Heterolytic dissociation

Figure S.U. Many gases dissociate heterolytically on oxide surfaces.

H—F. In the crudest approximation, one may say that the orbitals of C, N, O, and F are all approximately the same size and therefore the interaction matrix element hab will be approximately the same size for any A- pair. The dominant factor determining the heterolysis energy therefore is the difference in orbital energies in the denominator, and one has directly the prediction (Figure 4.2) that ease of heterolytic cleavage for C—X is in the order C > N > > F. The C—C bond is least likely to dissociate heterolytically and the C—F bond the most likely. In an absolute sense, of course, heterolytic cleavage is not a likely process for any of these bonds in the absence of other factors, as discussed below. 

It is noticed that when Fe3+OOR is formed in aprotic diluters (under strongly basic conditions) at low temperature, degradation products are synthesized by reaction (7.4) [29], However, in hydroxide diluters oxidants dissociate heterolytically by reaction (7.3) [30, 31], which is proved by the formation of the following products epoxides from alkenes and alcohols from ROOH. 

The question of iron(III) tetraphenylporphyrin mimic stabilization [44-46] is resolved by replacing all phenyl hydrogen atoms by halides, which significantly extends the lifetime of the catalyst. It is also proven that oxidants (H202, ROOH, peroxy acids and iodosylben-zene) in hydroxide solvents (diluters) dissociate heterolytically [30, 31],... 

The second reduction couple of (TPP)Con(—1.95 V) produces (TPPT)Co, which also facilitates the transformation of C02 to CO and CO2- (Figure 13.9). Scheme 13.1 outlines a reasonable mechanistic path that has the first-formed adduct, (TPP )ComC(0)0 , acting as a nucleophile toward a second C02 molecule. This adduct dissociates heterolytically to give CO2 and bound CO. 

Bond dissociation energies, described in Chapter 5, measure the energy required to homolytically break a bond. They are not the same as dissociation enthalpies, which measure the ability of a compound to dissociate heterolytically. Bond dissociation energies can be used to calculate dissociation enthalpies in the gas phase if other quantities are also known. 

Finally, the complexity of the thermal decomposition of disulfides is well illustrated by qualitative results on the pyrolysis of dibenzyl disulfide . At temperatures below 150 °C the major products are dibenzyl, thiobenzophenone and elemental sulfur above 200 "C a second reaction sets in, leading to the production of HjS, tetraphenylethane and tetraphenylethylene. It has even been suggested that at temperatures below 140 °C di- and polysulfides dissociate heterolytically into polar persulphenyl intermediates. 

Likewise the reaction of Na Ois) with CO2(g) yields covalently bonded sodium carbonate, which dissociates heterolytically in aqueous solutions... 

The foregoing mechanism shows that an El reaction has two steps. In the first step, the alkyl halide dissociates heterolytically, producing a carbocation. In the second step, the base forms the elimination product by removing a proton from a carbon that is adjacent to the positively charged carbon (i.e., from the /8-carbon). This mechanism agrees with the observed first-order kinetics. The first step of the reaction is the rate-determining step. Therefore, increasing the concentration of the base—which comes into play only in the second step of the reaction—has no effect on the rate of the reaction. 

For a dissociative heterolytic mechanism, the reactant is solvated, and heterolysis forms an ion pair (termed an intimate ion pair ) that is contained within the original solvation shell. The ions are then solvated separately but remain associated ( solvent-separated ion pair ). The ions dissociate, and the incoming nucleophile reacts by a reversal of this process with the electrophile. The presence of up to three intermediate types accounts for many of the complexities associated with dissociative reaction mechanisms. For example, the separated ions are stereochemically distinct from the reactant, and in the case of carbonium ions or metaphosphate intermediates the intermediate is expected to be planar. 

In particular it is well-known that the closed-shell single-determinant model of electronic structure is inappropriate for the description of bond-breaking situations in molecules the very fact that one insists on a set of doubly occupied MOs in this model means that a bond will dissociate heterolytically into closed-shell ions rather than homolytically into open-shell fragments. 

Z =...[4>b) 4>a) Z2 =. ..[4>b) 4>a) D=...[4>b) 4>a) and D =. . . 4>b) 4>a) The first two states are zwitterionic, since the bond dissociates heterolytically. Also, since both electrons occupy the same orbital, both zwitterionic states are singlet states and will be totally symmetric with respect to any symmetry operation which may be preserved in the dissociation. By our assumption that ((iij) < it is expected that... 

In the first step, a CO molecule inserts into the M-C bond of adsorbed ethyl. Subsequently, Hg adsorbs on the vacant ligand position. In the final step, H2 dissociates heterolytically and subsequently reacts with the CH3CH2CO fragment to form the corresponding aldehyde along with regeneration of the catalytically active carbonyl complex. 

The catalytic reduction of a particular intermediate involves the addition of hydrogen. Just as in oxidation catalysis, there is the issue of whether one or two hydrogen atoms from H2 are incorporated into the substrate through identical intermediate hydrogen atoms. In heterogeneous catalysis, this question translates into whether dissociation occurs ho-molytically or heterolytically. On a transition-metal surface H2 dissociation generates two equivalent hydrogen atoms such as we have seen in Chapter 3. As discussed in Chapter 4, however, Hg can dissociate heterolytically H2 H + H+. 

Hydrogen dissociates heterolytically to be adsorbed on Zn and O. The first step for CO hydrogenation is hydride transfer from the metal atom to the carbonyl ligand. 

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