Bromine flash photolysis

Tlie existence of the ylide 19, which can formally be interpreted as the deprotonation product from the corresponding salt 7a, has been claimed by trapping chlorocarbene with pyridine during the laser-flash photolysis of e do-7-chlorodibenzo[n,c]bicyclo[4.1.0]heptane (18) (96JPC18426). Bromination of l-vinyl-2-pyridone (20) yields the bicyclic pyridinium bro-... 

Chiang, Y. Kresge, A. J. Zhu, Y. Flash photolysis generation and study of p-quinone methide in aqueous solution. An estimate of rate and equilibrium constants for heterolysis of the carbon-bromine bond inp-hydroxybenzyl bromide. J. Amer. Chem. Soc. 2002,124, 6349-6356. 

The kinetics of the reaction of bromine atoms with simple aliphatic aldehydes have been measured by the fast-flow technique with resonance fluorescence detection, and by laser flash photolysis. 

Fig. 9. Electronically excited bromine atoms, Br(42Pyt), resulting from the flash photolysis of IBr. Pibt = 0.03 mm Hg, pAr = 100 mm Hg, E = 780 J.

The rates of ketonization are usually easier to determine (by flash photolysis) than the much slower rates of enolization that require laborious conventional methods such as measuring bromination kinetics and analysis of the reaction products. Thus the shape of the profile is conveniently explored by flash photolysis over a wide range of pH for kK, and only a single point on the lower curve is then required to determine the enolization constant Ke-... 

CHClBr2 - , CFBrj - - - - ", CHIj , CFCU . The formation of electronically excited bromine and iodine atoms during the flash photolysis of halomethanes has been observed by Kasper and Pimental and by Donovan and... 

Photoreductiom in Aqueous Solution. Solvated electrons form on the irradiation of natural water samples or of aqueous solutions of natural organic matter isolated from surface waters (12, 45, 46). The solvated electron is a powerful reductant that reacts rapidly with electronegative substances such as chlorinated, brominated, and iodinated compounds. The interaction of pho-toejected electrons and a halocarbon is demonstrated by the laser flash photolysis data shown in Figure 3. 

Formation of iodine is detected tens of picoseconds after the irradiation of CH2ICH2I or CF2ICF2I in acetonitrile solution. In a continuation of their work on 1,1-dihalo compounds, Phillips and his group have investigated the photochemical behaviour of 1,1-dibromoacetic acid. Irradiation provides evidence for the formation of the iso-dibromoacetic acid (120). This is akin to the observations with 1,1-diodomethane. The authors propose that this species (120) could exhibit carbenoid character. Laser flash photolysis of 1,2-dibro-moethane at 248 nm in water provides a source of bromine atoms. ""... 

Schrock and Schuster [46] have demonstrated that the quantum yield to loss of nitrogen from triplet phenyl azide is rather low. The fact that a strong transient spectrum of the brominated triplet phenyl nitrene is observed following laser flash photolysis thereby implies that the heavy atom effect has catalyzed intersystem crossing from the singlet to the triplet state of the nitrene rather than of the azide excited state. 

The results obtained with the eosin dianion are similar to fluorescein. In this case, the R absorptions at 369 m/x (monoanion) and 405 m/x (dianion) agree exactly with the flash photolysis assignments (7, 10, 16), while the X monoanion band at 450 m/x is reasonably close to the flash photolysis spectra reported at 462 (10) and 456 m/x (16). The eosin OH adduct maximum is located near 600 m/x compared with 570 20 m/x for fluorescein (4, 5). The initial yields at pH 9.0 are G(R) =3.3 0.3 and G(X) = 2.0 0.3 (22). The smaller contribution of the OH addition path in eosin compared with fluorescein is consistent with the occupation of four of the six xanthene ring sites by bromine atoms. The only information available on erythrosin from pulse radiolysis locates the R dianion at 450 m/x and the X monoanion at 470 m/x (6). 

The brominated diaryldiazomethane (7) was found to be stable enough to survive Suzuki coupling, thus allowing the synthesis of mono-, bis- and tris(diazo) compounds, e.g. (8). The photoproducts of these diazo compounds were characterized by EPR and SQUID measurements, which indicated that triplet, quintet and septet ground-state species were generated from the mono-, bis- and tris(diazo) precursors, respectively. Temperature-dependent EPR and UV-visible measurements and also laser flash photolysis studies showed that all three species are stable up to 160 K, and have half-lives of a few seconds in solution at room temperature. 

The interaction of OH radical with seawater yields oxidized bromine species. The identity of the oxidized bromine species ahd its subsequent reactions have been studied by extensive flash photolysis experiments and also by pulse radiolysis. OH appears to give a nearly quantitative yield of the dibromide ion-radical in seawater. This radical decays by parallel first- and second-order reactions, the latter process being well-known but irrelevant in nature. 

Of these rate constants, all except the last cited have been determined in discharge-flow systems. Bromine and iodine atom recombination rates have been measured using flash photolysis over a wide temperature range, and for a wide variety of third bodies M. Temperature coefficient measurements using discharge-flow methods have been reported for N, H and Cl atom recombinations. Data on Br atom recombination at 298 K have also been obtained using these methods. Limited data on the effect of third body on the rate constants have been obtained in these cases. However, there is much scope for the acquisition of good data with systematic variation of T and M, as well as extensions to hetero-atom recombination reactions. At present, quantitative kinetic data are available for only one reaction of the latter type... 

The kinetics and mechanism of the oxidation of aromatic aldehydes by bromine have been investigated, the rate of production of benzoic acid being little affected by substitution of the aldehyde. Radical bromine ions Bra" have also been used as an oxidant. Flash photolysis of 10 mol 1" sodium bromide solutions may be used to generate the radical anions ... 

The TT-complex of a chlorine atom with an aromatic molecule can approach the selectivity of a bromine atom. The observed selectivity of a chlorine atom in the presence of a complexing agent also depends upon the substrate concentration , because the rate of the reaction of Cl with the substrate and the complexing agent are of the same magnitude Scheme 4 presents the absolute rate constants that have been deduced by a study of the chlorination of DMB by the laser flash photolysis technique ... 

Trifluoroacetylketene (91) has been generated in aqueous solution by flash photolysis. Rates of hydration to form the enol of 4,4,4-trifluoroacetoacetic acid (92e) have been measured, and also rates of the subsequent ketonization to the /3-keto acid (92k). Extensive rate and equilibrium constant data are reported for these reactions and for the ionizations of the tautomers. For example, the enol (92e) has acidity constants (in -logio form) of 1.85 and 9.95, for the acid and enol OH groups, respectively. Rates of enolization of (92k) have also been measured (by bromination) and, combined with an estimate of the hydration constant (K = 2900) of (92k), suggest that the keto-enol tautomeric constant is ca 0.5, about 100 times greater than that of its unfluorinated analogue. 

Sander, S.P., Friedl, R.R. Kinetics and product studies of the reaction chlorine monoxide + bromine monoxide using flash photolysis-ultraviolet abstsption. J. Phys. Chem. 

The rate of recombination of bromine atoms is 2.4 x 10 l mole s in reasonably good agreement with the value of 3.6 x 10 found by using flash photolysis, and the photostationary method can thus be used to obtain reaction rates much greater than those obtained by other competitive methods. 

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