Propargyl radicals

Table 6 Vibrational assignment of the IR spectra of the free propargyl radical" and methylacetylene.

Alternatively, two propargyl radicals ( CH2-C=CH) can combine and undergo H-atom transfers, followed by cyclization, to yield benzene. 

Fig. 19. The unimolecular reaction channels for the propargyl radical together with the heats of reaction. Assuming that H2 loss is associated with a substantial reverse barrier, formation of cyclopropenylidene, c-C3H2, is the most likely channel. (From Deyerl et a/.143)...

In 1997, Nicholas and his co-workers developed a novel radical cyclization, where propargylic radical 50 is trapped intramolecularly by activated alkenes (Equation (26)). This reaction proceeds with exclusive... 

The second class of ring-forming reactions is recombination of resonantly stabilized free radicals, with subsequent rearrangement and ring formation. The most important such steps are believed to be recombination of two propargyl radicals,... 

Also addition of the propargyl radical may play a role in the formation of the larger PAH compounds. Reactions such as these are denoted Ring growth in Fig. 14.8. 

The lowest-lying potential energy surfaces for the 0(3P) + CH2=C=CH2 reaction were theoretically characterized using CBS-QB3, RRKM statistical rate theory, and weak-collision master equation analysis using the exact stochastic simulation method. The results predicted that the electrophilic O-addition pathways on the central and terminal carbon atom are dominant up to combustion temperatures. Major predicted end-products are in agreement with experimental evidence. New H-abstraction pathways, resulting in OH and propargyl radicals, have been identified.254... 

Alkynyl substituents stabilize a radical center by the same 12 kcal/mol that on average is achieved by alkenyl and aryl substituents. From the point of view of the VB model this is due to the fact that propargyl radicals exhibit the same type of resonance stabilization as formulated for allyl and benzyl radicals in the right column of Table 1.1. In the MO model, the stability of propargyl radicals rests on the overlap between the one correctly oriented n system of the C=C triple bond and the 2 pz AO of the radical center, just as outlined for allyl and benzyl radicals in Figure 1.5 (the other 7t system of the C=C triple bond is orthogonal to the 2pz AO of the radical center, thus excluding an overlap that is associated with stabilization). 

The formation of the propargyl radical indicates that hydrogen abstraction from allene has occurred. However it was shown in a separate experiment that the 1 -methylvinyl radical abstracts hydrogen very easily from allene to form the propargyl radical. (The 1-methylvinyl radical was prepared from 2-bromopropene and sodium and then bombarded with allene). Thus it is unnecessary to postulate that hydrogen atoms can abstract hydrogen directly from allene. 

Typically, these are bromide or iodide precursors such as bromoacetylene (CaHBr), vinyl bromide (C2H3Br), ° and propargyl bromide (C3H3Br) ° to yield supersonic beams of helium-seeded ethynyl, vinyl, and propargyl radicals, respectively. Briefly, the radical precursor is entrained in helium at a ratio of typically 1%. The gas mixture is released by a pulsed valve at 700 Torr backing pressure. A teflon extension with a slit located parallel to the expansion direction of the pulsed beam was... 

We have referred to this alcohol (p. 163) in connection with the unsaturated di-ine hydrocarbon 1-5-hexa-di-ine, CH = C—CH2—CH2 —C = CH, which is known as di-propargyl because it contains two of the groups present in the above alcohol, i.e., the propargyl radical (CH = C - CH2—). 

C-H bond fission and the production of ethynyl radicals. Butadiyne and vinyl acetate are formed when the T -shaped ethyne dimer is irradiated at 193 nm in argon or xenon. The dynamics of the photodissociation of propyne and allene have been studied. The H2 elimination from propyne is a minor route for propyne dissociation and the major path identified in this study is loss of the alkyne hydrogen. A study of the photodissociation dynamics of allene and propyne has been reported and this work has demonstrated that allene gives rise to a propargyl radical while propyne yields the propynyl radical. Other research has examined the photodissociation of propyne and allene by irradiation at 193 nm. ... 

Propargyl radical was produced by vacuum pyrolysis (900-1050°C, 10" -10 Torr) of propargyl iodide or of dipropargyl oxalate (Korolev et al., 1989) and it was frozen into an argon matrix at 12 K (15). Twelve bands were observed in the matrix IR spectrum of the C3H3 radical [only three bands were recorded earlier (Jacox and Milligan, 1974) for this radical] 3307, 3111, 3026, 2080, 1440, 1369, 1061, 1017, 686, 618, 532, 482 cm . Based on the experimental IR spectrum, a normal coordinate analysis has been carried out and a valence force field calculated (Table 6). Stretching frequencies of the carbon skeleton [v(C=C) 2080 cm and p(C—C)... 

In addition to these reactions of even carbon atoms, odd pathways involving the resonantly stabilized propargyl radical (C3H3) have also been identified as relevant contributions to benzene formation (Miller and Melius, 1992) ... 

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