Carbonic acid 51 heterolytic

Chemical Properties. The most impoitant reactions which tetraorganotins undergo are heterolytic, ie, electrophilic and nucleophilic, cleavage and Kocheshkov redistribution (81—84). The tin—carbon bond in tetraorganotins is easily cleaved by halogens, hydrogen hahdes, and mineral acids ... 

The small Hammett p value of +0.16 observed for a series of related meta- and para-substituted mandelic acids indicates that there is a very small negative charge development on the benzyl carbon in the transition state of the rate-determining step of the pyridine catalysed oxidation of mandelic acid. The large positive AS value (+24 e.u./mol) found for the catalysed reaction led Banetjee and coworkers to conclude that the transition state (Figure 5) is product-like . This conclusion is consistent with the small f n/f D that is observed in this reaction164. The Pb—O bond is shown to rupture in a heterolytic fashion because Partch and Monthony185 have demonstrated that pyridine diverts the reaction from a homolytic to a heterolytic mechanism. 

Another factor to be investigated in the metathesis process is the effect of bases in the reaction media. Bases such as triethylamine are added in the experimental conditions to stabilize the formic acid product because otherwise the product is thermodynamically less stable than the separate carbon dioxide and dihydrogen reactants. As discussed above, the o-bond methathesis involves the heterolytic H-H bond fission, which would be accelerated by the presence of the base. This effect was theoretically investigated in the four-center o-bond metathesis between RhOn1-... 

These equations involve bond cleavages of the type shown in color in figure 10.3d. Pyridoxal-5 -phosphate promotes these heterolytic bond cleavages by stabilizing the resulting electron pairs at the a- or /3-carbon atoms of a-amino acids. To do this, the aldehyde group of the coenzyme first reacts... 

The reaction chemistry of simple organic molecules in supercritical (SC) water can be described by heterolytic (ionic) mechanisms when the ion product 1 of the SC water exceeds 10" and by homolytic (free radical) mechanisms when <<10 1 . For example, in SC water with Kw>10-11 ethanol undergoes rapid dehydration to ethylene in the presence of dilute Arrhenius acids, such as 0.01M sulfuric acid and 1.0M acetic acid. Similarly, 1,3 dioxolane undergoes very rapid and selective hydration in SC water, producing ethylene glycol and formaldehyde without catalysts. In SC methanol the decomposition of 1,3 dioxolane yields 2 methoxyethanol, il lustrating the role of the solvent medium in the heterolytic reaction mechanism. Under conditions where K klO"11 the dehydration of ethanol to ethylene is not catalyzed by Arrhenius acids. Instead, the decomposition products include a variety of hydrocarbons and carbon oxides. 

In addition to catalyzing hydroformylation, the platinum SPO complexes are excellent hydrogenation catalysts for aldehydes (as already demonstrated by the side products of hydroformylation), in particular, in the absence of carbon monoxide. Further, in ibis process, the facile heterolytic splitting of dihydrogen may play a role. The hydrogenation of aldehydes requires the presence of carboxylic acids, and perhaps the release of alkoxides from platinum requires a more reactive proton donor than that available on the nearby SPO. For example, 4 hydrogenates 2-methylpropanal at 95 °C and 40 bar of H2 to give the alcohol, with a TOF of 9000 mol moN h (71). 

Under strongly acidic conditions C-H bond protolysis is not the only pathway by which hydrocarbons are heterolytically cleaved. Carbon-carbon bonds can also be cleaved by protolysis involving pentacoordinate ions [Eq. (6.16)]. 

---