Deuterium atoms, reaction

Xi M and Bent B E 1993 Reaction of deuterium atoms with cyclohexane on Cu(111)—hydrogen abstraction reactions by Eley-Rideal mechanisms J. Phys. Chem. 97 4167... 

Laser photolysis of a precursor may also be used to generate a reagent. In a crossed-beam study of the D + FI2 reaction [24], a hypertliennal beam of deuterium atoms (0.5 to 1 eV translational energy) was prepared by 248 mn photolysis of DI. This preparation method has been widely used for the preparation of molecular free radicals, both in beams and in experiments in a cell, with laser detection of the products. Laser photolysis as a method to prepare reagents in experiments in which the products are optically detected is fiirtlier discussed below. 

Only the a hydrogens are replaced by deuterium m this reaction The key intermediate IS the enolate ion formed by proton abstraction from the a carbon atom of cyclopen tanone Transfer of deuterium from the solvent D2O to the enolate gives cyclopentanone containing a deuterium atom m place of one of the hydrogens at the a carbon... 

Replacement of halides with deuterium gas in the presence of a surface catalyst is a less useful reaction, due mainly to the poor isotopic purity of the products. This reaction has been used, however, for the insertion of a deuterium atom at C-7 in various esters of 3j -hydroxy-A -steroids, since it gives less side products resulting from double bond migration. Thus, treatment of the 7a- or 7j5-bromo derivatives (206) with deuterium gas in the presence of 5% palladium-on-calcium carbonate, or Raney nickel catalyst, followed by alkaline hydrolysis, gives the corresponding 3j3-hydroxy-7( -di derivatives (207), the isotope content of which varies from 0.64 to 1.18 atoms of deuterium per mole. The isotope composition and the stereochemistry of the deuterium have not been rigorously established. 

Aliphatic sulfonyl chlorides that have a-hydrogen substituents, react with simple tertiary amines, such as trimethylamine, to generate sulfenes or perhaps their amine adducts 446). These species are suggested by the incorporation of one (but not more) deuterium atoms on reaction of sulfonyl chlorides with deuterated alcohols and triethylamine (447-450). A 2 1 adduct of sulfene and trimethylamine with proposed sulfonyl-sulfene structure could be isolated (451). 

Attempted Grignard reaction of cyclohexanone with test-butylmagnesiu 1 n bromide gives only about 1% yield of the expected addition product along with 99% tin reacted cyclohexanone. If D30+ is added to the reaction mix l ure after a suitable period, however, the "un reacted" cyclohexanone is found to have one deuterium atom incorporated into it. Explain. 

The deuterium isotope effect for each hydrogen atom ortho to the diazonio group ( H/ D = 1.22, Swain et al., 1973b) is the largest secondary aromatic hydrogen isotope effect yet observed. It is comparable to those observed for a-deuterium in reactions involving carbocation formation from secondary aliphatic esters. Ob-... 

On the basis of these correlations, Gold and Satchell463 argued that the A-l mechanism must apply (see p. 4). However, a difficulty arises for the hydrogen exchange reaction because of the symmetrical reaction path which would mean that the slow step of the forward reaction [equilibrium (2) with E and X = H] would have to be a fast step [equivalent to equilibrium (1) with E and X = H] for the reverse reaction, and hence an impossible contradiction. Consequently, additional steps in the mechanism were proposed such that the initial fast equilibrium formed a 7t-complex, and that the hydrogen and deuterium atoms exchange positions in this jr-complex in two slow steps via the formation of a a-complex finally, in another fast equilibrium the deuterium atom is lost, viz. 

The importance of the inductive effect in controlling the reaction rates was further shown by Streitweiser and Humphrey596, who measured the rates of dedeuteration of toluene (a, a-d2), (a, 2,4,6-g 4), and (a, 2,3,4,5,6-g 6) by lithium cyclohexylamide at 50 °C and found the rate to be reduced by 0.4 %, 0.4 %, and 1.8 % for a deuterium atom in the ortho, meta and para positions respectively. The retardation is consistent with the +1 effect of deuterium but the differential positional effect could not be rationalised in simple and general terms. 

It was of course not possible to introduce the deuterium atoms closer than three bonds away from the center of reaction , but the attenuation of the inductive effect with distance was not expected to affect the validity of the conclusions, especially considering the work of Streit-wieser and Klein (1964), who found that the isotope effect per deuterium in the solvolysis of benzhydryl chloride only decreased from 1 9% for deuterium in the ortho positions to l-5% for deuterium in the meta positions. 

With regard to the mechanism of the cycloisomerization, Fiirstner et al. found strong evidence of a metallacyclic intermediate. By labeling the allylic position of enynes 46 and 48, they showed that reactions yielding traws-annulated rings 47 transferred the deuterium atom to the exocychc double bond (eq. 1 in Scheme 10), whereas c -annulated rings 49 formed with complete preservation of the position of the deuterium atom (eq. 2 in Scheme 10). This corresponds well to a metallacycUc... 

As nuclear reactions are isotope specific, NRA may be used, for example, to distinguish the distribution of binary blends of polymers in a polymer film, where one of the polymers is labelled with deuterium. The depth distribution of the deuterium atoms can be established and hence that of the labelled polymers. 

Let us now discuss some of the characteristics of this quenching with mercaptans and disulfides. Interestingly, both sulfur derivatives are equally effective in inhibiting the photoreduction and are in fact interconverted during the reaction. The same equilibrium mixture of mercaptan and disulfide is obtained regardless of which was initially added to the reaction mixture. Furthermore, there appears to be no appreciable consumption of the sulfur compounds/64 When benzophenone is irradiated in the presence of isopropanol (OD) and mercaptan, isopropanol containing two deuterium atoms is isolated,... 

To summarize the exchange and donation mechanisms of Tetralin, pathway 1 in Figure 2 is the predominant pathway of exchange and donation as determined by preferential incorporation of protium into the a-position of Tetralin. Tetralin-di2 loses a deuterium atom from the a-position, and a protium atom is incorporated into the a-position. This equilibrium is the exchange pathway. Continued loss of deuterium from the a-Tetralinyl radical eventually leads to naphthalene, and this reaction is the donation pathway. 

The pressure P is contained in formula (62) not only in an explicit form but also in terms of the parameters e8 and e8+, as seen from (5), because s and e8+ are, generally speaking, functions of pressure. In our model, however, 8- and e8+ may be regarded as independent of P since the surface is supposed to be saturated with hydrogen and deuterium atoms (all the adsorption centers are assumed to be occupied). Thus, the hydrogen-deuterium exchange proves, in accordance with (63), to be a reaction of the first order with respect to hydrogen (deuterium), which is consistent with numerous experimental data (see Section III.A). 

The great majority of experimental data (see Section III.A) indicate that the hydrogen-deuterium exchange reaction belongs to the class of acceptor reactions (i.e., reactions that are accelerated by electrons and decelerated by holes). This means that the experimenter, as a rule, remains on the acceptor branch of the thick curve in Fig. 8a, on which the chemisorbed hydrogen and deuterium atoms act as donors. Here a donor impurity must enhance the catalytic activity, while an acceptor impurity must decrease it. This is what actually occurs, as we have already seen (see Section III.A). 

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