Photolysis alkanes

Figure 8 Connection between primary C-H and C-C bond ruptures during radiolysis and photolysis. Alkanes (1) propane, (2) w-butane, (3) -pentane, (4) -hexane, (5) w-heptane, (6) n-octane, (7) w-decane, (8) isobutane, (9) neopentane, (10) 3-methylpentane, (11) 2,2-dimethylbutane, (12) isooctane, (13) cyclopentane, (14) cyclohexane, (15) cycloheptane, (16) cyclooctane, (17) cyclodecane, (18) methylcyclopentane, (19) methylcyclohexane, (20) ethylcyclohexane, (21) 1,1-dimethylcyclohexane, (22) cis-l,2-dimethylcyclohexane, (23) fraw5-l,2-dimethylcyclohexane, (24) cis-1,3-dimethylcyclohexane, (25) trarw-l,3-dimethylcyclohexane, (26) cw-l,4-dimethylcyclohexane, (27) trawi-l,4-dimethylcyclohexane. (From Refs. 18, 29, 91, 92, 99, 100, 108, 110, 111, 113, 114, and 160.)...

In another reductive coupling, substituted alkenes (CH2=CH Y Y = R, COOMe, OAc, CN, etc.) can be dimerized to substituted alkanes (CH3CHYCHYCH3) by photolysis in an H2 atmosphere, using Hg as a photosensitizer. Still another procedure involves palladium-catalyzed addition of vinylic halides to triple bonds to give 1,3-dienes. ... 

The chain length, i.e. number of RH —> RC1 conversions per Cl produced by photolysis, is wlO6 for CH4, and the reaction can be explosive in sunlight. Chlorination can also be initiated thermolytically, but considerably elevated temperatures are required to effect Cl2 — 2C1, and the rate of chlorination of C2H6 in the dark at 120° is virtually indetectable. It becomes extremely rapid on the introduction of traces of PbEt4, however, as this decomposes to yield ethyl radicals, Et, at this temperature, and these can act as initiators Et- + Cl—Cl —> Et—Cl + Cl. Chlorination of simple alkanes such as these is seldom useful for the preparation of mono-chloro derivatives, as this first product readily undergoes further attack by the highly reactive chlorine, and complex product mixtures are often obtained. 

The origin of the persistent radicals which are produced in low yields is of some interest. Simple photochemical silicon-carbon bond cleavage lacks precedent and is not consistent with the failure to observe any alkane or 1-alkene in the photolysis mixture. While the latter could not be expected to survive in the irradiated solution, the former would. A possible route involving photochemical 1,1-reductive elimination is described below. 

The insertion products obtained by the photolysis of diazomethane in the gas phase in the presence of alkanes also include products originating from ethyl radicals, the formation of which must be explained by postulating vibrationally excited species The relative rates of abstraction of CH2 from alkanes are... 

Sensitization, which can populate the triplet manifold, was used in a number of instances. Sensitization with benzophenone was used in the photolysis of diazomethane to generate triplet methylene. The triplet methylene thus produced, however, failed to abstract much hydrogen from alkanes (cyclohexene), but... 

A further possibility of inducing the elimination of alkanes from transition metal alkyl complexes is photolysis [398,424-427]. Two examples of photolytic a-eliminations leading to non-heteroatom-substituted alkylidene complexes are shown in Figure 3.7. 

Carbenes and transition metal carbene complexes are among the few reagents available for the direct derivatization of simple, unactivated alkanes. Free carbenes, generated, e.g., by photolysis of diazoalkanes, are poorly selective in inter- or intramolecular C-H insertion reactions. Unlike free carbenes, acceptor-substituted carbene complexes often undergo highly regio- and stereoselective intramolecular C-H insertions into aliphatic and aromatic C-H bonds [995,1072-1074,1076,1085,1086],... 

Cyclohexyl xanthate has been used as a model compound for mechanistic studies [43]. From laser flash photolysis experiments the absolute rate constant of the reaction with (TMS)3Si has been measured (see Table 4.3). From a competition experiment between cyclohexyl xanthate and -octyl bromide, xanthate was ca 2 times more reactive than the primary alkyl bromide instead of ca 50 as expected from the rate constants reported in Tables 4.1 and 4.3. This result suggests that the addition of silyl radical to thiocarbonyl moiety is reversible. The mechanism of xanthate reduction is depicted in Scheme 4.3 (TMS)3Si radicals, initially generated by small amounts of AIBN, attack the thiocarbonyl moiety to form in a reversible manner a radical intermediate that undergoes (3-scission to form alkyl radicals. Hydrogen abstraction from the silane gives the alkane and (TMS)3Si radical, thus completing the cycle of this chain reaction. 

The photodecomposition of -alkanes at excitation energies slightly above the absorption onset involves both C-H and C-C bond decompositions [18]. The dominant process is the C-H scission, (H2) 0.8-0.9, and the contribution of C-C decomposition is small. In the photolysis of cyclohexane, cycloheptane, cyclooctane, and cyclodecane, however, only hydrogen evolution was observed [[Pg.375]

The mechanism of H2 formation involves both unimolecular H2 elimination, when both of the H atoms of a hydrogen molecule originate from the same alkane molecule, and H atom elimination with subsequent H atom abstraction reaction. We show the mechanism on the example of cyclohexane photolysis hv = 7.6 eV) [91] ... 

The quantum yields and G values of H2 elimination for a larger group of alkanes are eolleeted in Table 4 the values were mostly determined by using Eq. (10). Because in the photolysis and radiolysis of the -alkanes shown in the table various kinds of radicals are produced simultaneously (e.g., -propyl and ec-propyl from propane), the weighted averages of several k /kc values were used in Eq. (10). The ratios can be determined by suppressing the radiolytic alkene and dimer yields in the presence of radical scavenger (e.g., I2) ... 

Alkane elimination has a low yield during the photolysis of liquid n-alkanes (e.g., n-pentane [104,106]). This reaction takes place with high yield only for branched alkanes where it is likely to be a main primary-decomposition step [105,107]. 

Table 5 Quantum Yield of Alkane Elimination in the Photolysis of Some Liquid Alkanes...

Similarly to the fluorescence quantum yields, the yields of individual primary decomposition steps generally show considerable excitation energy dependence the yields of the unimolecular H2 and alkane eliminations and also those of the radical-type decompositions show a continuous variation with photon energy [27,39,42,107,115]. In cyclohexane photolysis the sum of the quantum yields of the two primary decompositions described by Reactions (5) and (6) is practically unity between photon energies 7.6 and 11.6 eV yield decreases with the energy, [Pg.382]

In contrast to the results obtained with -alkanes and cycloalkanes, H2 elimination is found to be an unimportant process in the photolysis of neopentane [107]. At 7.6 eV the methane elimination and direct C-C bond cleavage to radicals are the predominant processes ... 

The radical-forming reactions are suggested to take place mostly after an Si T type ISC the reactions have nonactivated character. The homolytic split to H atom and alkyl radical has a considerable yield in the photolysis of n-alkanes and cycloalkanes, while the scission to two radicals is characteristic of the decay of excited branched alkane molecules. 

The Jovian moon, lo, shows an orange hue (Fig. 6.1), which may be due to long-chain alkane radical cations. The atmosphere of lo consists mostly of methane deep UV photolysis proceeds with electron ejection thus, the molecular ion of methane was perhaps the earliest organic radical cation, generated by solar irradiation aeons ago. 

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