Reaction mechanisms heterolytic-type

When drawing arrows to illustrate movement of electrons, it is important to remember that electrons form the bonds that join atoms. The following represent heterolytic-type reaction mechanisms ... 

The following represents a heterolytic-type reaction mechanism ... 

The acid-base catalysis is carried out in BRC, with H+ or 2H+ ion transfer. Poltorak notes [99] that when H+ and e or H+ and H are transferred, the reaction mechanisms relate to H+-dependent redox type. BRC with electron transfer describes heterolytic oxidative processes. 

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. 

Further research into the reaction mechanism revealed that the reaction rate was correlated with the electron structure of the sulfoxide the more electropositive sulfoxides were the better oxygen donors. Excellent correlation of the reaction rates with the heterolytic benzylic carbon-hydrogen bond dissociation energies indicated a hydride abstraction mechanism in the rate-determining step to yield a carbocation intermediate. The formation of 9-phenylfluorene as by-product in the oxidation of triphenylmethane supports this suggestion. Further kinetic experiments and NMR showed the formation of a polyoxometalate-sulfoxide complex before the oxidation reaction, this complex being the active oxidant in these systems. Subsequently, in a similar reaction system, sulfoxides were used to facilitate the aerobic oxidation of alcohols [29]. In this manner, benzylic, allyUc, and aliphatic alcohols were all oxidized to aldehydes and ketones in a reaction catalyzed by Ke jn-type... 

Another type of reaction was seen for dalvastatin (8.151), a prodrug that bears an unsaturated side chain. The hydrolysis of dalvastatin to the active acid competes with epimerization at C(6), the rate of the reaction being independent of pH above pH 2 [192], The mechanism is believed to be one of heterolytic cleavage of the C(6)-0 bond to generate a C-centered carbonium ion stabilized by the extended conjugated system characteristic of this compound. In the pH range 2 - 7, the rate of epimerization was found to be ca. 100 times faster than hydrolysis. Above pH 7, base catalysis accelerates hydrolysis, the rate of which increases ca. 100-fold between pH 7 and 9. These facts serve only to complicate the design of HMG-CoA reductase inhibitors and the interpretation of their pharmacokinetic behavior. 

Many convenient methods for the introduction of carbon-carbon double bonds into a saturated carbon chain involve the removal of two atoms or groups from adjacent carbon atoms. Usually, but not invariably, one of these groups is hydrogen (i.e. the removal of HX). Two main types of elimination reactions are recognised - heterolytic processes in solution and pyrolytic reactions in the gas phase. A detailed discussion of the mechanisms of these reactions may be found in all standard and advanced textbooks in each of the reactions discussed below the probable mechanism is noted in relation to the aim of obtaining good yields of regio- or stereoisomerically pure compounds. 

We shall discuss several types of no-mechanism reactions first and some-mechanism reactions second. From an orbital viewpoint, all heterolytic rearrangements would be subsumed under sigmatropic reactions heterolytic substitutions and additions (eliminations) turn out to be versions of cycloadditions (cycloeliminations). Nevertheless, we have retained categories on the basis of mechanism, because these are familiar, and because the trend is from the relatively simple to the complex mechanism. 

These Hughes-Ingold rules can be used for making qualitative predictions about the effect of solvent polarity on the rates of all heterolytic reactions of known mechanisms. For nucleophilic substitution reactions of types (5-11) and (5-12)... 

In reactions of this type, the direction of the electron flow between the radical X and the substrate molecule S depends, of course, on the oxidizing or reducing properties of X and the ability of S either to donate or to accept an electron. Irrespective of electron flow direction, transfer between X and S can proceed by the outer-sphere mechanism [37], Eq. 2, or by inner-sphere or bonded paths (Eqs 3 and 4) [38, 39]. The stoichiometry in Eqs 3 and 4 is the same as in Eq. 2. In the bonded mechanism, however, a heterolytic fragmentation of the bond joining X and S is the step in which the electron transfer actually occurs ... 

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