Trap experiment

The velocity distribution/(v) depends on the conditions of the experiment. In cell and trap experiments it is usually a Maxwell-Boltzmann distribution at some well defined temperature, but /(v) in atomic beam experiments, arising from optical excitation velocity selection, deviates radically from the nonnal thennal distribution [471. The actual signal count rate, relates to the rate coefficient through... 

Pryolysis of soHd Cp2Ti(CD2)2 yields CD H but not CD. Pyrolysis of (C D )2Ti(CH2)2 yields CH D. These results show that the radical attacks the Cp rings (301,302). Pyrolysis of Cp2Ti(CgH )2 proceeds via a ben2yne intermediate, as shown by trapping experiments involving cycloadditions (293,303-306). 

All other spectroscopic methods are applicable, in principle, to the detection of reaction intermediates so long as the method provides sufficient structural information to assist in the identification of the transient species. In the use of all methods, including those discussed above, it must be remembered that simple detection of a species does not prove that it is an intermediate. It also must be shown that the species is converted to product. In favorable cases, this may be done by isolation or trapping experiments. More often, it may be necessary to determine the kinetic behavior of the appearance and disappearance of the intermediate and demonstrate that this behavior is consistent with the species being an intermediate. 

The first evidence for thionitrosoarenes 4-XC6H4N=S (X = H, Cl, Br) was gleaned from trapping experiments in which the Diels-Alder... 

By trapping experiments, the intermediate radical species have been identified, thus supporting the mechanism as formulated above. 

Diradical species 4 is more stable than diradical 5, and the oxetane 6 is thus formed preferentially oxetane 7 is obtained as minor product only. Evidence for diradical intermediates came from trapping experiments, as well as spectroscopic investigations. ... 

Further mechanistic evidence comes from trapping experiments. When bromobenzene is treated with KNH2 in the presence of a diene such as furan, a Diels-Alder reaction (Section 14.5) occurs, implying that the symmetrical intermediate is a benzyne, formed by elimination of HBr from bromobenzene. Ben-zyne is too reactive to be isolated as a pure compound but, in the presence of water, addition occurs to give the phenol, in the presence of a diene, Diels-Alder cycloaddition takes place. 

The efficacy of RF in stabilizing small rings is well illustrated by the fact that the azete from trifluoro-l,2,3-triazine is considerably more reactive. Trapping experiments were unsuccessful and a polymer was isolated at room temperature. The dimer (41) forms an observable anion with CsF, which confirmed the endo structure [87CC1699 90JCS(P1)975, 90JCS(P1)983], In contrast, trifluoro-l,2,4,-triazine is resistant to vapor phase photoysis and flow pyrolysis [87JCS(P1) 1251]. 

Thermal initiation of styrene has been shown to be third order in monomer. The average rate constants for third order initiation determined by Hui and Hamielec is k = 105 34 e(,j8iaT) (M V).- "0 The rate constant for formation of the Mayo dimer determined in trapping experiments with nitroxidcs (Scheme 3.63) or acid (Scheme 3.64) as kn = 104 4 (M ls 1)j21 is substantially higher than is... 

Donor free silylene complexes are reactive intermediates in a variety of chemical reactions. In many cases, evidence for the coordinated silylenes involved has been obtained indirectly by means of trapping experiments [49-60]. 

In a similar way, a set of disilanyl and polysilanyl complexes has recently been synthesized and exposed to photochemical deoligomerization reactions [137]. The photolytically obtained reactive silylene complexes have been identified by trapping experiments [138, 139]. 

Similar results were obtained by Ogino and co-workers. After the initial cleavage of the CO from the metal, a silyl group migration occurs. It was shown by cross experiments that in this case the silyl substituent migrates and not the silylene. In a subsequent step, the silylene is then displaced by CO. Unfortunately, no trapping experiments of the silylenes in solution have been reported. 

A careful investigation of the reaction kinetics and detailed trapping experiments allow the conclusion that in this case a a-bond metathesis reaction mechanism applies. The polymerization reaction of PhSiH3 by CpCp Hf(SiH2Ph)Cl has been monitored by H-NMR spectroscopy. The data k(75 °C) = 1.1(1) x 10-4 M 1 s AH = 19.5(2) kcal mol" AS = -21(l)euandkH/fcD = 2.9(2) (75 °C) are in good agreement with the proposed mechanism with a metallacycle as transition state [164],... 

Most of the subsequent work on this reagent was concerned with the formation of aryl radicals (see review by Cadogan, 1971). However, 2-terf-butyl-A-nitrosoacet-anilide was found to decompose in benzene to give, instead of 2-tert-butylbiphenyl, as expected for a substitution of benzene by a 2-tert-butylphenyl radical, a mixture of isomeric tert-butylphenyl acetates. A careful reexamination (Cadogan and Hib-bert, 1964) suggested that the ratio of 2- and 3-tert-butylphenyl acetates was consistent with the involvement of 2-tert-butylbenzyne, i.e., the product of an ionic dediazoniation, as an intermediate. This was later confirmed by trapping experiments designed to detect aryne intermediates. 

Morkovnik et al. (1989) found experimentally that the addition of an equimolar amount of 4-morpholino- or 4-dimethylaminoaniline to a suspension of nitrosyl perchlorate in 100 % acetic acid, dioxan, or acetonitrile yields a mixture of the diazonium perchlorate and the perchlorate salt of the amine radical cation, with liberation of gaseous nitric oxide. Analogous results in benzene, including evidence for radicals by ESR spectroscopy and by spin trapping experiments, were obtained by Reszka et al. (1990). 

Evidence for arynes as unstable intermediates in dediazoniation comes from product analyses (Scheme 8-23) and from aryne-trapping experiments. A benzenediazonium ion with a substituent in the 4-position gives the two products indicated in Scheme 8-23 when reacted with a Bronsted acid HY. Analogously, two or three isomers respectively are obtained with benzenediazonium ions substituted in the 2-or 3-position. 

Mechanistically there is ample evidence that the Balz-Schiemann reaction is heterolytic. This is shown by arylation trapping experiments. The added arene substrates are found to be arylated in isomer ratios which are typical for an electrophilic aromatic substitution by the aryl cation and not for a homolytic substitution by the aryl radical (Makarova et al., 1958). Swain and Rogers (1975) showed that the reaction takes place in the ion pair with the tetrafluoroborate, and not, as one might imagine, with a fluoride ion originating from the dissociation of the tetrafluoroborate into boron trifluoride and fluoride ions. This is demonstrated by the insensitivity of the ratio of products ArF/ArCl in methylene chloride solution at 25 °C to excess BF3 concentration. 

As to the question of intra- or intermolecularity of the rearrangement there are three opinions, one which states that the reaction is completely intermolecular148, another supports a concurrent intra- and intermolecular mechanism149, whilst a third claims that the reaction is completely intramolecular150. Evidence for the intermolecular mechanism is based on trapping experiments such as the reaction of meta-tolyl acetate (CVIII) in the presence of we/a-chlorobenzoyl chloride (CIX) when the acylation product (CX) is formed rather than the products of... 

This chapter deals with systems in which a product of one reaction becomes a reactant in the next. The intervening species, known as an intermediate, sometimes rises to a concentration comparable to those of the main species, the reactants and products. In other circumstances, it remains at a much smaller concentration. Whether at high concentration or low, the intermediate may be directly detectable. If not, its existence can perhaps be inferred from the kinetic equations or trapping experiments. 

A careful distinction must be drawn between transition states and intermediates. As noted in Chapter 4, an intermediate occupies a potential energy minimum along the reaction coordinate. Additional activation, whether by an intramolecular process (distortion, rearrangement, dissociation) or by a bimolecular reaction with another component, is needed to enable the intermediate to react further it may then return to the starting materials or advance to product. One can divert an intermediate from its normal course by the addition of another reagent. This substance, referred to as a trap or scavenger, can be added prior to the start of the reaction or (if the lifetime allows) once the first-formed intermediate has built up. Such experiments are the trapping experiments referred to in Chapters 4 and 5. 

The difference in behaviour between pentyl and butyl cation systems (Figs. 3 and 4) has also been encountered in trapping experiments with carbonium ions, primarily formed from alkanes and SbFs, by CO (Hogeveen and Roobeek, 1972). In the case of n-butane the secondary butyloxocarbonium ion is the main product, whereas in the case of n-pentane only the tertiary pentyloxocarbonium ion is found. 

Before our studies, high temperatures (>600°C) had usually been used to generate dichlorocarbene in the gas phase. Based on trapping experiments we have shown that the trihalomethyl mercury derivatives RHgCHals, which were successfully used earlier as sources of dihalocarbenes in solution (Seyferth, 1972), are also convenient precursors of carbenes in the gas phase (Mal tsev etaL, 1971a,b). 

The reaction of OH radicals with dimethyl sulfoxide in aqueous solution was studied already in 1964 by Norman and coworkers . They used the system Tj -HjOj to produce OH radicals and using ESR/rapid mixing techniques they were able to demonstrate elimination of a methyl radical during the OH induced oxidation. Further studies showed the formation of sulfinic radicals in this reaction either directly or by spin trapping experiments . ... 

The adsorbent also needs to be spiked directly with the chemical to determine the recovery from the adsorbent, and this recovery value can be used to adjust the recovery from the adsorbent after air sampling. For compounds with extremely low volatilities, the chemical deposit can be heated to promote volatilization and to minimize the time required for the trapping experiments. 

These results are in excellent agreement with the alcohol trapping experiments of Platz and co-workers, who isolated essentially only carbene derived adducts, ... 

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