Atomic reaction with methane

Kandel S A and Zare R N 1998 Reaction dynamics of atomic chlorine with methane importance of methane bending and tortional excitation in controlling reactivity J. Chem. Phys. 109 9719-27... 

Chlorination of Methane. Methane can be chlorinated thermally, photochemicaHy, or catalyticaHy. Thermal chlorination, the most difficult method, may be carried out in the absence of light or catalysts. It is a free-radical chain reaction limited by the presence of oxygen and other free-radical inhibitors. The first step in the reaction is the thermal dissociation of the chlorine molecules for which the activation energy is about 84 kj/mol (20 kcal/mol), which is 33 kJ (8 kcal) higher than for catalytic chlorination. This dissociation occurs sufficiendy rapidly in the 400 to 500°C temperature range. The chlorine atoms react with methane to form hydrogen chloride and a methyl radical. The methyl radical in turn reacts with a chlorine molecule to form methyl chloride and another chlorine atom that can continue the reaction. The methane raw material may be natural gas, coke oven gas, or gas from petroleum refining. 

The destruction of 03 by chlorine and bromine can be short-circuited by removing either Cl and Br or, alternatively, CIO and BrO. For chlorine atoms, this occurs by reaction with methane that has been transported from the troposphere ... 

The reaction of CBr4 with potassium is reported to generate free C atoms and the rate constants for reaction with methane, ethylene, and benzene have been reported. The reaction of nitrogen atoms with CN radicals has also been used as a C atom source. Carbon atoms have also been produced by passing organics through a microwave discharge. ... 

Use of bond-dissociation energies gives a calculated AH0 of—26 kcal for this reaction, which is certainly large enough, by our rule of thumb, to predict that Kqq will be greater than 1. Attack of a methyl radical on molecular chlorine is expected to require a somewhat more oriented collision than for a chlorine atom reacting with methane (the chlorine molecule probably should be endwise, not sidewise, to the radical) but the interatomic repulsion probably should not b much different. 

These reactions supply atomic hydrogen to lower parts of atmospheres and thus initiate numerous reactions of carbon, oxygen, and nitrogen species. The most important are the reactions with methane (equations 8.8-8.10), which finally yield... 

This is probably because 0 atoms produced in primary process (45) react much more rapidly with C2H6 than with N20. Several products are formed including ethylene, butane, carbon monoxide, hydrogen, methane, and probably ethanol and acetaldehyde. More ethylene is formed than one would expect from the amount of butane. It was found that 0 atoms react rapidly with ethylene, which is one of the photolytic products. The reaction-rate constant of O atoms with ethylene is estimated to be about 330 times as rapid as that with ethane.82 Complete elucidation of the mechanism of O-atom reaction with ethane is complicated because of the rapid reaction of O atoms with one or more of the products. 

In a free-radical chain reaction, every propagation step must occur quickly, or the free radicals will undergo unproductive collisions and participate in termination steps. We can predict how quickly the various halogen atoms react with methane given relative rates based on the measured activation energies of the slowest steps ... 

A chlorine atom may go through this cycle many thousands of times before it is removed in the form of HC1 following reaction with methane ... 

Many of the above complexes have been examined by photolysis in inert and reactive gas matrices. These experiments, in general, provide evidence for the photochemical generation of the 16-electron coordinatively unsaturated intermediates, their weak interaction with inert gas atoms or methane, and in several cases their eventual reaction with methane by C-H activation. The applicability of this method to a particular system depends upon the volatility of the metal complex precursor, as the species must go into the gas phase during deposition in the matrix. Several examples are given below. 

Compare the second reaction in each pair methyl radical reacting with chlorine is more exothermic than methyl radical reacting with iodine this does not explain how iodine prevents the chlorination reaction. Compare the first reaction in each pair chlorine atom reacting with iodine is very exothermic whereas chlorine atom reacting with methane is slightly endothermic. Here is the key chlorine atoms will be scavenged by iodine before they have a chance to react with methane. Without chlorine atoms, the reaction comes to a dead stop. 

None of the neutral second-row transition metal atoms react with methane at 300 K, but some of them (Rh and Pd) react with ethane and larger alkanes [15]. Ab initio calculations [15] indicate that Y, Zr, Nb, Ru and Rh atoms produce very stable insertion products H-M-CH3 in the reaction (1) with an activation barrier ( 0 4-20 kcal/mol), which is much lower than the C-H bond dissociation energy (103 kcal/mol). Some metals (Nb, Ru, Rh) have to change spin multiplicity [15] in the course of the reaction (1). 

Bromine is potentially able to interact with stratospheric ozone in the same manner as chlorine (Wofsy et al., 1975). The catalytic cycle for bromine is expected to be quite efficient, because its reaction with methane is slower than that of Cl atoms in addition, the reaction of OH with HBr is faster than that of OH with HC1. The major bromine compound in the troposphere is methyl bromide, which has a natural origin and occurs with a mixing ratio of about 10 pptv (see Table 6-14). This seems small enough to neglect bromine to a first approximation. 

Several of our studies were designed to investigate the reactions of atomic ground-state silicon ions with hydrocarbon gases that are potentially present in plasma systems. The first of these involved reactions with methane (Boo et al., 1990). The results, illustrated in Fig. 5, show that only endothermic processes are... 

Formed in superacid media species exhibiting electrophilic properties are able to attack alkanes primarily via electrophilic addition to C-H bond followed by other reactions. In particular, Olah et al. observed O atom insertion in hydrogen peroxide reaction with methane in Magic Acid above 0 °C to produce methanol with very high (>95%) selectivity [54a]. The particle (OH)"", which may be considered to be a protonated oxygen atom in the singlet state is apparently the active species in the reaction. Methyl alcohol formed is immediately protonated to methyloxonium ion, and this prevents further... 

In the cases when metal atoms in their ground state do not react with alkane at low temperature, an active species may be generated by photoexcitation of metal atoms. The excited atoms which are formed are capable of inserting into the C-H bond of alkane [28], For example, irradiated (X. < 360 nm)iron atoms react with methane to produce the species CHj-Fe-H. Analogously, atoms of Mn, Co, Cu, Zn, Ag and Au insert into the C-H bond of methane. However, atoms of Ca, Ti, Cr and Ni are inactive in this reaction [29]. Species containing M-C bonds can be detected by IR spectroscopy (Table V.3) [29]. 

By international agreement, chlorofluorocarbons are being replaced by environmentally acceptable alternatives that are less harmful with respect to the destruction of ozone in the atmosphere. E. W. Kaiser, T. J. WalUngton, and M. D. Hurley [Int. J. Chem. Kinet., 27, 205-218 (1995)] studied several of the reactions that are thought to be involved in the overall reaction network. Of particular interest is their stndy of the relative rates at which chlorine atoms react with methane and CF3I. The data tabulated below correspond to reaction at 2TC in a batch reactor illuminated with nltraviolet radiation. The reactions of interest are... 

Chou and Rowland demonstrated that photodionically generated tritium atoms are capable of promoting both substitution (S3) and hydrogm abstraction (54) in reaction with methane, the ratio of the yields of processes (53) and (54) being 0.27 for atoms of initial energy 2.8 eV. produced by photolysis of TBr at 185 nm. The thresliold for substitution of T for D in CD is about 1.5 eV, comparable with a rough value for T-for-H replacement in cyclohexane and appreciably... 

In terms of nonequilibrium interaction conditions the lack of the intramolecular deuterium isotope effect in the hot tritium atoms reaction with partial deuteriated methane, implies that the greater chance of encounter between fast moving... 

The reaction (20.13) occurs via hydrogen atom transfer step followed by the CH3 addition to the next available FeO site. The stoichiometry of the FeO reaction with methane was measured and it was found to be very close to 2 1 [100], Therefore, FeO behavior is similar to oxygen radical [80]. 

The Infrared spectra also indicated evidence for the formation of the blnuclear species AlaCO and A1sa. The matrix isolation technique has been employed to show that ground state aluminium atoms react with methane, though less efficiently than the spontaneous reaction observed with boron, whilst gallium and indium atoms do not. However, some controversy exists as to whether or not photoactivation is necessary. Electron spin resonance studies of the reaction of ground state aluminium atoms with buta-1,3-dlene show that two major paramagnetic species, a v-... 

Effective bimolecular rate constants for reaction of the gas phase, neutral, and ground state Ir and Pt atoms with small linear alkanes at 300 K in 0.5-1,1 Torr of He buffer gas have been measured.In these experiments Ir shows no reaction with methane, ethane, propane, or n-butane. In contrast, Pt reacts with all four alkanes. The reaction efficiency increases with the size of the alkanes, from 0.01 for methane to 0.5 for n-butane. Thus far, ground state Pt is unique among the neutral transition metal atoms in its ability to react with methane at 300 K. 

This cycle is terrninated by the reaction of chlorine atoms with methane Cl + CH — HCl + CH3. The importance of this cycle depends on the avadabiLity of oxygen atoms and varies with altitude as well as the time of year it accounts for only 5% of the halogen-controUed loss at 15 km, but increases to 25% at 21 km. 

Reaction with triplet oxygen 0( P) atoms [17778-80-2] gives high yields of CO and chloroacetaldehyde [107-20-0], with smaller amounts of acetyl chloride [75-36-5], HCl, methane [74-82-8], and polymer. The rate of the gas-phase reaction of vinyl chloride with 0( P) atoms has also been reported (41). 

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