Carbon—hydrogen bonds triplet carbenes

A second process that has a central position in the analysis of the chemical properties of carbenes is their reaction with hydrocarbons. As is the case for alcohols, singlet and triplet carbenes react with hydrocarbons in distinctive ways. It has long been held that very electrophilic singlet carbenes can insert directly into carbon-hydrogen bonds (11) (Kirmse, 1971). On the other hand, triplet carbenes are believed to abstract hydrogen atoms to generate radicals that go on to combine and disproportionate in subsequent steps (12)... 

It is generally observed that the isotope effect for abstraction of hydrogen by a triplet carbene is 2-9, whereas insertion into a carbon-hydrogen bond by a singlet carbene proceeds with an isotope effect of 1-2. ... 

Products which can be ascribed to the intermediate formation of radicals have long been observed in carbene reactions. In the gas phase these products could arise by homolytic decomposition of excited primary products before collisional deactivation rather than from radicals generated in the course of insertion. This is not so in solution. It is found that, in the thermal decomposition of diphenyldiazomethane (Bethell et al., 1965) or photolysis of diphenylketene (Nozaki et al., 1966) in toluene solution, the product of insertion of diphenylmethylene into the benzylic carbon-hydrogen bonds, 1,1,2-triphenylethane, is accompanied by substantial amounts of 1,1,2,2-tetraphenylethane and bibenzyl. This is a strong indication that discrete diphenylmethyl and benzyl radicals are formed, and, taken in conjunction with EPR (Section IIB) and other evidence (Etter et al., 1959) that diphenylmethylene is a ground-state triplet, would support the view that equation (20) is an adequate representation of triplet insertion. 

Atomic Carbon.—Skell and Plonka have extended their studies of the chemistry of atomic carbon, which they generate in a low-intensity carbon arc. The mechanism of the conversion of carbon-hydrogen bonds to carbon-methyl bonds has been investigated thoroughly the detection of triplet carbenes as intermediates infers that the inserting carbon atoms are in the state. 

Considering the simplest carbene, methylene, it is clear that it could have a linear structure (D -symmetry) or be bent ((72s-symmetry). In the former case, using the ls-orbitals of hydrogen and the 2s- and 2 -orbitals of carbon, two a molecular orbitals can be constructed and these can clearly accommodate the C—H bond electrons. A degenerate pair of orbitals, similar to carbon atomic p-orbitals, are the ones of next-lowest energy. The linear structure of methylene would thus correspond to a triplet state. 

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