Trapping hydride

A. G. Howard, C. Salou, Cysteine enhancement of the cryogenic trap hydride AAS determination of dissolved arsenic species, Anal. Chim. Acta, 333 (1996), 89-96. 

As the main pollutant of the secondary circuit is hydrogen, the secondary auxiliary circuits have to be optimised in order to measure hydrogen contents and in order to trap hydrides it would be useful to have data about tritium diffusion from the primary circuit. 

An alternative method for ascertaining the end of the reaction, which does not involve the removal of the cover, is to conduct the exit gas through an empty wash bottle (to eict as a trap in case of sucking back ) and then collect a sample in a test-tube over water. If an inflammable gas (hydrogen) is absent, the reaction may be considered complete. Under no circumstances should the reaction be stopped until all the sodium has completely reacted too early arrest of the reaction may result in the product containing sodium hydride, which appears to be partially responsible for the explosive properties of the impure substance ... 

He did not put on the fire gear and proceeded to open the vessel and hose it out with a fire hose. An explosion resulted when water f dislodged crusted-over sodium aluminum hydride trapped in a nozzle. The worker was burned, requiring a two-week hospital stay and several months of recuperation. 

A 2-1. two-necked, round-bottomed flask equipped with a magnetic stirrer (Note 1) is fitted with a 250-ml. pressure-equalizing constant-rate dropping funnel and a condenser, the top of which is connected to a mercury trap to prevent the entrance of air during the reaction and for the detection of gas evolution. The dropping funnel is removed, and 35 g. (0.85 mole) of sodium hydride dispersed in mineral oil is added (Note 2). The mineral oil is removed by washing the dispersion four times with 100-ml. portions of benzene (Note 3). The benzene is removed with a pipet after the sodium hydride is allowed to settle (Note 4). 

The wide variety of methods available for the synthesis of orga-noselenides,36 and the observation that the carbon-selenium bond can be easily cleaved homolytically to give a carbon-centered radical creates interesting possibilities in organic synthesis. For example, Burke and coworkers have shown that phenylselenolactone 86 (see Scheme 16), produced by phenylselenolactonization of y,S-unsaturated acid 85, can be converted to free radical intermediate 87 with triphenyltin hydride. In the presence of excess methyl acrylate, 87 is trapped stereoselectively, affording compound 88 in 70% yield 37 it is noteworthy that the intramolecular carbon-carbon bond forming event takes place on the less hindered convex face of bicyclic radical 87. 

UV irradiation of Pt(PEt3)2(C204) under a dihydrogen atmosphere yields a hydride by trapping [52] ... 

Metal hydride trapping agents have been used extensively in studying the reaction of alkyl radicals with monomers.489 400... 

Alkyl mercuric hydrides are generated in situ by reduction of an alkyl mercuric salt with sodium borohydridc (Scheme 3.91). Their use as radical traps was first reported by Hill and Whitesides491 and developed for the study of radical-olefin reactions by Giese,489490 Tirrell492 and coworkers. Careful choice of reagents and conditions provides excellent yields of adducts of nucleophilic radicals (e.g. -hexyl, cyclohexyl, /-butyl, alkoxyalkyl) to electron-deficient monomers (e.g. acrylics). 

The subsequent fate of this intermediate primary vinyl cation 28 seems to be in doubt and dependent upon the exact reaction conditions. In concentrated (98%) Hi SO4, ion 28 seems to undergo a rapid hydride migration to form the more stable vinyl cation 29, which in turn is trapped by water to give ketone 25... 

Iron porphyrins containing vinyl ligands have also been prepared by hydromet-allation of alkynes with Fe(TPP)CI and NaBH4 in toluene/methanol. Reactions with hex-2-yne and hex-3-yne are shown in Scheme 4. with the former giving two isomers. Insertion of an alkyne into an Fe(III) hydride intermediate, Fe(TPP)H, formed from Fe(TPP)Cl with NaBH4, has been proposed for these reactions. " In superficially similar chemistry, Fe(TPP)CI (present in 10 mol%) catalyzes the reduction of alkenes and alkynes with 200 mol% NaBH4 in anaerobic benzene/ethanol. For example, styrene is reduced to 2,3-diphenylbutane and ethylbenzene. Addition of a radical trap decreases the yield of the coupled product, 2,3-diphenylbutane. Both Fe(lll) and Fe(II) alkyls, Fe(TPP)CH(Me)Ph and [Fe(TPP)CH(Me)Ph] , were propo.sed as intermediates, but were not observed directly. ... 

---