Lone pairs Lewis acids

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. 

Lewis s definition of an acid is also straightforward. Clearly, it has to be an entity that can attach to a base by sharing the latter s lone pair. Lewis proposed that that ability alone is sufficient for a species to be regarded as an acid. That is, according to Lewis, an acid is any species that can stick to a lone pair of electrons. Notice that the proton is not mentioned any suitably accepting species can be called an acid the proton is only one of a myriad possibilities. 

We have seen that a base can be defined as combining with a proton and, therefore, requires at least one lone pair of electrons. A more general definition of acids and bases, due to G. N. Lewis, describes a base as any species (atom, ion or molecule) which can donate an electron pair, and an acid as any species which can accept an electron pair— more simply, a base is an electron-pair donor, an acid an electron-pair acceptor. Some examples of Lewis acids and bases are ... 

Boron trioxide is not particularly soluble in water but it slowly dissolves to form both dioxo(HB02)(meta) and trioxo(H3B03) (ortho) boric acids. It is a dimorphous oxide and exists as either a glassy or a crystalline solid. Boron trioxide is an acidic oxide and combines with metal oxides and hydroxides to form borates, some of which have characteristic colours—a fact utilised in analysis as the "borax bead test , cf alumina p. 150. Boric acid. H3BO3. properly called trioxoboric acid, may be prepared by adding excess hydrochloric or sulphuric acid to a hot saturated solution of borax, sodium heptaoxotetraborate, Na2B407, when the only moderately soluble boric acid separates as white flaky crystals on cooling. Boric acid is a very weak monobasic acid it is, in fact, a Lewis acid since its acidity is due to an initial acceptance of a lone pair of electrons from water rather than direct proton donation as in the case of Lowry-Bronsted acids, i.e. 

Neutral compounds such as boron trifluoride and aluminum chloride form Lewis acid-base complexes by accepting an electron pair from the donor molecule. The same functional groups that act as lone-pair donors to metal cations can form complexes with boron trifluoride, aluminum chloride, and related compounds. 

A coordination compound, or complex, is formed when a Lewis base (ligand) is attached to a Lewis acid (acceptor) by means of a lone-pair of electrons. Where the ligand is composed of a number of atoms, the one which is directly attached to the acceptor is called the donor atom . This type of bonding has already been discussed (p. 198) and is exemplified by the addition compounds formed by the trihalides of the elements of Group 13 (p. 237) it is also the basis of much of the chemistry of the... 

The basic concept of activation in hetero-Diels-Alder reactions is to utilize the lone-pair electrons of the carbonyl and imine functionality for coordination to the Lewis acid. The coordination of the dienophile to the Lewis acid changes the FMOs of the dienophile and for the normal electron-demand reactions a decrease of the LUMO and HOMO energies is observed leading to a better interaction with... 

Notice in the list of Lewis bases just given that some compounds, such as carboxylic acids, esters, and amides, have more than one atom ivith a lone pair of electrons and can therefore react at more than one site. Acetic acid, for example, can be protonated either on the doubly bonded oxygen atom or on the singly bonded oxygen atom. Reaction normally occurs only once in such instances, and the more stable of the two possible protonation products is formed. For acetic add, protonation by reaction with sulfuric acid occurs on... 

A Lewis base donates an electron pair to a Lewis acid. We therefore need to locate the electron lone pairs on acetaldehyde and use a curved arrow to show the movement of a pair toward the H atom of the acid. 

Lewis base (Section 2.11) A substance that donates an electron lone pair to an acid. All nucleophiles are Lewis bases. 

As discussed in Sections 5.1-5.3, arenediazonium ions are Lewis acids in which the (3-nitrogen forms the center of electrophilic character. This was demonstrated by the addition of hydroxide ions and water molecules. Other nucleophiles can also be added and, in principle, these reactions display the same mechanistic characteristics as those with OH and H20. According to the nature of the atom of the nucleophile that provides the lone pair of electrons, O-, S-, Se-, N-, P-, or C-coupling can occur. With N- and C-coupling, important and large groups of compounds are formed, namely azo compounds (mainly important as azo dyes) and triazenes, respectively. These compounds will be discussed in Chapters 12 and 13, respectively. 

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 Lewis acid is an electron pair acceptor a Lewis base is an electron pair donor. A proton is a Lewis acid that attaches to a lone pair provided by a Lewis base. 

Boric acid, B(OH) , is a white solid that melts at 171°C. It is toxic to bacteria and many insects as well as humans and has long been used as a mild antiseptic and pesticide. Because the boron atom in B(OH)3 has an incomplete octet, it can act as a Lewis acid and form a bond by accepting a lone pair of electrons from an H20 molecule acting as a Lewis base ... 

Silicon compounds can also act as Lewis acids, whereas carbon compounds typically cannot. Because a silicon atom is bigger than a carbon atom and can expand its valence shell by using its d-orbitals, it can accommodate the lone pair of an attacking Lewis base. A carbon atom is smaller and has no available d-orbitals so in general it cannot act as a Lewis acid. An exception to this behavior is when the carbon atom has multiple bonds, because then a Tt-bond can give... 

Carbon monoxide is a colorless, odorless, flammable, almost insoluble, very toxic gas that condenses to a colorless liquid at — 90°C. It is not very reactive, largely because its bond enthalpy (1074 kj-mol-1) is higher than that of any other molecule. However, it is a Lewis base, and the lone pair on the carbon atom forms covalent bonds with J-block atoms and ions. Carbon monoxide is also a Lewis acid, because its empty antibonding Tr-orbitals can accept electron density from a... 

Silicon reacts directly with chlorine to form silicon tetrachloride, SiCl4 (this reaction was introduced in Section 14.17, as one step in the purification of silicon). This compound differs strikingly from CC14 in that it reacts readily with water as a Lewis acid, accepting a lone pair of electrons from H20 ... 

Water is also a Lewis base, because an HzO molecule can donate one of its lone pairs to a Lewis acid and form complexes such as Fe(H20)63+. Water s ability to act as a Lewis base is also the origin of its ability to hydrolyze substances. The reaction between water and phosphorus pentachloride mentioned in Section 15.2 is an example. 

Group 4 metal complexes of the dianion [ BuNP( -N Bu)2PN Bu] polymerize ethylene in the presence of a co-catalyst, but they are readily deactivated [10,14]. This behaviour is attributed to coordination of the lone-pair electrons on the phosphorus(III) centers to Lewis acid sites, which initiates ring opening of the ligand [15]. 

The synthesis of three silaketenimines 105a-c prompted Tokitoh and Okazaki to calculate the optimized geometry of a model compound, PhiSiCNPh. This model reinforced that 105a-c are truly Lewis acid-base pairs, with the isocyanide donating its carbon lone pair to an empty p-orbital perpendicular to the lone pair... 

Trimethylboron is an example of one type of Lewis acid. This molecule has trigonal planar geometry in which the boron atom is s hybridized with a vacant 2 p orbital perpendicular to the plane of the molecule (Figure 21-11. Recall from Chapter 9 that atoms tend to use all their valence s and p orbitals to form covalent bonds. Second-row elements such as boron and nitrogen are most stable when surrounded by eight valence electrons divided among covalent bonds and lone pairs. The boron atom in B (CH ) can use its vacant 2 p orbital to form a fourth covalent bond to a new partner, provided that the new partner supplies both electrons. Trimethyl boron is a Lewis acid because it forms an additional bond by accepting a pair of electrons from some other chemical species. 

The simplest type of Lewis acid-base reaction is the combination of a Lewis acid and a Lewis base to form a compound called an adduct. The reaction of ammonia and trimethyl boron is an example. A new bond forms between boron and nitrogen, with both electrons supplied by the lone pair of ammonia (see Figure 21-21. Forming an adduct with ammonia allows boron to use all of its valence orbitals to form covalent bonds. As this occurs, the geometry about the boron atom changes from trigonal planar to tetrahedral, and the hybrid description of the boron valence orbitals changes from s p lo s p ... 

Every Lewis base has one or more lone pairs of valence electrons. A Lewis acid can have vacancies in its valence shell, or it can sacrifice aiTrbond to make a valence orbital available for adduct formation. To... 

The Lewis structures verity that each species identified as a Lewis base possesses lone pairs of electrons and that the Lewis acids have orbitals available to accept electrons. 

With its empty 3 d orbitals, SF4 also acts as a Lewis acid. This molecule forms adducts with pyridine and with fluoride ion. Each adduct has square pyramidal geometry, with a lone pair of electrons completing an octahedral... 

Cations at the surface possess Lewis acidity, i.e. they behave as electron acceptors. The oxygen ions behave as proton acceptors and are thus Bronsted bases. This has consequences for adsorption, as we will see. According to Bronsted s concept of basicity, species capable of accepting a proton are called a base, while a Bronsted acid is a proton donor. In Lewis concept, every species that can accept an electron is an acid, while electron donors, such as molecules possessing electron lone pairs, are bases. Hence a Lewis base is in practice equivalent to a Bronsted base. However, the concepts of acidity are markedly different. 

Figure 5.10. Defects consisting of oxygen vacancies constitute adsorption sites on a Ti02 (110) surface. Note how CO binds with its lone-pair electrons on a Ti ion (a Lewis acid site). O2 dissociating on a defect furnishes an O atom that locally repairs the defect. CO2 may adsorb by coordinating to an O atom, thus forming a carbonate group. [Figure adapted from W. Gopel, C. Rocher and R. Feierabend, Phys. Rev. B 28 (1983) 3427.]...

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