Hydrogen atom, reactions

The 0 -phenyl-0 -piperidyl-(2)-acetic acid methylester of BP 135° to 137°C under 0.6 mm pressure is obtained in theoretical yield by hydrogenation of 50 g of 0 -phenyl-0 -pyridyl-(2)-acetic acid methylester in glacial acetic acid in the presence of 1 g of platinum catalyst at room temperature, while taking up 6 hydrogen atoms. Reaction with HCI gives the hydrochloride. Resolution of stereoisomers is described in U.S. Patent 2,957,880. 

The vinyl + hydrogen atom reaction is a rich example of competition between abstraction and addition-elimination pathways... 

Gutterez M, Henglein A, Dohrmann JK (1987) Hydrogen atom reactions in the sonolysis of aqueous solutions. J Phys Chem 91(27) 6687-6690... 

The photochemical and thermal decompositions of this compound have received very little attention. Most kinetic data have come from studies of the synthesis and chlorine and hydrogen atom reactions. 

Most of the calculations discussed in this chapter, and in Chapter 11, have been carried out using the TST model. However the calculations on the hydro-genic molecule-hydrogenic atom reactions reported in Table 10.1, which include... 

The remaining steps consist in the union of the adsorbed olefin and its derivative, the half-hydrogenated state, with an adsorbed hydrogen atom, reactions which occur with retention of configuration of the substituted carbon atom, reactions (3) and (4),... 

Because many of the alternates and replacements for CFCs have an abstractable hydrogen atom, reaction with OH in the troposphere dominates their loss. Table 13.4 gives some rate constants for the reaction of OH with these compounds the kinetics summary of De-More et al. (1997) should be consulted for other compounds. It is seen that the rate constants at 298 K are typically in the range of 10-l3-10-ls cm3 molecule-1 s-1, depending on the degree of halogen substitution and the nature of the halogen, e.g., F, Cl, or Br. Typical A factors are of the order of 1 X 10 12 cm3 molecule-1 s-1 per H atom (DeMore, 1996). 

If hydroxyl were to react by H atom abstraction in the case of ethane, and by addition with ethylene, an inverse dependence would be observed for the activation energy, namely, a decrease from ethane to ethylene, as is the case for hydrogen atom reactions. The activation energies for reactions of oxygen atoms with ethane and ethylene decrease from ethane... 

By this technique these authors have determined the rate constants and collision yields for a number of simple olefins, substituted olefins, and some aromatic hydrocarbons. For a number of years these determinations represented the only extensive set of rate constants of hydrogen atom reactions with olefins. The technique did not differentiate between addition and abstraction by hydrogen atoms from the olefins and the rates were the sum of the two. 

In this section, we discuss reactions of hydrogen atoms, understanding that the isotopes of hydrogen are also included. A summary of various experiments measuring energy disposal in the products of hydrogen atom reactions is given in Table 1. 

The reaction of hydrogen and chlorine is one of the so called chain reactions. We may assume that the combining of every two molecules is followed by a whole chain of reactions innumerably reproduced. At a normal temperature and under low pressures this chain reaction is started and kept going by chlorine or hydrogen atoms reactions of this type are described as mass chains. 

The intermediate states in this two-hydrogen atom reaction are not known, although Teichner et al, basing their view on the overall kinetics, have suggested that H3 is involved in the reaction H3 has been observed spectroscopically in other studies (125). Alternatively, because of the low concentration of spiltover hydrogen on the surface, a two-dimensional-three-center (2Hsp + OMe) reaction seems improbable. It may be that the reaction occurs stepwise by the association of spiltover hydrogen with a methoxyl as a first step, i.e. ... 

Mezyk S, Bartels DM. (1997) Rate of hydrogen atom reaction with ethanol, ethanol-dj, 2-propanol, and 2-propanol-dy in aqueous solution. /Phys Chem A 101 1329-1333. 

The energetics of the H abstraction reaction [reaction (27)], AG= -11.4 kcal moH, and the /3-scission [reaction (28)], AG = -1-8.1 kcal mol are from DFT calculations. The two-step mechanism was excluded because of the endothermic step in reaction (28). However, a concerted reaction [combining reactions (27) and (28)] would be spontaneous, but with only a small exothermicity, AG ncerted = -3.3 kcal mol Therefore, the source of these radicals appears to be a bimolecular homolytic substitution (Sjj2) of the acetamide, by hydrogen atoms [reaction (29)] ... 

In addition to these steps, the mechanism must include radical-radical termolecular recombination reactions. The key feature of this mechanism is the competition between H and N for C2H5 radicals as indicated in steps (8) and (9). The addition of hydrogen atom reactions to the mechanism brings the NO titration and C2H4 reaction estimates of N atom concentrations into fair agreement. 

A second variation, which is extremely useful for measuring rate constants of hydrogen atom reactions in aqueous solution, is the following immediately after... 

Studies of the rare earth hydrides have been largely concerned with the stabilities of the phases formed, and the thermodynamic properties of the solid solutions [16-18], The decompositions of hydrides of metals with pronoimced catalytic properties (Ni, Pd) are often limited by surface steps involving hydrogen atom reactions, which are sensitive to poisoning and resemble rate processes occurring at surfaces of heterogeneous catalysts. In contrast, the decomposition of hydrides of other metals (Be, Zn) apparently proceed by nucleation and growth mechanisms. 

Due to the relatively small mass of the proton it is assumed that tunneling effects play a crucial role in many proton and hydrogen atom reactions, and that they determine many specific properties of H-bonded systems in the gas and condensed phases [38], It should be noted, however, that proton tunneling has been proved experimentally for only a relatively small number of H-bonded systems in the gas phase and in crystals. 

Therefore, we conclude that the attack of oxygen atoms on the compounds in Table I may be of an electrophilic nature, but that the rather small differences in comparison with the analogous hydrogen atom reactions leaves some doubt as to the meaning and correctness of this conclusion. 

The Hydrogen Atom. Hydrogen atom reactions can be studied in isolation by irradiation of N2-saturated aqueous solutions of 2-methyl-2-propanol adjusted to acid pH. The hydroxyl radicals are deactivated by the alcohol, as in the e q experiments. Since e q is the conjugate base of H , can be converted... 

K31 = 10 dm mol (SCN)2 is rapidly formed and significantly stable. Hydrogen-atom reactions in this system are slow and can be ignored. 

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