Boron acid-base reaction

The electrophilic character of boron is again evident when we consider the oxida tion of organoboranes In the oxidation phase of the hydroboration-oxidation sequence as presented m Figure 6 11 the conjugate base of hydrogen peroxide attacks boron Hydroperoxide ion is formed m an acid-base reaction m step 1 and attacks boron m step 2 The empty 2p orbital of boron makes it electrophilic and permits nucleophilic reagents such as HOO to add to it... 

Look closely at the acid-base reaction in Figure 2.5, and note how it is shown. Dimethyl ether, the Lewis base, donates an electron pair to a vacant valence orbital of the boron atom in BF3, a Lewis acid. The direction of electron-pair flow from the base to acid is shown using curved arrows, just as the direction of electron flow in going from one resonance structure to another was shown using curved arrows in Section 2.5. A cuived arrow always means that a pair of electrons moves from the atom at the tail of the arrow to the atom at the head of the arrow. We ll use this curved-arrow notation throughout the remainder of this text to indicate electron flow during reactions. 

Borane is very reactive because the boron atom has only six electrons in its valence shell. In tetrahydrofuran solution, BH3 accepts an electron pair from a solvent molecule in a Lewis acid-base reaction to complete its octet and form a stable BH3-THF complex. 

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 ... 

An essential feature of [Tc=N] + complexes is the Lewis basicity of the nitrido group, which can react with strong Lewis acids such as trivalent boron compounds. Recently the first nitrido bridges between technetium and boron were produced by an acid-base reaction between... 

Boron and aluminum atoms need five electrons to complete their octets. However, they may be unable to acquire that number of electrons from the atoms to which they are bonded. As a result, the compounds these elements form have special chemical characteristics. Moreover, they introduce an important class of reactions that are called Lewis acid-base reactions. ... 

Thus the reaction of acetone with BF3 is a Lewis acid-base reaction in which a lone pair of the ketone oxygen atom is donated to an unfilled valence orbital of BF3. Bond formation is accompanied by the development of formal charges on both oxygen and boron. 

Boron(III) oxide is an exceedingly weak base, so that the forward acid/ base reaction does not progress far. The reverse reaction - the hydrolysis of the covalent, molecular boron(III) chloride - is, of course, highly favourable. However, compare this with the following ... 

Cerium(III) oxide is quite a strong base compared with boron(IIl) oxide (see Table 9.1), and the acid/base reaction is strongly exothermic. No appreciable hydrolysis occurs for CeCl3, which dissolves in water to give Ce3+(aq). A detailed thermochemical analysis of the factors which lead to the dramatic difference in behaviour between B203 and Ce203 is not straightforward, but the considerations set out in Section 10.2 in the discussion of Table 10.1 are relevant. 

According to the Lewis definition, an acid is an electron pair acceptor and a base is an electron pair donor. All Bronsted-Lowry bases are also Lewis bases. However, Lewis acids include many species that are not proton acids instead of H+, they have some other electron-deficient species that acts as the electron pair acceptor. An example of a Lewis acid-base reaction is provided by the following equation. In this reaction the boron of BF3 is electron/deficient (it has only six electrons in its valence shell). The oxygen of the ether is a Lewis base and uses a pair of electrons to form a bond to the boron, thus completing boron s octet. 

The coordination chemistry of boron was reviewed some time ago and the structure and properties of compounds of the general formula BX3 L, where X and L can be one of a wide variety of substituents and electron pair donors, respectively (15). Indeed, the reactions of tricoordinate boron compounds in general are thought to proceed via addition of the reaction partner in a Lewis acid-base reaction to yield a tetracoord-inate intermediate that then undergoes further reaction. Stable tetra-coordinate boron compounds are subject to ligand displacement reactions for which a variety of mechanisms obtain (16). The coordination chemistry of transition metals is vast and includes not only structimal facts (17) but considerable information on the mechanistic behavior of these species as well (18). In our brief comparison we will restrict ourselves to low oxidation state chemistry and group 16 metals (19). 

Both are acid-base reactions involving in the case of boron the trans-cient BH3 molecule (16) and in the case of iron the unsaturated transient Fe(CO)4 (20). These two species are isolobal (21) in that they both... 

The movement of electrons in Lewis acid-base reactions can be seen clearly with electrostatic potential maps. In the reaction of boron trifluoride with dimethyl ether, for instance, the ether oxygen atom becomes more positive and the boron becomes more negative as electron density is transferred and the B-0 bond forms (Figure 2.6). 

The reaction of boron trichloride with ammonia is a typical Lewis acid-base reaction. 

The significance of the Lewis concept is that it is much more general than other definitions. Lewis acid-base reactions include many reactions that do not involve Brpnsted acids. Consider, for example, the reaction between boron trifluoride (BF3) and ammonia to form an adduct compound (Figure 15.11) ... 

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 ... 

It might surprise you to learn that the reaction of gaseous boron trifluoride (BF3) with gaseous ammonia (NH3) to form BF3NH3 is a Lewis acid-base reaction. 

The most general view of acids and bases was advanced by G. N. Lewis. In this model, acids are substances which have an affinity for lone electron pairs, and bases are substances which possess lone electron pairs. Water and ammonia are the most common substances which possess lone electron pairs, and therefore behave as bases in the Lewis scheme. The reaction of silver ion, Ag with cyanide ion, CN , and boron trifluoride, BF3 (an electron-deficient compound), with ammonia, NH3, are two examples of Lewis acid-base reactions. The Lewis acid-base concept is most useful in chemical reactions in nonaqueous solvents. We will not find it useful in our study of ionic equilibria in water. 

Arts. A Lewis acid is a molecule or ion which has room for a lone pair of electrons in its valence shell. A Lewis base is a molecule or ion which possesses a lone pair of electrons in its valence shell. An example of a Lewis acid-base reaction is the reaction between ammonia and boron trifluoride ... 

Fusion processes can be grouped into acid-base reactions (carbonates, borates, hydroxides, disulfates, fluorides, and boron oxide) and redox reactions (alkaline fusion agent plus oxidant or reductant). Common fluxes are listed in Table 4.2. Fluoride-pyrosulfate and carbonate-bisulfate fusions are used to decompose soil and fecal samples. 

---