Deuterium atoms, addition

Hydrogen as it occurs in nature is predominantly composed of atoms in which the nucleus is a single proton. In addition, terrestrial hydrogen contains about 0.0156% of deuterium atoms in which the nucleus also contains a neutron, and this is the reason for its variable atomic weight (p. 17). Addition of a second neutron induces instability and tritium is radioactive, emitting low-energy particles with a half-life of 12.33 y. Some characteristic properties of these 3 atoms are given in Table 3.1, and their implications for stable isotope studies, radioactive tracer studies, and nmr spectroscopy are obvious. 

One useful feature of this reducing system is its apparent ability to allow deuteration of double bonds without scrambling. Although the precise stereochemistry of the addition remains to be established, the incorporation of only two deuterium atoms per double bond has been clearly demonstrated (7). 

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. 

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. 

Farkas and Farkas suggested that the critical step in hydrogenation involved the simultaneous addition of two hydrogen atoms to an adsorbed benzene molecule, whereas exchange with deuterium required the prior dissociation, on the surface, of benzene to form a phenyl radical and a hydrogen atom. The phenyl radical then combined with a deuterium atom, which had been produced by the dissociation of a deuterium molecule, and the monodeuterobenzene was desorbed. 

The addition mechanism was studied using both vinyl monomers specifically labeled with deuterium atoms on the methylene group and disubstituted CHA=CHB olefins. The polymerization of the two stereoisomers, cis and trans. 

Every adsorbed ethylene molecule, so formed, will revert to an adsorbed ethyl radical by the addition of a deuterium atom to one of the two carbon atoms, both of which have equal probability of gaining the deuterium atom. 

The nearly equal amount of cis and trans products formed from 1,5-dimethylcyclo-hexene is explained by the almost equal degree of hindrance of the homoallylic methyl group with the catalyst surface in the alternate adsorption modes.63 64 Another interesting example is the platinum-catalyzed hydrogenation of isomeric octalins.65-67 If syn addition to the double bond is assumed, in principle, both cis- and mms-decalin are expected to result from l(9)-octalin, but only the cis isomer from 9(10)-octalin. In contrast with these expectations, the isomers are produced in nearly the same ratio from both compounds. Transformation in the presence of deuterium revealed that most of the products contained three deuterium atoms. This was interpreted to prove that the very slow rate of hydrogenation of 9(10)-octalin [Eq. (11.9)] permits its isomerization to the 1(9) isomer. As a result, most of the products are formed through l(9)-octalin [Eq. (11.10)] ... 

The cleavage of the S = C thioaldehyde bond was observed when the manganese thioketone complex related to 8 (Scheme 2 R1 = R2 = NMe2) was treated with LiAlD4. The C(C6H4OMe-p)2-moiety of the complex was recovered as tetra(p ra-methoxyphenyl)ethane bearing a deuterium in the ortho position of two vicinal phenyl groups each and additionally two deuterium atoms on the ethane skeleton.127... 

The first two equations represent the fact that the D-D reaction can follow either of two paths, producing tritium and one proton or hehum-3 and one neutron, with equal probability. The products of the first two reactions form the fuel for the third and fourth reactions and are burned with additional deuterium. The net reaction consists of the conversion of six deuterium nuclei lnlo two helium nuclei, two hydrogen nuclei, and two neutrons along with a net energy release of 43.1 MeV. The reaction products—helium, hydrogen, and neutrons—are harmless as contrasted with the myriad fission products obtained in a fission reactor. The neutrons produced may be absorbed in sodium to produce an additional 0.25 MeV per cycle. Therefore, the D-D reaction produces at least 7 MeV per deuterium atom (deuteron) and, with absorption in sodium, more than 10 MeV per fuel atom. 

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