Propane with radicals

The shift of curves, as shown in Fig. 3.9, is unsurprising since the larger fuel molecules and their intermediates tend to break down more readily to form radicals that initiate fast reactions. The shape of the propane curve suggests that branched chain mechanisms are possible for hydrocarbons. One can conclude that the character of the propane mechanism is different from that of the H2—02 reaction when one compares this explosion curve with the H2—02 pressure peninsula. The island in the propane-air curve drops and goes slightly to the left for higher-order paraffins for example, for hexane it occurs at 1 atm. For the reaction of propane with pure oxygen, the curve drops to about 0.5 atm. 

Photolytic. When synthetic air containing propane and nitrous acid was exposed to artificial sunlight (X = 300-450 nm), propane photooxidized to acetone with a yield of 56% (Cox et al., 1980). The rate constants for the reaction of propane and OH radicals in the atmosphere at 298 and 300 K were 1.11 x 10cmVmolecule-sec (DeMore and Bayes, 1999) and 1.3 x lO" cmVmolecule-sec (Hendry and Kenley, 1979). Cox et al. (1980) reported a rate constant of 1.9 x 10cmVmolecule-sec for the reaction of gaseous propane with OH radicals based on a value of 8 X 10cmVmolecule-sec for the reaction of ethylene with OH radicals. 

Figure 4-9 shows the free-radical reaction of propane with bromine. Notice that this reaction is both heated to 125 °C and irradiated with light to achieve a moderate rate. The secondary bromide (2-bromopropane) is favored by a 97 3 product ratio. From this product ratio, we calculate that the two secondary hydrogens are each 97 times as reactive as one of the primary hydrogens. 

The free-radical reaction of propane with bromine. This 97 3 ratio of products shows that bromine abstracts a secondary hydrogen 97 times as rapidly as a primary hydrogen. Bromination (reactivity ratio 97 1) is much more selective than chlorination (reactivity ratio 4.5 1). 

The photolysis of n-butane follows a pattern similar to that of propane, with many corresponding reactions. As found for previous hydrocarbons the photolysis includes both molecular and free-radical processes. The molecular elimination of Hj and Dj from C4H10-C4D10 mixtures was first shown by Sauer and Dorfman, who concluded that at 1470 A more than 90 % of the hydrogen came from molecular processes. On the basis of a study of the decomposition of excited -butane molecules generated by electron impact , they attributed hydrogen, methane, ethylene, and other hydrocarbon products to molecular processes, and concluded that free-radical reactions were minimal. 

The mechanism of the radiolysis of the higher paraffins is generally less clearly understood than that of methane, ethane and propane. The same types of reaction are found, however, and although the system may be more complicated owing to the presence of an increased number of reactive species the pattern of reaction is similar to that with the lower paraffins. Free-radical reactions continue to comprise an important fraction of the overall reaction for the uninhibited radiolysis. In view of the complexity in the presence of free radicals many studies have been made with radical scavengers added to the system so that the ion, ion-molecule and excited-molecule reactions might be studied. 

The anion radical of 2-methyl-2-nitropropane being formed by reduction with alkali metal or electrolysis fragmentates to tert-butyl radicals and nitrite ions. Thus formation of di-tert-butylaminyloxide is initiated85. Treatment of 2-methyl-2-nitro-propane with sodium phenyl gives tert-butyl-phenylaminyloxide the reaction proceeding through an intermediate hydroxylamine oxide salt 65 which is even isolable86. ... 

Similar radical intermediates were formed by treatment of alkylsulfanyl(chloro)methylcyclo-propanes with tributyltin hydride this approach also produced butenyl sulfides in good yields. ... 

The radical reaction of propane with chlorine yields (in addition to more highly halogenated compounds) 1-chloropropane and 2-chloropropane. 

It is well-known that propylene acts in the capture of free radicals in the same manner as nitric oxide and propylene inhibits the rate of reaction, which proceeds through the chain mechanism. Therefore, in the progress of a reaction such as the pyrolysis of propane in which one of the main products is propylene, the inhibition effect to the reaction will be observed. In order to analyze the kinetics of the inhibited reaction, it will be necessary to investigate how much propylene influences the rate of pyrolysis of other hydrocarbons. There are, however, a few quantitative works about this effect. Stubbs Hinshelwood (9) and Laldler Wojciechowski (7) have researched the thermal decomposition of propane, sufficiently inhibited by propylene, and were able to discuss the reaction mechanisms of the thermal decomposition of normal paraffin-hydrocarbons. Kershenbaum and Martin (5) have carried out experiments on pyrolysis of propane with small amounts of propylene in the feed and diluted with nitrogen, and they noted the effect of propylene, if any, on the pyrolysis. 

Data obtained for the nitration of propane with NO in the gas phase at <300 "C in the presence of oxygen shows [3] that a rise in oxygen concentration in the reaction zone by increasing the opportunities for radical formation appreciably increases the yield of nitroalkanes. If the same process is carried out at 350 C, there is a sharp fall in the yield and extent of conversion, and there is an appreciable increase in the carbon monoxide (CO) content of the exit gases. It would appear that, in the high-temperature nitration of alkanes, conditions are created under which the rate of the reaction ... 

In the course of gas-phase nitration of alkanes with NO, increase in the yield of nitroalkane can be attained by increase in the rate of radical formation and retardation of oxidative reactions in which the free radicals take part. It is probable that the catalytic effect of molecnlar iodine on the nitration of propane with NO is associated mainly with the tendency of inactive iodine atoms to retard the oxidation of hydrocarbons [11]. In the presence of 0.15% I, the CO content of the reaction products is reduced from 22.1 % to 5.2%. Simultaneously, the yield of nitro compounds is increased by 10%. 

The effect of NO is particularly notable in the nitration of propane with nitric acid at 420 °C [10]. Being a powerfnl inhibitor of chain reactions (Chapter 3), NO reduces the rate of radical formation and the rate of recombination of radicals with NO. On the other hand, the increase observed in the content of CO and Cp of the exit gases indicates the development of processes of hydrocarbon breaking. The addition... 

Moreover, Bols et al. developed another methodology for the synthesis of carbamoyl azides from aldehydes by treatment with iodine azide at reflux in acetonitrile [41]. The carbamoyl azides are obtained in 70-97 % yield from the aliphatic and aromatic aldehydes (Scheme 5.4). When the reaction of phenyl-propanal with IN3 at 25 °C was performed in the presence of the radical trap, no acyl azide was observed, which was taken as support for a radical reaction mechanism. The mechanism shown in Scheme 5.6 is proposed for the reaction. Iodine radicals are formed by homolysis of the weak iodine-azide bond, abstracting the aldehyde hydrogen atom. The resulting carbon-centered radical reacts with iodine azide to produce an acyl azide. The following Cuitius rearrangement provides carbamoyl azides. 

Ullerstam, M., S. Danger, and E. Ljungstrdm (2000), Gas phase rate coefficients and activation energies for the reaction of butanal and 2-methyl-propanal with nitrate radicals, Int. J. Chem. Kinet, 32, 294-303. 

Irradiation of ethyleneimine (341,342) with light of short wavelength ia the gas phase has been carried out direcdy and with sensitization (343—349). Photolysis products found were hydrogen, nitrogen, ethylene, ammonium, saturated hydrocarbons (methane, ethane, propane, / -butane), and the dimer of the ethyleneimino radical. The nature and the amount of the reaction products is highly dependent on the conditions used. For example, the photoproducts identified ia a fast flow photoreactor iacluded hydrocyanic acid and acetonitrile (345), ia addition to those found ia a steady state system. The reaction of hydrogen radicals with ethyleneimine results ia the formation of hydrocyanic acid ia addition to methane (350). Important processes ia the photolysis of ethyleneimine are nitrene extmsion and homolysis of the N—H bond, as suggested and simulated by ab initio SCF calculations (351). The occurrence of ethyleneimine as an iatermediate ia the photolytic formation of hydrocyanic acid from acetylene and ammonia ia the atmosphere of the planet Jupiter has been postulated (352), but is disputed (353). 

In the second paper the models were amplified for ethane, 49 reactions with 11 molecular species and 9 free radicals for propane, 80 reactions with 11 molecular species and 11 free radicals. The second paper has a list of 133 reactions involving light hydrocarbons and their first- or second-order specific rates. 

The substitution of one hydroxyl radical for a hydrogen atom in propane produces propyl alcohol, or propanol, which has several uses. Its molecular formula is C3H7OH. Propyl alcohol has a flash point of 77°F and, like all the alcohols, bums with a pale blue flame. More commonly known is the isomer of propyl alcohol, isopropyl alcohol. Since it is an isomer, it has the same molecular formula as propyl alcohol but a different structural formula. Isopropyl alcohol has a flash point of 53 F. Its ignition temperamre is 850°F, while propyl alcohol s ignition temperature is 700 F, another effect of the different stmcture. Isopropyl alcohol, or 2-propanol (its proper name) is used in the manufacture of many different chemicals, but is best known as rubbing alcohol. 

As a result, the central radical R3 is formed, and the fragment with the end methyl group breaks down into propane and a new fragment with the end vinyl group. 

The mass spectrum of a compound is typically presented as a bar graph with masses (m/z values) on the x axis and intensity, or relative abundance of ions of a given m/z striking the detector, on the y axis. The tallest peak, assigned an intensity of 100%, is called the base peak, and the peak that corresponds to the unfragmented cation radical is called the parent peak or the molecular ion (M+). Figure 12.2 shows the mass spectrum of propane. 

Two of the many products of ethylene radiolysis—methane and propane—show no or only negligible variation with field strength. Methane is produced by a molecular elimination process, as evidenced by the inability of oxygen or nitric oxide to quench its formation even when these additives are present in 65 mole % concentration (34). Propane is completely eliminated by trace amounts of the above scavengers, suggesting methyl and ethyl radicals as precursors ... 

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