Pentyl radicals, reactions

In addition, Kozuka and Lewis measured the tritium isotope effect for the reaction between the n -hexyl, the 2-hexyl and the 2-methy 1-2-pentyl radicals with triphenyltin hydride and triphenyltin hydride-t see the last three entries in Table 11. The isotope effect of 2.55 found for the triphenyltin hydride-w-hexyl radical reaction was slightly smaller... 

A report considers the reactions of 1-butoxy and 1-pentoxy radicals with oxygen (eqs 1 and 2) and of their isomerizations by 1,5-H-shift (eqs 3 and 4) using direct and time-resolved monitoring of the formation of NO2 and HO radicals in the laser flash-initiated oxidation of 1-butyl and 1-pentyl radicals. ... 

The flash vacuum pyrolysis of alkynes, arynes, and aryl radicals has been reviewed. A discussion of secondary reactions and rearrangements is included. The pyrolysis of cyclopentadienes has also been examined. The rates for the initial C—H bond fission and the decomposition of C-C5H5 have been calculated. A single-pulse shock study on the thermal decomposition of 1-pentyl radicals found alkene products that are formed by radical isomerization through 1,4- and 1,3-hydrogen migration to form 2- and 3-pentyl radicals. The pyrrolysis of f-butylbenzene in supercritical water was the subject of a report. ... 

O3yhydroperoxides. Peroxides of the oxyhydro type are obtained by the addition of hydrogen peroxide to ketones. High yields of alkyl radicals are then often obtained by reaction with ferrous salts. 1-Meth-oxycyclohexyl hydroperoxide is easily obtained from cyclohexanone and hydrogen peroxide in methanol. It gives rise to the 5-(methoxy-carbonyl)-pentyl radical, which has been used to alkylate protonated heteroaromatic bases in high yield [Eq. (6)]. 

For pentyl radical, internal H-atom transfers can occur regardless of whether further oxidation occurs. These unimolecular reactions can directly compete with oxidation steps and so have implications for low-temperature combustion. For instance, n-pentyl radical can quickly isomerize to iso-pentyl radical via 1,4-H atom transfer each of these radicals can undergo p-scission reactions to yield a new alkyl radical + alkene ... 

Several experimental studies, over the past several decades, have modeled the overall combustion of this system. Jitariu et al. have calculated unimolecular rates for reactions of pentyl radical (i.e., intramolecular H-atom transfer, p-scission, and elimination) noting that isomerizations have lower barriers than p-scissions. ° ... 

A 1,4 shift of this type was postulated by Duncan and Cvetanovid27 to explain some of their results on the reaction of CH2 with isobutene and butene-2. Previously, Gordon and McNesby56 demonstrated that intramolecular hydrogen abstraction occurs in the case of n-pentyl radicals ... 

The methyl radical produced in primary Reaction 2 can abstract a hydrogen atom from an unreacted butene molecule in a chain propagation step or it can add to a butene molecule to provide a pentyl radical, which is the precursor for the observed Cr, products (see Reactions 7 and 8). Methyl radical addition (Reaction 8) is favored at low conversion... 

H-transfer is always ca. 10 faster than 1 4 H-transfer at 600 °K (see Table 3), so it will predominate when the molecular structure of the fuel permits. Simple estimation of the relative concentrations of the hydroperoxyalkyl radicals derived from propane, n-butane and n-pentane illustrates this. Thus, if the relative frequency of attack by OH at primary, secondary and tertiary C—H bond is taken as 2 3 5 [102], then the relative concentrations of propyl, butyl and pentyl radicals may be obtained. The equilibrium constant for reaction (3)... 

The behavior of diethylamino radicals is akin to that of alkyl radicals, in that both dimerization and disproportionation occur and at approximately equal rates. However we have no accurate numerical data for the corresponding sec-pentyl radical which may be used to provide the required contrast. The occurrence of DEMA, methane, and ethane in the reaction products suggests that the methyl radical is an essential precursor and the decomposition of diethylamino radicals... 

The analysis of the primary decomposition products from the decomposition of methyl-cyc/o-pentane requires the addition of only a few reactions to the scheme shown in Fig. 6. Three methyl-cyc/o-pentyl radicals were already formed from the successive isomerization reactions of cyc/o-hexy 1-radical, and thus only the tertiary radical, with its isomerization and decomposition reactions to form methyl-allyl radical and ethylene, needs to be included... 

However, although radical reactions can explain dimerization, they cannot account for the production of trimers. It is possible to explain the formation of polymers in liquid-phase radiolysis by the following sequence. Reaction of a pentyl radical with a pentene molecule would yield a decyl radical (10). 

Let us calculate the rate of both reactions when a sample of solid pentane irradiated at a dose of 10 Mrads is allowed to melt. Assuming the yield of pentyl radicals is twice that of the dimers and using the experimental G values, concentrations of 3.5 X 10 5 mole/gram and of 1.2 X 10 3 mole/gram are found respectively for the olefins and the pentyl radicals. The ratio of the rates of the two reactions is then ... 

Reaction of pentyl radicals with olefins may thus be neglected at 143 °K. 

Furthermore, relatively high yields of light olefins are formed during the radiolysis—e.g., ethylene yield is found to be about 0.60 molecules/ 100 e.v. If addition of pentyl radicals on olefins was possible, one would find at least traces of dodecane among the radiolysis products. The absence of products with molecular weights intermediate between those of dimers and pentadecanes excludes occurrence of this reaction. 

When the temperature is raised, the decyl radicals react with pentyl radicals yielding pentadecane by combination (Reaction 11), decanes, decenes, pentenes, and pentane by disproportionation (15, 16). 

Dimers and trimers are produced by radiolysis of n-pentane in the solid phase. The yields of the various dimers have been measured. These yields suggest a much higher production of 1-pentyl radicals than does the ESR study of these transformations. Occurrence of non-radical processes in dimer production would account for this discrepancy. Trimer production strongly suggests the occurrence of ion-molecule reactions. These reactions would also yield dimers. Dimers would thus be formed by ionic and by radical processes. 

With this information in hand, it seemed reasonable to attempt to use force field methods to model the transition states of more complex, chiral systems. To that end, transition state.s for the delivery of hydrogen atom from stannanes 69 71 derived from cholic acid to the 2.2,.3-trimethy 1-3-pentyl radical 72 (which was chosen as the prototypical prochiral alkyl radical) were modeled in a similar manner to that published for intramolecular free-radical addition reactions (Beckwith-Schicsscr model) and that for intramolecular homolytic substitution at selenium [32]. The array of reacting centers in each transition state 73 75 was fixed at the geometry of the transition state determined by ah initio (MP2/DZP) molecular orbital calculations for the attack of methyl radical at trimethyltin hydride (viz. rsn-n = 1 Si A rc-H = i -69 A 6 sn-H-C = 180°) [33]. The remainder of each structure 73-75 was optimized using molecular mechanics (MM2) in the usual way. In all, three transition state conformations were considered for each mode of attack (re or ) in structures 73-75 (Scheme 14). In general, the force field method described overestimates experimentally determined enantioseleclivities (Scheme 15), and the development of a flexible model is now being considered [33]. 

Scheme 14. Calculated enantioselectivity data for reactions of the 2,2,3-trimethyl-3-pentyl radical 72 with stannanes 69-71 derived from cholic acid...

The n-pentyl radical is the largest alkyl radical for which Arrhenius parameters have been determined for a gas-phase metathetical reaction. Problems with volatility of reactants and dimer products are considerable in studies involving radicals larger than C4. The few results available for n-pentyl are given in Table 21. but in fact for only the first of the four reactions listed was there an experimental determination the other three results were obtained from data for the reverse reactions and the equilibrium constants derived from thermodynamic data. The selection of the rate coefficient for the n-pentyl dimerization reaction, upon which to base the absolute data for the reaction... 

A somewhat different method has been employed for the diazomethane-isopentane system . The formation of pentenes is attributed to the disproportionation of pentyl radicals and, therefore, indicative of abstraction by triplet methylene. The assumption that all of the products eliminated by added oxygen arise through reactions involving triplet methylene leads to the expression... 

In the first step (equation 50) pentane is oxidized to a cation radical which on losing a proton becomes a radical. Pentyl radicals can react to form a C p-dimer (equation 51) or disproportionate with pentane and pentene formation (equation 52). Pentene may be alkylated also forming a C.jQ-dimer intermediate (equation 53). The final products of the reaction are formed as a result of C Q-isomerization and decomposition. 

Table 5 gives the propagation reactions (not including metatheses) of the lumped primary mechanism of the oxidation of n-pentane. It must be noted that this scheme only includes 4 free radicals instead of 24 for the detailed mechanism 3 pentyl radicals, 3 peroxy radicals, 9 hydroperoxypentyl radicals and 9 hydroperoxypentyl-peroxy radicals the dihydroperoxypentyl radicals have not been considered. 

Combining ife(OH n-C5Hn0C(0)CH3) = 7.39 x lO- cm molecule- at 298 K with a diurnal average of [OH] 1.0 x 10 molecule cm gives an estimate of about 1.5 days for the atmospheric lifetime with respect to reaction with OH radicals. Reaction with OH radicals dominates the atmospheric fate of n-pentyl acetate. Using the structure-activity relationship method outlined by Kwok and Atkinson (1995) with F(—OC(O)H) = 0.6 and k(—OC(O)H) = 0.9 x 10 cm molecule" s , it can be estimated that attack of OH radicals on n-pentyl acetate occurs 98% at the n-pentyl group, and 2% at the acetate group. 

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