Mobile hydrogen

The concept of mesohydric tautomerism was advanced by Hunter and his associates in a series of papers which appeared between 1940 and 1950 (e.g., references 15 and 16). This concept was based on the fact that in all cases where the mobile hydrogen atom would be bonded to oxygen, sulfur, or nitrogen atoms in both possible tautomers, the individual forms had not been isolated. It was further established that many of these compounds were associated both in the liquid state and in solution, and it was concluded that the individual tautomers did not exist. The actual molecules were thought to be intermolec-ularly hydrogen-bonded, the mobile hydrogen atom being bonded equally to both of the hetero atoms. This concept has been useful and has led to clarification of the tautomerism which occurs in solids and... 

The foregoing conclusions are further supported by a refined X-ray analysis of pyrid-2-one, which indicated that the mobile hydrogen atom is attached to the nitrogen atom in the solid state and that individual molecules are bound into helices by N—H- -0 hydrogen bonds. An oxo structure is also indicated by the molar refractivity of pyrid-2-one. The dipole moment of 4-methoxypyridine is ca. 3.0 debyes in dioxane, whereas the values for pyrid-4-one and its 1-methyl derivative are much higher, ca. 6.0 debyes indicating the... 

In 1882 Baeyer and Oekonomides advanced formula 72 (R = H) for isatin on chemical grounds, but shortly thereafter the dioxo structure 73 (R H) was proposed since the ultraviolet spectrum of isatin resembled that of the N—Me derivative (73, R Me) and not that of the O—Me derivative (72, R = Me). " It was later shown, despite a conflicting report, that the ultraviolet spectrum of isatin is very similar to the spectra of both the O— and N—Me deriva-tives - the early investigators had failed to take into consideration the facile decomposition of the O—Me derivative. Although isolation of the separate tautomers of isatin has been reported, - these claims were disproved. A first attempt to determine the position of the mobile hydrogen atom using X-ray crystallographic techniques was inconclusive, but later X-ray work," dipole moment data, and especially the infrared spectrum demonstrated the correctness of the... 

If the nitrogen atoms of this type of compound do not carry a substituent, tw o mobile hydrogen atoms and eight tautomeric forms (72-... 

One of the most important applications of diazomethane depends on its ability to replace a mobile hydrogen atom by a methyl group. In comparison with other methylation agents, diazomethane occupies a rather special place this is, in part, because only rather strongly acidic protons are replaced. Because of the simple method of working up the reaction mixture, the reaction is especially well applicable for sensitive compounds and for small amounts. If tautomeric or potentially tautomeric compounds are treated with diazomethane, two... 

The classical theory of methylation with diazomethane was developed by Arndt from a different basis. It depends on the postulate (which can be traced back to von Pechmann " 0 of direct methylation mobile hydrogen in an acid compound is directly replaced by the methyl group, i.e., the methyl group appears in the place which the hydrogen previously occupied. For the reaction of tautomeric substances with diazomethane, the following equation is applicable ... 

Substituent groups on a polyphosphazene chain containing mobile hydrogen atoms (4-isopropylphenol [715,716], 4-benzylphenol [293,718], etc.) showed a completely different photochemical reactivity both in solution and in sohd state under accelerated conditions, based mostly on the fight-induced oxidation of these groups and radical formation reactions. 

Ion binding reduces the repulsive forces between the charged groups on the polyanion but, unless the counterions are site-bound, the repulsive osmotic forces are not affected. At full neutralization the coulombic forces along the polymer chain become zero. However, the polymer does not contract, because the osmotic forces remain unless, of course, all the cations become site-bound. (Of course, in the case of a free weak acid the concentration of mobile hydrogen ions is very small and the polymer adopts a compact form.)... 

Acid chlorides and anhydrides, especially the former, are the most reactive carbonyl compounds, hence the most dangerous. The dangerous reaction that is involved in accidents usually corresponds to the equation below, in which X is a chlorine atom or a O2C-R group and H-X a compound with a mobile hydrogen ... 

Isocyanates react violently with water, alcohols and more generally compounds with a mobile hydrogen. 

Ion-ion tautomerism Functional groups with a mobile hydrogen atom present in the ammonium cation can result in one more type of tautomerism, as in the case of /3-oxyethyl(triethyl)ammonium 1,3,2,5-dioxaborataphosphorinane (128) (88IZV155). In this case the equilibrium between two ionic forms (acyclic and cyclic) has been observed [Eq. (94)]. 

Bis(a-aminomethyl)phosphines having a mobile hydrogen atom could be expected to undergo a borylation reaction with borane, hydrogen being evolved. In fact, bis(Af-phenylaminomethyl)-phenylphosphine (199) and its sulfide (200) appeared to interact with borane under mild conditions, yielding a new type of phosphorus-boron-containing heterocycle—2-... 

The above compounds contain both cyclic and acyclic P—C—N—B fragments. Complexes formed at the first stage of the reaction undergo a series of successive transformations depending on the electronic and structural characteristics of the substituents and the presence of a mobile hydrogen at heteroatoms. 

Figure 3 shows a graph of a as a function of the ratio i/n(0) of the surface concentration of mobile hydrogen to the concentration of traps I. The passivation depth L is given, according to (50), by... 

There is interesting physics, not all of it yet elucidated, in the interpretation of a number of the experiments listed in Table 2. We shall give a brief discussion of some of the issues in Sections 3b through 3e, with particular attention to a few studies that have inferred especially large diffusion coefficients (as opposed to the typical lower limits, of order say 10 12 cm2/ sec at 150°C, obtained from many experiments). But in order to present the first main conclusion of this section as quickly as possible, we shall start out with a discussion of the one experiment to date that can use specific empirical evidence on the density of mobile hydrogen in order to extract a specific diffusion coefficient. 

The time constant involved in this approach to a stable profile should be essentially the rc of (99) if the mobile hydrogen is all H+ or this times (n0 + n+ )/n+ if interconverting H° and H+ are present. Of course, n0,n+, and nB are dependent on space and time, so a more elaborate theory would be needed for quantitative accuracy, but the simple use of (99) should give at least an order-of-magnitude value for D+, or more properly for D+n+/(n0 + n+). 

Fig. 26. A schematic diagram illustrating a possible hydrogen diffusion path, as in Fig. 25. The diagram indicates the Si—H bonds, the distribution of weak Si—Si bonds that can be broken by the insertion of a hydrogen atom, and mobile hydrogen interstitial states (Street etal., 1988). 

As shown in Scheme 1, metallacycles can be ring-opened by hydrogen transfer, generally brought about with the aid of compounds (HX) having mobile hydrogen-like acids, amines, etc. In some cases the HX compound adds to the organic chain. 

Some distinctive features of the insertion reactions reported in Table VII can be summarized as follows First, carbon monoxide gives rise by insertion (5, 195a) to acyl bonds which are easily cleaved by water, alcohols, or compounds with mobile hydrogen. The metal is thus easily removed from the organic part and, being eliminated in its reduced state, can undergo a further oxidative addition, leading to a catalytic cycle. Thus, use of CO is very favorable for catalytic reactions. 

MHDV [Mobil Hydrogen Donor Visbreaking] A modified visbreaking process in which a hydrogen donor stream from the oil refinery is added to the heavy hydrocarbon stream before thermal cracking. Developed by Mobil Corporation... 

An analogous reaction is known with aromatic nitroso-compounds, but for it an exceptionally mobile hydrogen atom must be present in the ketone and hence no condensation occurs with simple ketones such Us acetone. The products of the reaction are, of course, azomethines. This condensation has made possible the synthesis of 1 2 3-triketones (F. Sachs), e.g. 

Condensations with alkyl nitrites and nitrates, however, are not so generally applicable as the true ethyl acetoacetate reaction, and the possibility is not excluded that they proceed in another way compounds with mobile hydrogen might first he added to the inorganic part of the ester by means of an aldol condensation. The fact that fluorene, which contains no active double bond at all, combines with ethyl nitrate (as well as with ethyl oxalate) and sodium ethoxide in the same way, yielding oci-nitrofluorene, seems to support this second theory (W. Wislicenus). 

The compound which couples adds itself to the double bond of the diazo-hydroxide either in virtue of (a) the mobile hydrogen atom or (h) the double bond ... 

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