Bimolecular trapping

A simple extension of the competition technique is to the comparison of scavenger efficiencies. Thus pairs of spin traps have been allowed to compete for a variety of radicals, including t-butoxyl, phenyl, and primary alkyl. Much more revealing, however, is the type of experiment in which the bimolecular trapping of a radical is allowed to compete with some other reaction of that radical whose absolute rate constant is known. In this way, the rate constant for the trapping reaction itself is accessible. 

These conclusions were anticipated by product studies. Alkyl azides are readily available and their thermal and photochemical decomposition reactions have been studied. In general, light and heat induced decomposition of methyl azide does not produce a MeN species that can be intercepted in respectable yields with a bimolecular trap. For example, attempts to trap MeN with cyclohexane as solvent produced only a 0.4% yield of adduct. Photolysis of CH3N3 or CD3N3 at cryogenic temperatures fails to produce an IR spectrum attributable to triplet methylnitrene. The IR spectrum of CH2=NH (or CD2=ND) is observed instead. " ... 

At higher concentrations of acetaldehyde, bimolecular trapping of the enolate in Scheme 4.7 will become faster, so, at some stage, this will compete effectively with the reprotonation of the enolate. When the bimolecular capture of enolate by another acetaldehyde molecule becomes much faster than the reprotonation of the enolate, i.e. when /c4[CH3CHO] k, 1 + k-2[H+] + /c 3[BH+], another limiting approximation to the complex rate equation predicted from the mechanism (Equation 4.17) is obtained, Equation 4.19 ... 

These results are complemented by theoretical calculations and computer simulations [110, 111] for J = 1,2 and 3 of bimolecular trapping/annihilation reaction A-l-A—> 0, A-fT - At and A -I- Ay -> T (T is an immobile trap making A particle to become immobile too) and unimolecular trapping/annihilation, A -I- A 0, A —> Ay, A -f Ay 0. It was found that the kinetics of trapped particles can be described by the mean-field theory for bimolecular but not for unimolecular reactions. The kinetics of free A s is described by mean-field theory at short times, but at long times and low trap concentrations the concentration of free A s decays as (2.1.106). 

This is in agreement with the general observation that the equilibration of isomeric cyclopropyl radicals is always faster than any of the known bimolecular trapping reactions of these radicals in solution. 

Bimolecular trapping of phenoxonium ions by nucleophilic yr-bonds can lead to cyclized products when attack occurs at the or /io-position. For instance, a Ritter-type reaction occurs when MeCN is used as solvent in the absence of other nucleophiles, leading to a useful synthesis of benzoxazoles (XLV) [48] ... 

Mott [22] suggested that the nucleation step required either the bimolecular combination of two interstitial barium ions, or the bimolecular trapping of two conduction electrons. The rate-determining step proposed for growth was the transfer of an electron fi om the azide valence band to the metal, subsequently attracting an interstitial barium ion into the developing nucleus. The positive holes (Nj) generated diffiised to the surface and reacted to form Nj. Objections to this mechanism resulted from inconsistencies between the measured conductivity of the... 

With the assumption that the undetected precursors to triplet 1- and 2-naphthylnitrenes are the azirines seen in the low temperature experiments, we can reach useful conclusions about the photochemistry of polynuclear aromatic azides. First, unlike phenyl azide where the closed-shell singlet intermediate formed in room temperature irradiations is dehydroazepine [46, 49, 69], the intermediates formed from both 1- and 2-naphthyl azide are azirines. The difference in the chemistry of 1- and 2-naphthyl azides is traced to a difference in the lifetime of the respective azirines. The azirine from 2-naphthyl azide survives at least 200 times longer than does the azirine formed from 1-naphthyl azide. The increase in lifetime permits the bimolecular trapping reaction by diethylamine to compete with isomerization to the triplet nitrene in the case of the 2-naphthyl but not the 1-naphthyl azides. 

In turn, photolysis of carbonyl azides gives rise to two types of reactions. The photo-Curtius rearrangement proceeds to form isocyanate. In addition, bimolecular trapping products, typical of the reactions of singlet carbonylnitrenes, are also observed. 

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