Reactions of CH2 with

Bawn and Milsted6 and Bawn and Tipper6 attempted to study the reaction of CH2 with H2 by the technique of generating CH2 from the reaction of CH2C12 with Na vapor in a flow system at 300 °C. With N2 as the carrier gas the major product was ethylene, with a small amount of CH4, but when H2 was used as the carrier gas the product composition was 92% CH4 and 8% C2H4, and H2 was consumed. Formation of CH4 was accounted for by the direct association... 

Vanpee and Grard133 made a quantitative study of the formation of saturates (mainly ethane) in the photolysis of CH2CO with added methane (ratios CH4/CH2CO = 1 to 7) at 28 to 250 °C. and found that the results could be explained by a mechanism involving competition between CH4 and CH2CO for methylene by reactions of the first order in methylene. The rate of reaction of CH2 with CH4 was found to be 0.183 that of the reaction with ketene. Decomposition of excited ethane by the reaction... 

Holroyd and Noyes66 and Strachan and Noyes129 studied the photolysis of CH2CO in the presence of 02, but with ketene in large excess. The products were CO, C02, C2H4, HCHO, and HCOOH, and the yields were nearly independent of 02 concentration. The C2H4 yield was reduced to about two-thirds of the Octree value. The lack of dependence of the product yields on 02 concentration suggested either quantitative reaction of CH2 with ketene or quantitative reaction with 02. Since there is evidence that the reaction of CH2 with ketene is very rapid,46 70 73 the former seems likely, and the oxidation products observed must therefore be attributed to secondary reactions and not the reaction of CH2 with 02. 

Addition of 02 has been observed to reduce the formation of byproducts in the reaction of CH2 with ethylene46 and methane,9 but does not seem to affect the major reactions of methylene with these substrates. 

Failure of added 02 to affect CH2 reactions in the experiments described above does not prove that the reaction of CH2 with 02 is slow, because methylene reacts with CH2CO, olefins, H2, and CH4 very rapidly, and in most cases 02 was added in relatively small amount. The reaction... 

The insertion reaction seems to be quite general for C—H bonds in all classes of molecules. The reaction of CH2 with CH4, discussed separately in Sec. IV-B, is a special case of the insertion reaction. 

Considerable evidence shows that the reaction is fast. For example, Vanpee and Grard133 found the reaction of CH2 with CH4 to be only about five times slower than the reaction with ketene, which has a collision efficiency > 10 2. Further evidence is found in the results of the... 

It has been assumed 42 that triplet CH2 does not undergo the insertion reaction with C—H bonds, but there is no firm evidence to support this contention. Reaction of CH2 with isobutene, cis and trans butene-2, and cyclohexene (see Sec. IV-E) under conditions favoring formation of triplet methylene gave relatively higher yields of C=C addition products, but the insertion products were nevertheless present in each case. We believe that reduced yields of the insertion products result from lower methylene energy rather than a fundamental difference in reaction mechanism. 

Abstraction Reactions. Many of the reactions of CH2 show indications of partial free-radical character. Evidence for the presence of CH3 radicals in the reactions of CH2 with H2 and CH4 has been discussed in Sects. IV-A and IV-B. Also, ethane and other by-products are reported in the reaction of CH2 with ethylene,46,120 ketene,20 diazomethane,42 allene,43 butene-2,120 and isobutene.39 The results of Doering and Prinzbach26 (see Sec. IV-E-3) provide very convincing evidence of partial free-radical participation in the reaction of CH2 with the C—H bonds of isobutene, especially in the gas phase. 

Alkanes. The indiscriminate reaction of CH2 with primary, secondary, and tertiary C—H bonds as discovered by Doering et al.24 constituted early evidence for the extreme reactivity of methylene. Under some conditions, however, selectivity is shown. The results of studies of tire reaction of CH2 from various sources and under different conditions are summarized in Table I. 

The reaction of CH2 with cyclopropane1617 46177 gives excited methyl-cyclopropane which is deactivated or undergoes structural isomerization to butene-1, butene-2 (m and trans), and isobutene. The lifetime of methylcyclopropane wa6 found to depend on the CH2 source. 

Although no absolute rates are available, the reaction of methylene with olefins is very fast. For example, Frey40 has shown that CH produced by CH2N2 photolysis (4358 A.) reacts with trans butene-2 at a rate 2.4 times slower than with CH2N2, and it is believed that the rate of reaction of CH2 with CH2N2 is comparable to the collision rate.64... 

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 ... 

Ketene. The reaction of CH2 with CH2CO has been studied in connection with the photoiysis of ketene. The major products are CO and ethylene,96 formed in the ratio of about 2.2.73 102 129 Ethane, methane,... 

Diazomethane. Although CH2N2 photolysis has been widely used as a methylene source, the reaction of CH2 with CH2N2 has received little attention. The quantum yield in the photochemical decomposition of CH2N2 is about four at both 4360 A. and 3650 A.96 The mechanism is believed to involve a short energy chain, in addition to the simple process... 

Kistiakowsky and Sauer74 photolyzed ketene in the presence of excess C02 and found evidence for a reaction of CH2 with C02 giving CO and probably HCHO. The reaction with C02 was about 50 times slower than the reaction with CH2CO. The following mechanism was proposed ... 

The rates of reaction of CH2 with benzene and toluene were comparable to the rate of reaction with CH2CO. 

The foregoing mechanism for addition of methylene to the double bond is substantiated by the observation of Jacox and Milligan69 that allene but not cyclopropene is produced upon reaction of CH2 with acetylene in an argon matrix at 4°K. It is quite reasonable to expect in this case that the rate of ring closure would be slow compared to the rate of hydrogen migration. 

The reactions of CH2 with paraffins proceed with efficiencies of 0.02 to 0.05, while those of 3CH2 occur with efficiencies of 10 5 to I0 h. 

The reaction of CH2 with C02 was first postulated by Kistiakowsky and Sauer (13) as taking place via an a-lactone intermediate. The occurrence of this reaction was subsequently demonstrated by Milligan and Jacox (14) in low temperature matrices. These low temperature matrix isolation experiments, however, could not determine definitely the structure of the CH2 C02 intermediate. The result of our laser absorption experiment shows that the CO is vibrationally excited up to v — 4 with a distribution close to the one predicted by a statistical model assuming the existence of a long-lived CH2 C02 complex. This calculation, however, is insensitive to the structure of the complex assumed. Since the ground state triplet CH2 is known to be less reactive and kinetically behaves like CHj (15,16), which does not react readily with C02, the singlet A CH2 is assumed to be involved in the reaction. 

Table II. Average CO Product Vibrational Energies Measured for the Reactions of CH2 with 0(3P), 02 and C02...

P) atoms with 1-alkynes and from the reactions of CH2 with 0(3P), 02 and C02. These reactions can not be readily studied with other techniques such as chemiluminescence (due to low product concentrations as well as the difficulty in producing radical species in flow experiments). With the aid of simple statistical models, the dynamics and branching ratios of these reactions can be reasonably interpreted and crudely estimated. More detailed discussion of the application of this technique to many other examples including energy transfer reactions has recently been reviewed elsewhere... 

The observation that propane is also formed parallels the results of the direct photolysis at 1236 A. and the explanation for the appearance of propane may be nearly the same as suggested in the case of the direct photolysis, i.e., the reaction of CH2 with the product ethane. 

The reaction between CH2 and CH3 fragments is the only one that is preferred on steps, all others being favored on terraces. Noteworthy is the very low activation energy of 0.22 eV for the CH2—CH2 recombination on surface step sites. As indicated by similar barriers, the recombination reaction of CH2 with CH3 on steps competes with the recombination reactions of C with CH2 and CH with CH2 on flat surfaces. 

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