Acid-base reactions covalent bonding

Where the bond between the metal and the ring is covalent, the usual arenium ion mechanism operates. Where the bonding is essentially ionic, this is a simple acid-base reaction. For the aliphatic counterpart of this reaction, see Reaction 12-23. 

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 ... 

A base has the ability to donate a pair of electrons and an acid the ability to accept a pair of electrons to form a covalent bond. The product of a Lewis acid-base reaction may be called an adduct, a coordination compound or a coordination complex (Vander Werf, 1961). Neither salt nor conjugate acid-base formation is a requirement. 

The general Lewis-acid-base reaction (3.95) exemplifies the two-electron stabilizing donor-acceptor interaction of Fig. 1.3 (namely the nN->-nB interaction for (3.94)), which may be distinguished from the complementary bi-directional donor-acceptor interactions of covalent-bond formation (Section 3.2.1). However, this leaves open the question of whether (or how) the equilibrium bond reflects the formal difference between heterolytic (3.95) and homolytic (3.96) bond formation. 

In the same year that Bronsted and Lowry proposed their definition of acids and bases, an American chemist named Gilbert Lewis proposed an alternative definition that not only encompassed Bronsted-Lowry theory but also accounted for acid-base reactions in which a hydrogen ion isn t exchanged. Lewis s definition relies on tracking lone pairs of electrons. Under his theory, a base is any substance that donates a pair of electrons to form a coordinate covalent bond with another substance, while an acid is a substance that accepts that electron pair in such a reaction. As we explain in Chapter 5, a coordinate covalent bond is a covalent bond in which both of the bonding electrons are donated by one of the atoms forming the bond. 

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... 

The BF3 molecule is the Lewis acid, the F ion is the Lewis base, and the BF4 ion is the Lewis acid-base complex. The bond formed in a Lewis acid-base reaction is a coordinate covalent bond. 

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. 

On a molar basis, most organic compounds contain similar amounts of hydrogen and carbon, and processes involving transfer of hydrogen between covalently bound sites rank in importance in organic chemistry second only to those involving the carbon-carbon bond itself. Most commonly, hydrogen is transferred as a proton between atoms with available electron pairs (l), i.e. Bronsted acid/base reactions. The alternative closed shell process, hydride transfer or shift, involves motion of a proton with a pair of electrons between electron deficient sites (2). These processes have four and two electrons respectively to distribute over the three atomic centres in their transition structures. It is the latter process, particularly when the heavy atoms are both first row elements, which is the subject of this review. The terms transfer and shift are used here only to differentiate intermolecu-lar and intramolecular reactions. 

Coordinate Covalent Nonmetals and a cation Both electrons in the bond came from one element. Yes Weaker. Can be removed in acid-base reactions. Ex. H301+... 

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 ... 

The acetylide anions discussed in Chapter 11 are another example of organometallic compounds. These reagents are prepared by an acid-base reaction of an alkyne with a base such as NaNH2 or NaH. We can think of these compounds as organosodium reagents. Because sodium is even more electropositive (less electronegative) than lithium, the C-Na bond of these organosodium compounds is best described as ionic, rather than polar covalent. 

Here, the Lewis base CaO donates an electron pair (one of the lone pairs of the oxygen atom) to the Lewis acid (CO2) to form a coordinate covalent bond in the CO ion. Similar Lewis acid-base reactions can be written for other acid-base anhydride pairs. Sulfur trioxide, for example, reacts with metal oxides to form sulfates ... 

The autoionization of water (Section 10-5) was described in terms of Bronsted-Lowry theory. In Lewis theory terminology, this is also an acid-base reaction. The acceptance of a proton, H, by a base involves the formation of a coordinate covalent bond. 

Throughout our consideration of the formation of the double helix, we have dealt only with the noncovalent bonds that are formed or broken in this process. Many biochemical processes entail the formation and cleavage of covalent bonds. Of these, a particularly important class of reactions prominent in biochemistry is acid-base reactions. 

In Section 10.4 we saw that the B atom in BF3 is xp -hybridized. The vacant, unhybridized 2p orbital accepts the pair of electrons from NH3. So BF3 functions as an acid according to the Lewis definition, even though it does not contain an ionizable proton. Note that a coordinate covalent bond is formed between the B and N atoms, as is the case in all Lewis acid-base reactions. 

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