Acid-base reactions orbitals

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... 

The fact that a Lewis acid is able to accept an electron pair means that it must have either a vacant, low-energy orbital or a polar bond to hydrogen so that it can donate H+ (which has an empty7 Is orbital). Thus, the Lewis definition of acidity includes many species in addition to H+. For example, various metal cations, such as Mg2+, are Lewis acids because they accept a pair of electrons when they form a bond to a base. We ll also see in later chapters that certain metabolic reactions begin with an acid-base reaction between Mg2+ as a Lewis acid and an organic diphosphate or triphosphate ion as the Lewis base. 

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. 

The most common reaction of carbanions is combination with a positive species, usually a proton, or with another species that has an empty orbital in its outer shell (a Lewis acid-base reaction) ... 

According to this theory, an acid is defined as a proton donor and a base as a proton acceptor (a base must have a pair of electrons available to share with the proton this is usually present as an unshared pair, but sometimes is in a 7t orbital). An acid-base reaction is simply the transfer of a proton from an acid to a base. (Protons do not exist free in solution but must be attached to an electron pair). When the acid gives up a proton, the species remaining still retains the electron pair to which the proton was formerly attached. Thus the new species, in theory at least, can reacquire a proton and is therefore a base. It is referred to as the conjugate base of the acid. All acids have a conjugate base, and all bases have a conjugate acid. All acid-base reactions fit the equation... 

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 ... 

Figure 10.1. Orbitals for a simple acid-base reaction involving proton abstraction. Charges are ignored.

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... 

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. 

The first step in this mechanism is a relatively slow reaction. (The activation energy for this step is roughly 80 kJ/mol.) If this reaction is done in water, the next step is extremely fast. The (CH3)3C+ ion is a Lewis acid because it has an empty orbital that can be used to accept a pair of electrons. Water, on the other hand, is a reasonably good Lewis base. A Lewis acid-base reaction therefore rapidly occurs in which a pair of nonbonding electrons on a water molecule are donated to the carbocation to form a covalent C—O bond. 

As you should recall from general chemistry, a favorable equilibrium constant is not sufficient to ensure that a reaction will occur. In addition, the rate of the reaction must be fast enough that the reaction occurs in a reasonable period of time. The reaction rate depends on a number of factors. First, the reactants, in this case the acid and the base, must collide. In this collision the molecules must be oriented properly so that the orbitals that will form the new bond can begin to overlap. The orientation required for the orbitals of the reactants is called the stereoelectronic requirement of the reaction. (,Stereo means dealing with the three dimensions of space.) In the acid-base reaction, the collision must occur so that the atomic orbital of the base that is occupied by the unshared pair of electrons can begin to overlap with the is orbital of the acidic hydrogen. 

If, instead, the collision were to occur so that the orbital on the nitrogen bumped into some other part of the acetic acid molecule, that collision would not lead to an acid-base reaction. 

The interaction between a metal ion and a ligand can be viewed as a Lewis acid-base reaction, with the ligand donating a lone pair of electrons to an empty orbital on the metal ion to form a coordinate covalent bond ... 

In acid/base, or donor/acceptor, reactions, bonding results form the overlap of filled orbitals on the "donor" and empty orbitals on the "acceptor". Surface cations are generally Lewis acids and act as electron acceptors, while surface O ions are Lewis bases and can donate electrons to acceptor adsorbates. In lower oxides of the transition metals (i.e., in which the cations are in an oxidation state lower than their maximal valency), cations may also be able to donate electrons in an acid/base reaction. Although one talks of donating and accepting electrons in acid/base reactions, the electrons are in no sense free, and there is no actual electron transfer involved. This type of bonding can be either molecular or dissociative. 

The molecular orbital description of acid-base reactions mentioned in Section 6-2-4 uses frontier molecular orbitals (those at the occupied-unoccupied frontier), and can... 

In most acid-base reactions, a HOMO-LUMO combination forms new HOMO and LUMO orbitals of the product. We can see that orbitals whose shapes allow significant overlap and whose energies are similar form useful bonding and antibonding orbitals. On the other hand, if the orbital combinations have no useful overlap, no net bonding is possible (as shown in Chapter 5) and they cannot form acid-base products. ... 

Similar reactions can be described for other species, and the adducts formed in the acid-base reactions can be quite stable or very unstable, depending on the exact relationship between the orbital energies. 

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