation of the specific ratio of syn and anti product from any given reaction requires assessment of several variables (1) What is the stereochemical composition of the enolate (2) Does the Lewis acid promote tight coordination with both the carbonyl and enolate oxygen atoms and thereby favor a cyclic TS (3) Does the TS have a chairlike conformation (4) Are there additional Lewis base coordination sites in either reactant that can lead to reaction through a chelated TS Another factor comes into play if either the aldehyde or the enolate, or both, are chiral. In that case, facial selectivity becomes an issue and this is considered in Section 2.1.5. 

Both hydroxide ion and chloride ion have extra lone pairs of electrons these two are the electron pair donors, the Lewis bases. Al(OH)3 and SnCl4 have additional spaces in their structures to accept pairs of electrons, which is what occurs when they form complex anions. Al(OH)3 and SnCl4 are the Lewis acids in these reactions. 

These qualitative explanations, whether they be hard-soft or ionic-covalent or Class A-Class B, all suffer from the arbitrary way in which they can be employed. All Lewis acid-base type interactions are composed of some electrostatic and some covalent properties, i.e., hardness and softness are not mutually exclusive properties. Predictions are straightforward when dealing with the extremes, but with other more ambiguous systems, one can be very arbitrary in explaining results and the predictive value is impaired. What is needed is a quantitative assessment of the essential factors which can contribute to donor strength and acceptor strength. Proper combination of these parameters should produce the enthalpy of adduct formation. Until this can be accomplished, one could even question the often made assumption that the strength of the donor-acceptor interaction is a function of the individual properties of a donor or acceptor. 

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