Frontier orbitals acid-base reactions and

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.  

Reactant A is taken as a reference water is a good example. The first combination of reactants, A -I- B, has all the B orbitals at a much higher energy than those of water (Ca, for example the alkali metals react similarly but have only one electron in their highest s orbital). The energies are so different that no adduct can form, but a transfer of electrons can take place from B to A. From simple electron transfer, we might expect formation of H20 , but reduction of water yields hydrogen gas instead. As a result, water is reduced to H2 and OH, Ca is oxidized to Ca  

In this reaction, water is the acceptor, and the LUMO used is an antibonding orbital centered primarily on the hydrogen atoms (the chloride HOMO is one of its lone pairs from a 3p orbital). 

A reactant with orbitals lower in energy than those of water (for example, Mg  

FIGURE 6.8 Boron Trifluoride-Ether Adduct Formation from a Lewis Perspective. 

In less common cases, the orbitals with the required geometry and energy do not include the HOMO this possibility should be kept in mind. When this happens, the HOMO is usually a lone pair that does not have the geometry needed for bonding with the acid. 

Water plays different roles that can be rationalized from the perspective of frontier orbital interactions. 

---