Acid-base reactions, frontier orbitals

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... 

The hydration of metal ions is a process that belongs to what we call the acid-base reaction in chemistry. The acid-base reaction involves no electron transfer between separate partners but a localized electron rearrangement to make up or break down a hybrid molecular orbital between an acid particle and a base particle, that is, the formation or the dissolution of a bonding molecular orbital due to the interaction between the frontier donor orbital of a particle Lewis base) and the frontier acceptor orbital of another particle Lewis acid). In order for an acid-base reaction to occur between a base particle and an acid particle, the electronic energy levels of the frontier orbitals for both acid and base particles are required to be close enough to each other for the orbital hybridization to prevail [3]. 

Let us consider, as examples for redox and acid-base reactions, water molecules, H20, in the chemical reactions with four different chemical species of metallic iron, Fe, chloride ions, Cl-, ferrous ions, Fe2+, and fluorine, F2, in aqueous solution. In the cases of Fe and F2, the uppermost frontier orbital level is sufficiently far away from that of H20, and hence the complete electron transfer takes place for these two species out of or into the water molecules. Water, H20, is an oxidant against metallic iron, Fe, and is a reductant against fluorine, F2 ... 

The molecular orbital description of acid-base reactions in Section 6.4 uses frontier molecular orbitals, those at the occupied-unoccupied frontier, which can be further illustrated by NH3 -F NH4. In this reaction, the a orbital containing the lone-pair electrons of the ammonia molecule (Figure 5.30) combines with the empty li orbital of the... 

In most Lewis acid-base reactions, a HOMO LUMO combination forms new HOMO and LUMO orbitals of the product. Frontier orbitals whose shapes and symmetries allow significant overlap, and whose energies are similar, form useful bonding and antibonding orbitals. If the orbital combinations have no useful overlap, no net bonding is possible, and they cannot form acid-base products. ... 

To summarize how acid-base reactions do work on the basis of molecular orbitals perturbation theory, we have reported on Figure 10.2.1, the relative energies (as perturbed by the field of the other reactant) of the frontier orbitals HOMO and LUMO of a hypothetical species A and of the frontier orbitals of several hypothetical reaction partners B, C, D, E and F. This figure is intended to represent possible variations of donor-acceptor properties in the broadest possible context i.e. not only those species encountered in aqueous solution but also those stabilized by non-aqueous environments. 

Fig. 6.1. Frontier orbital diagram for an acid-base reaction the HOMOs are filled with an electron pair the LUMOs are empty. The Y compound presents an amphoteric character it is a base in the XY conjugate pair, and an acid in the YZ pair. When the energy difference between the base HOMO and the acid LUMO is small, the reaction is controlled by the frontier orbitals. Otherwise, it is charge-controlled.

Frontier Orbitals and Chemical Reactivity. Chemical reactions typically involve movement of electrons from an electron donor (base, nucleophile, reducing agent) to an electron acceptor (acid, electrophile, oxidizing agent). This electron movement between molecules can also be thought of as electron movement between molecular orbitals, and the properties of these electron donor and electron acceptor orbitals provide considerable insight into chemical reactivity. 

The SC descriptions of the electronic mechanisms of the three six-electron pericyclic gas-phase reactions discussed in this paper (namely, the Diels-Alder reaction between butadiene and ethene [11], the 1,3-dipolar cycloaddition offulminic acid to ethyne [12], and the disrotatory electrocyclic ring-opening of cyclohexadiene) take the theory much beyond the HMO and RHF levels employed in the formulation of the most popular MO-based treatments of pericyclic reactions, including the Woodward-Hoffmarm mles [1,2], Fukui s frontier orbital theory [3] and the Dewar-Zimmerman model [4—6]. The SC wavefunction maintains near-CASSCF quality throughout the range of reaction coordinate studied for each reaction but, in contrast to its CASSCF counterpart, it is very much easier to interpret and to visualize directly. 

Further examination of the results indicated that by invocation of Pearson s Hard-Soft Acid-Base (HSAB) theory (57), the results are consistent with experimental observation. According to Pearson s theory, which has been generalized to include nucleophiles (bases) and electrophiles (acids), interactions between hard reactants are proposed to be dependent on coulombic attraction. The combination of soft reactants, however, is thought to be due to overlap of the lowest unoccupied molecular orbital (LUMO) of the electrophile and the highest occupied molecular orbital (HOMO) of the nucleophile, the so-called frontier molecular orbitals. It was found that, compared to all other positions in the quinone methide, the alpha carbon had the greatest LUMO electron density. It appears, therefore, that the frontier molecular orbital interactions are overriding the unfavorable coulombic conditions. This interpretation also supports the preferential reaction of the sulfhydryl ion over the hydroxide ion in kraft pulping. In comparison to the hydroxide ion, the sulfhydryl is relatively soft, and in Pearson s theory, soft reactants will bond preferentially to soft reactants, while hard acids will favorably combine with hard bases. Since the alpha position is the softest in the entire molecule, as evidenced by the LUMO density, the softer sulfhydryl ion would be more likely to attack this position than the hydroxide. 

Baciocchi et al43 have reported the existence of the pH-dependent mechanistic dichotomy for the deprotonation of 4-methoxybenzyl alcohol radical cation in aqueous solution. In neutral and acidic solutions the 4-MeOC6H4CH2OH + radical cation undergoes C-H deprotonation, while in basic solution (pH 10), the reaction is initiated by deprotonation of the OH group. DFT calculations were carried out and reveal that the OH induced O-H deprotonation is consistent with the charge controlled reaction, while the C-H deprotonation, observed when the base is HjO, appears to be effected by frontier orbital interactions43. 

The reaction of the acid chloride phenylhydrazone (11) with base gives the nitrile-imine 1,3-dipolar compound (12) which reacts with potassium thiocyanate to give the A2-thiadiazo-line (13 Scheme 1). Thus the cycloaddition occurs at the C=S and not the C=N bond. This regioselectivity can be explained in terms of the frontier orbital treatment. Due to the electron rich nature of the thiocyanate anion, its reaction with (12) is expected to be controlled by the LUMO and HOMO of (12) and the thiocyanate respectively. As the HOMO of the thiocyanate anion has the larger orbital coefficient on the sulfur atom, it can be concluded that the larger orbital coefficient in the LUMO of (12) is on the carbon atom. This is also in agreement with other dipolar cycloadditions (82H( 19)57). 

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