Hydrogen ortho-para reaction

On the basis of information on the properties of the nickel-hydrogen and nickel-copper-hydrogen systems available in 1966 studies on the catalytic activity of nickel hydride as compared with nickel itself were undertaken. As test reactions the heterogeneous recombination of atomic hydrogen, the para-ortho conversion of hydrogen, and the hydrogenation of ethylene were chosen. 

His researches and those of his pupils led to his formulation in the twenties of the concept of active catalytic centers and the heterogeneity of catalytic and adsorptive surfaces. His catalytic studies were supplemented by researches carried out simultaneously on kinetics of homogeneous gas reactions and photochemistry. The thirties saw Hugh Taylor utilizing more and more of the techniques developed by physicists. Thermal conductivity for ortho-para hydrogen analysis resulted in his use of these species for surface characterization. The discovery of deuterium prompted him to set up production of this isotope by electrolysis on a large scale of several cubic centimeters. This gave him and others a supply of this valuable tracer for catalytic studies. For analysis he invoked not only thermal conductivity, but infrared spectroscopy and mass spectrometry. To ex-... 

The reaction of benzoxazine in die presence of 2,6-xylenol does not occur until 135 C, presumably because die hydrogen-bonded intermediate depicted for the 2,4-xylenol reaction (Fig. 7.19) cannot occur. All three types of linkages are obtained in diis case. Para-para methylene-linked 2,6-xylenol dimers, obtained from the reaction of 2,6-xylenol with formaldehyde, formed in the decomposition of the benzoxazine (or with other by-products of that process) dominate. Possible side products from benzoxazine decomposition include formaldehyde and CH2=NH, either of which may provide the source of methylene linkages. Hie amount of ortho-para linkages formed by reaction of 2,6-xylenol with benzoxazine is low. Ortho-ortho methylene-linked products presumably form by a decomposition pathway from benzoxazine (as in Fig. 7.18). 

Integrated thermal conductivity cells (see Fig. 12.8) allow a quantitative determination of the corresponding ortho/para ratios of the dihydrogen. The enriched parahydrogen is well-suited for in-situ NMR studies of hydrogenation reactions that yield nuclear spin polarization due to symmetry breaking during the reaction. The same apparatus has also been used successfully to enrich ortho- and paradeuterium mixtures. 

Steiner and Rideal45 determined the Arrhenius parameters for (22), log k22 = 10.86 — 5200/4.58 T, from a study of the HCl-catalysed ortho-para hydrogen reaction in the temperature range 600-770°. When allowance is made for the better data for the hydrogen dissociation available to Steiner and Rideal, their results agree well with earlier work by Rodebush and Klingelhoefer46. 

The steric rather than the inductive origin of the secondary deuterium KIE is also suggested because kH/kD = 0.994 per deuterium found in the per-deuteropyridine-methyl iodide reaction is smaller (less inverse) than the kH/kn = 0.988 per deuterium found for the 4-deuteropyridine reaction. A secondary inductive KIE should be more inverse when a deuterium is substituted for a hydrogen nearer the reaction centre, i.e. at the meta- or ortho-rather than at the para-position of the pyridine ring. Thus, if the KIE were inductive in origin, the KIE in the perdeuteropyridine reaction should be more inverse than that observed for the 4-deuteropyridine reaction. If the observed KIE were the result of a steric KIE, on the other hand, a less inverse KIE per deuterium could be found in the perdeuteropyridine reaction, i.e. a less inverse KIE per deuterium would be expected if there were little or no increase in steric hindrance around the C—H(D) bonds as the substrate was converted into the SN2 transition state. Since the KIE per D for the perdeuteropyridine reaction is less than 1%, the transition state must not be sterically crowded and the KIE must be steric in origin. Finally, the secondary deuterium KIEs observed in the reactions between 2-methyl-d3-pyridine and methyl-, ethyl- and isopropyl iodides (entries 3, 7 and 9, Table 17) are not consistent with an inductive KIE. If an inductive KIE were important in these reactions, one would expect the same KIE for all three reactions because the deuteriums would increase the nucleophilicity of the pyridine by the same amount in each reaction. The different KIEs for these three reactions are consistent with a steric KIE because the most inverse KIE is observed in the isopropyl iodide reaction, which would be expected to have the most crowded transition state, and the least inverse KIE is found in the methyl iodide reaction, where the transition state is the least crowded. 

Ortho-para deuterium, 27 25, 50 Ortho-para hydrogen conversion, 27 23 Oscillatory catalytic reactions, 37 213-215, 271-272 see also Platinum catalytic CO oxidation on Pt(l 11) and Pt(llO) surfaces COj formation, 37 216-217 kinetic oscillation mechanism, 37 220-228... 

The effect of monofluorination on alkene or aromatic reactivity toward electrophiles is more difficult to predict Although a-fluonne stabilizes a carbocation relative to hydrogen, its opposing inductive effect makes olefins and aromatics more electron deficient. Fluorine therefore is activating only for electrophilic reactions with very late transition states where its resonance stabilization is maximized The faster rate of addition of trifluoroacetic acid and sulfuric acid to 2-fluoropropene vs propene is an example [775,116], but cases of such enhanced fluoroalkene reactivity in solution are quite rare [127] By contrast, there are many examples where the ortho-para-dueeting fluorine substituent is also activating in electrophilic aromatic substitutions [128]... 

If in the elementary step a change of total spin occurs, the reaction is forbidden, e.g. in the ortho/para conversion of the hydrogen molecule or the decomposition of N20 into nitrogen and oxygen (see section on this reaction). Materials containing paramagnetic centres could act as catalysts for this type of reaction, and many examples are actually known. 

The interesting aspect of reactions (1) and (2) is that on ortho/para conversion at a paramagnetic centre no H—H bond is broken, and consequently no H/D exchange occurs. On the other hand, when H/D exchange does occur, a hydrogen to hydrogen bond has been broken in the process, and O/P concer-sion occurs as a consequence. Therefore comparison of the rate of the two processes provides valuable information. 

Examples of first-order reversible reactions are gas phase cis-trans isomerization, isomerizations in various types of hydrocarbon systems, and the racemization of a and (3 glucoses. An example of a catalytic reaction is the ortho-para hydrogen conversion on a nickel catalyst. 

In Section V below we show that spillover can induce catalytic activity on the support. The nature of the active site created on the support may result from the surface reduction, or the adsorbed hydrogen may be a center and site for reaction (123). On the other extreme, spiltover hydrogen has been shown to inhibit ortho-para conversion over sapphire and ruby surfaces... 

It is interesting to note that C-H activation on ruthenium NHC complexes is not limited to intramolecular protons located in the N-sidechain of the carbene, but occurs inter-molecularly as well. Leimer et al. reacted [MesIRuH PCyj] with toluene-dg at ambient temperature and observed a rapid H/D exchange reaction involving the four hydride hydrogen atoms on ruthenium, the methyl protons of the mesityl substituents of the carbene ligand and the deuterium atoms on the meta positions of toluene-dg. The ortho-, para- and methyl-deuterium atoms of the solvent did not participate [145]. 

The earliest attempt to detect the presence of free radicals in this way was through the catalytic conversion of ortho-para hydrogen mixtures. At equilibrium at room temperature, ordinary hydrogen consists of a mixture of 75 per cent ortho-H2 (nuclear spins parallel) and 25 per cent para-H2 (nuclear spins antiparallel). At low temperatures (<90°K) equilibrium mixtures may be prepared which contain up to 100 per cent pure para-H2. The latter mixtures are metastable below 500 C and are slowly converted to the stable composition above that temperature. The thermal reaction has been well studied " and corresponds to a catalytic conversion by H atoms present at these temperatures. 

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