Radical reactions heterolysis

As mentioned earlier, at 500° C and 34.5 MPa supercritical water has a small dielectric constant, a very low ion product, and behaves as a high temperature gas. These properties would be expected to minimize the role of heterolysis in the dehydration chemistry. As shown in Table 1, the conversion of ethanol to ethylene at 500° C is small, even in the presence of 0.01M sulfuric acid catalyst. The appearance of the byproducts CO, C02) CH i+ and C2H6 points to the onset of nonselective, free radical reactions in the decomposition chemistry, as would be expected in the high temperature gas phase thermolysis of ethanol. 

In polar reactions, heterolytic (unsymmetrical) bond cleavage (heterolysis) and bond formation occur, while homolytic (symmetrical) bond cleavage (homolysis) and bond formation occur in radical reactions as shown below (Scheme a). 

The functionalization reaction as shown in Scheme 1(A) clearly requires the breaking of a C-H bond at some point in the reaction sequence. This step is most difficult to achieve for R = alkyl as both the heterolytic and homolytic C-H bond dissociation energies are high. For example, the pKa of methane is estimated to be ca. 48 (6,7). Bond heterolysis, thus, hardly appears feasible. C-H bond homolysis also appears difficult, since the C-H bonds of alkanes are among the strongest single bonds in nature. This is particularly true for primary carbons and for methane, where the radicals which would result from homolysis are not stabilized. The bond energy (homolytic dissociation enthalpy at 25 °C) of methane is 105 kcal/mol (8). 

In parallel with the development of the heterolysis of b-substituted alkyl radicals, a rearrangement reaction was observed and extensively studied in organic solvents. This rearrangement was first noted for b-(acyloxy)alkyl radicals (Scheme 5) by Surzur et al. [48] and, later, for b-(phosphatoxy)alkyl radicals by the Crich and Giese groups [49,50]. 

At one time considered as two distinct reactions occurring by different mechanisms [51], the fragmentations of Scheme 2 and the rearrangments of Scheme 5 are now seen as different facets of the same fundamental heterolysis of -substituted alkyl radicals into alkene radical cations, with the eventual outcome determined by the reaction conditions [52],... 

Equation lb, b is the defining equation for the addition-elimination route for one-electron transfer between X" and Y. It is important to note that although X-Y is a radical and the overall reaction results in the transfer of a single electron, in the actual electron transfer step an electron pair is shifted rather than a single electron [5]. This means that electron transfer is the consequence of a heterolysis reaction in which the electron pair joining X and Y ends up at... 

Concerning the general reaction Scheme 1, attention is restricted to two special areas A), cases where X is a carbon-centered radical and Y is an oxygen atom joined by a double bond to some center Z (Eq. 4), and B), cases where X is a hetero atom, in most cases oxygen centered radical and Y is a carbon (Eq. 5) [11]. One is then dealing with formation and heterolysis of a bond between a carbon- and a hetero-atom. Of the two, the hetero-atom is of course always more electron-affinic and therefore in the heterolysis the electron pair joining the two will go to the hetero-atom. 

At pH < 7 the nitroxyl radicals do not undergo an observable heterolysis (khs 10 s ), but decay by bimolecular reactions. However, in basic solution an OH -catalyzed heterolysis takes place to yield the radical anion of the nitrobenzene and an oxidized pyrimidine. In the case of the nitroxyls substituted at N(l) by H (i.e. those derived from the free bases), the OH catalysis involves deprotonation at N(l) which is adjacent to the reaction site [= C(6)] (cf. Eq. 15) [26] ... 

Deprotonation provides the necessary electron push to kick out the electron pair joining C(6) with the nitrobenzene oxygen. If, however, N(l) is alkylated (as with the nucleosides and nucleotides), OH catalysis is much less efficient since it now proceeds by deprotonation from N(3) (with the uracils) or from the amino group at C(4) (with the cytosines). In these cases the area of deprotonation is separated from the reaction site by a (hydroxy)methylene group which means that the increase in electron density that results from deprotonation at N(3) is transferable to the reaction site only through the carbon skeleton (inductive effect), which is of course inefficient as compared to the electron-pair donation from N(l) (mesomeric effect) [26]. Reaction 15 is a 1 1 model for the catalytic effect of OH on the heterolysis of peroxyl radicals from pyrimidine-6-yl radicals (see Sect. 2.4). 

The 5,6-dihydropyrimidine-6-yl radicals discussed above behave, in their reactions with nitrobenzenes, like the simpler radicals CH2OH and CH(al-kyl)Oalkyl do, i.e. they react exclusively by addition to give nitroxyl radicals and uncatalyzed heterolysis is not observed (khs < 10 s ). If, however, a methyl group is introduced at C(6) (= CJ of the pyrimidine-6-yl radical, the corresponding nitroxyl radicals heterolyze with rate constants at 20 °C of 10 to 5 X 10 s depending on the structure of the pyrimidine and of the nitrobenzene (Eq. 16). This SnI type reaction is characterized by activation enthalpies of 30-40 kJ mol and activation entropies of — 89 to — 7 Jmol K (entropy control) [27]. The rate-enhancing effect of the methyl group is, of course, due to... 

Stabilization of the (incipient) C 6) carbocation developing in the C-O heterolysis. The rate constants for the heterolysis reaction are a measure of the reducing power of 5,6-dihydro-6-methylpyrimidine-6-yl radicals. On this basis, the cytosine radicals are better reductants than the corresponding uracil radicals, and the radicals derived by hydrogen atom addition to pyrimidines are stronger reductants than those formed by OH radical addition [27]. 

With these radicals, spontaneous C(6)-0 heterolysis is slow ( < 10 s" ). However, if the electron density of the system is increased by OH -induced de-protonation of N(l)-H, 02 elimination is observed [23, 24, 25]. With the peroxyl radical from 5,6-dihydrouracil-6-yl, the heterolysis rate constant is 8.3 X 10 s the reaction leading to the isopyrimidine derivative shown [37]. The reaction is perfectly analogous to the eliminations of the radical anions of nitrobenzenes (Eq. 15) or anthraquinone-2,6-disulfonate (Eq. 18). 

A further support for the identification of the species responsible for the unimolecular conductance increase in terms of a chlorine-containing radical is the fact that in a blank experiment, i.e. one in which chloride is left out, a slow conductance increase is not observed and the overaU conductance yield is only half of that in the presence of chloride. Since in isobutene-saturated aqueous solution the lifetime of SOi is only 90 ns due to its rapid reaction with the alkene (as determined by optical experiments at 450 nm) [46], the non-observability of a unimolecular conductance increase means that the rate constant for heterolysis of the SO adduct to isobutene is 10 s" (cf. Eq. 32) ... 

Figure 4.18. Radical heterolysis reactions. The third example is a possible reaction pathway for 1,2-migrations of ester groups.

The initial transient formed, rearranges in a reaction that involves the ring contraction step in reaction (74). The lifetime of this intermediate is considerably longer than that reported for any other intermediate with a copper(II)-carbon bond in aqueous solution (85-87,101,136), suggesting the stabilized structure featuring the metallocycle. This intermediate decomposes via heterolysis of one of the copper(II)-carbon -bonds followed by homolysis of the second to form the cyclopentyl-methanol radical in reactions (75) and (76), which reacts with Cu + to form the final product cyclopentanecarbaldehyde (89). 

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