Electron-deficient atoms, Lewis acids

The question as to whether the acid properties of cracking catalysts are due to protons (Bronsted acids) or to electron-deficient atoms (Lewis acids) is somewhat more difficult to answer. Before considering this question, the origin of acid centers in silica-alumina compositions should be discussed. It is generally believed that acid centers, either the... 

One of the characteristic properties of organometallic reagents 1 is that the carbon atom serves as a nucleophile in chemical reactions. In contrast, when a carbon atom is bonded to more electronegative elements such as the halogen atom in the alkyl halide 2 (X = Cl, Br, or I) or oxygen atom in the carbonyl compound 3, it is electron-deficient or Lewis acidic and possesses a partial positive charge. Such carbon atoms then serve as electrophiles in chemical reactions. 

It is clear from the above observations that pyridine molecule interacts on the catalyst surface in the following three modes (1) interaction of the N lone pair electron and the H atom of the OH group, (2) transfer of a proton from surface OH group to the pyridine forming a pyridinium ion (Bronsted acidity), and (3) pyridine coordination to an electron deficient metal atom (Lewis acidity). Predominant IR bands, vga and vigb, confirms that the major contribution of acidity is due to Lewis acid sites from all compositions. Between the above two modes of vibrations, Vsa is very sensitive with respect to the oxidation state, coordination symmetry and cationic environment [100]. A broad feature for v a band on Cu containing... 

The Br0nsted-Lowry definition of acids and bases depends on the transfer of a proton from the acid to the base. The base uses a pair of nonbonding electrons to form a bond to the proton. G. N. Lewis reasoned that this kind of reaction does not need a proton. Instead, a base could use its lone pair of electrons to bond to some other electron-deficient atom. In effect, we can look at an acid-base reaction from the viewpoint of the bonds that are formed and broken rather than a proton that is transferred. The following reaction shows the proton transfer, with emphasis on the bonds being broken and formed. Organic chemists routinely use curved arrows to show the movement of the participating electrons. 

Acidity and basicity are relative properties. Many compounds are amphoteric and behave as acids or as bases according to a partner. Metal oxides are classified as acidic, amphoteric or basic. Experimentally, this classification corresponds to the adsorption of probe molecules[7, 8]. NH3 is a base probe molecule that reacts with the electron deficient metal atoms (Lewis acid) or the protons adsorbed on the hydrated surface, CO2 is usually considered as acidic and thus it is expected to adsorb more strongly on basic sites. According to this classification, Ti02 belongs to an amphoteric species and MgO to a basic species. A general difficulty for such classifications is that the order can vary with the choice of the probe. The Hard and Soft Bases and Acids theory[9, 10] responds to the necessity to refine the model with a second scale it is better to couple... 

In the Lewis acid-base definition, an acid is any species that accepts a lone pair to form a new bond in an adduct. Thus, there are many more Lewis acids than other types. Lewis adds include molecules with electron-deficient atoms, molecules with polar multiple bonds, and metal cations. 

Lewis Acids with Electron-Deficient Atoms Some molecular Lewis acids contain a central atom that is electron deficient, one surrounded by fewer than eight valence electrons. The most important of these acids are covalent compounds of the Group 3A(I3) elements boron and aluminum. As noted in Chapters 10 and 14, these compounds react to complete their octet. For example, boron trifluoride accepts an electron pair from ammonia to form a covalent bond in a gaseous Lewis acid-base reaction ... 

The Lewis acid-base definition focuses on the donation or acceptance of an electron pair to form a new covalent bond in an adduct, the product of an acid-base reaction. Lewis bases donate the electron pair, and Lewis acids accept it. Thus, many species that do not contain El are Lewis acids. Molecules with polar double bonds act as Lewis acids, as do those with electron-deficient atoms. Metal ions act as Lewis acids when they dissolve in water, which acts as a Lewis base, to form an adduct, a hydrated cation. Many metal ions function as Lewis acids in biomolecules. 

Nucleophiles (a) and (d). Both I and S in these species contain one or more lone pairs, making them Lewis bases, capable of attacking electron-deficient atoms such as those found in Lewis acids. 

Electrophile An electron-deficient atom, ion, or molecule that has an affinity for an electron pair, and will bond to a base or nucleophile. In general, electrophiles (literally electron-lover) are positively charged or neutral species that participate in a chemical reaction by accepting an electron pair in order to bond to a nucleophile. Because electrophiles accept electrons, they are Lewis acids. 

In the Bronsted/Lowry definition, a base donates an electron pair to a proton to form a covalent B—bond (a positive charge, if involved, will reside on B). In the Lewis definition, a base donates a pair of electrons to an electron deficient atom (other than a proton). An acid does not donate a proton but accepts a base to form a new bond. An electron pair is reqnired for forming a covalent or a dative bond. 

Besides doping, co-doping in where two or more elements are present on the G sheet is also interesting from the point of view of using Gs as catalysts. One case that could be of interest is the presence of electron deficient atoms such as B and heteroatoms with excess of electrons such as N. In a certain way the simultaneous presence of B and N could be similar to the introduction of acid and basic Lewis sites. The concept of bifunctional acid-base solid catalyst has found wide application in heterogeneous catalysis of solids such as aluminophosphates containing NH groups (ALPONs) due to the cooperative activity of both sites in the reaction mechanism. 

The fundamental mechanistic concept around which the course of aldol reactions under conditions of kinetic control has been interpreted involves the postulation of a cyclic transition state in which both the carbonyl and the enolate oxygen are coordinated to a Lewis acid. We will use the Li cation in our discussion, but another metal cation or electron-deficient atom may play the same role. 

If the acid-base reaction is written with the electron pairs and the arrows, as shown for water and HCl, the Lewis base definition is quite useful. The electron-rich molecule is the base, and the electron-rich atom donates two electrons. The molecule bearing the electron-deficient atom (hydrogen) is the acid. For reactions of organic molecules, it is essential to identify electron-rich and electron-poor components of molecules, to understand the electron flow, and to understand how to predict the products. That process begins with making the transition to thinking in terms of Lewis bases/Lewis acids rather than Br0nsted-Lowry acids and bases. 

Amines are good Lewis bases, donating two electrons to electron-deficient atoms to form a dative bond in the expected ate complex (see Chapter 2, Section 2.5). Diethylamine (iV-ethylethanamine, 40) readily reacts with the strong Lewis acid to form ate complex 41. Note the dative bond between nitrogen and aluminum and the direction of the arrow from the electron-rich nitrogen atom to the electron-deficient aluminum atom. Similarly, tertiary amine 42 (iV-ethyl-iV-methylethanamine) reacts with boron trifiuoride (BF3) to form ate complex 43. Amines will be used as Lewis bases several times in this book. 

Chapter 10 discusses the reaction of a nucleophile with a carbocation in the second step of a two-step mechanism, where the carbocation is generated by the reaction of a mineral acid with an alkene (Section 10.2). In Chapter 6 (Section 6.7), a nucleophile is defined as a species that donates two electrons to carbon, to form a new covalent a-bond to carbon. A carbocation has a positively charged carbon atom, and a nucleophile such as chloride ion, bromide ion, or iodide ion donates two electrons to that electron-deficient atom. The Lewis base analogy for a nucleophile is appropriate in these reactions, in which a carbocation 1 reacts with a nucleophile, iodide ion, to form a new C-I bond in 2. 

An electrophilic center is an electron-deficient atom that is capable of accepting a pair of electrons. Notice that this definition is very similar to the definition for a Lewis acid. In fact, the terms electrophile and Lewis acid are synonymous. 

Any electron-deficient atom can act as a Lewis acid. Many compounds containing group IIIA elements such as boron and aluminum are Lewis acids because group IIIA atoms have only a sextet of electrons in their outer shell. Many other compounds that have atoms with vacant orbitals also act as Lewis acids. Zinc and iron(III) halides (ferric halides) are frequently used as Lewis acids in organic reactions. 

In this reaction, the unshared electron pair of the base forms a coordinate covalent bond (or dative bond or dipolar bond) with an electron-deficient atom of the acid. The archetype of a Lewis acid/base reaction is... 

For a given molecule, the most Lewis acidic atom = most electron-deficient atom. 

In a generalized sense, acids are electron pair acceptors. They include both protic (Bronsted) acids and Lewis acids such as AlCb and BF3 that have an electron-deficient central metal atom. Consequently, there is a priori no difference between Bronsted (protic) and Lewis acids. In extending the concept of superacidity to Lewis acid halides, those stronger than anhydrous aluminum chloride (the most commonly used Friedel-Crafts acid) are considered super Lewis acids. These superacidic Lewis acids include such higher-valence fluorides as antimony, arsenic, tantalum, niobium, and bismuth pentafluorides. Superacidity encompasses both very strong Bronsted and Lewis acids and their conjugate acid systems. 

Acid Halides (Lewis Acids). AH metal haUde-type Lewis catalysts, generally known as Friedel-Crafts catalysts, have an electron-deficient central metal atom capable of electron acceptance from the basic reagents. The most frequendy used are aluminum chloride and bromide, followed by... 

Metal Alibis and Alkoxides. Metal alkyls (eg, aluminum boron, sine alkyls) are fairly active catalysts. Hyperconjugation with the electron-deficient metal atom, however, tends to decrease the electron deficiency. The effect is even stronger in alkoxides which are, therefore, fairly weak Lewis acids. The present discussion does not encompass catalyst systems of the Ziegler-Natta type (such as AIR. -H TiCl, although certain similarities with Friedel-Crafts systems are apparent. 

The coordination of the dienophile to a Lewis acid (in the calculations a proton was used as the Lewis acid) leads also to an increase in regioselectivity. The re-gioselectivity of reactions of electron-rich, or conjugated dienes, with electron-deficient dienophiles is also controlled hy the diene HOMO-dienophile LUMO interaction. From Fig. 8.2 it appears that the difference in magnitudes of the LUMO coefficients at carhon atoms 1 and 2 of acrolein (Ci -C2 = 0.20) is smaller than the same difference for protonated acrolein (Ci -C2 = 0.30-0.43) so that the reaction of the latter should he considerable more regioselective than the former in accordance with the experimental results [3]. 

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