Fast hydrogen atom reactions

Under 300 K ambient conditions, reactions of photolytically produced fast hydrogen atoms with C02 are known to yield OH + CO products via vibrationally excited HOCOt intermediates. Since the initial report by Oldershaw and Porter (1969), many groups have studied reaction (1) by using fast hydrogen atoms and... 

When alkanes are dissolved in superacids, they are transformed into various products [52]. In protic media, the first step of the reaction is the protonation to form in the case of methane, the methonium ion, CHs. For the theoretical description of such a structure see refs. [53]. The ground state of this ion has symmetry which may be regarded as a complex of CHi cation with Hi molecule (H-H distance has been found to be 0.936 A which suggests formation of a bond). The ground state is only insignificantly different ca. 1 kcal moP ) from the excited state of C4v symmetry. Even closer are two transition states of other symmetries [53c], This finding corresponds to extremely fast hydrogen atom movements in CH5 and other similar carbonium ions therefore, fast isomerization, e.g., epimerization of alkane molecules, is expected to proceed in... 

First calculations on the dynamics of the reaction (-2) using fast hydrogen atoms are now available [43]. Such calculations should allow microscopic interpretations of the experimental results described here. 

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 reaction is very fast in both directions, and so is always at equilibrium in water and in aqueous solutions. In every glass of water, protons from the hydrogen atoms are ceaselessly migrating between the molecules. This type of reaction, in which one molecule transfers a proton to another molecule of the same kind, is called autoprotolysis (Fig. 10.9). 

This type of catalyst is not limited to nickel other examples are Raney-cobalt, Raney-copper and Raney-ruthenium. When dry, these catalysts are pyrophoric upon contact with air. Usually they are stored under water, which enables their use without risk. The pyrophoric character is due to the fact that the metal is highly dispersed, so in contact with oxygen fast oxidation takes place. Moreover, the metal contains hydrogen atoms and this adds to the pyrophoric nature. Besides the combustion of the metal also ignition of organic vapours present in the atmosphere can occur. Before start of the reaction it is a standard procedure to replace the water by organic solvents but care should be taken to exclude oxygen. Often alcohol is used. The water is decanted and the wet catalyst is washed repeatedly with alcohol. After several washes with absolute alcohol the last traces of water are removed. 

This reaction is the reverse of the initial ketyl radical formation by the benzophenone triplet and is therm Q4ynamically favorable. The experiments using optically active alcohols as source of hydrogen atoms show, however, that under normal conditions this reaction is unimportant. This is probably due to other, more efficient pathways for reaction of the ketyl radicals or perhaps to diffusion rates which separate the radicals before reverse transfer can occur. That this reaction can be important in some cases even without the presence of sulfur compounds was shown by studying the photoreduction of benzophenone in optically active ethers.<68) Although the reaction of benzophenone in methyl 2-octyl ether is only 0.17 times as fast as that in isopropanol, ethers can be used as sources of hydrogen atoms for photoreduction ... 

The reaction of H atoms can be studied in acidic solution if the OH radicals are scavenged by t-butyl alcohol, in a very fast reaction, while hydrogen atoms react only very slowly with this alcohol (equations 5 and 6). The radical produced in equation 5 is relatively unreactive and does not interfere with the study of the reaction of H atoms with the solute. 

The pressure dependence of the reaction between butane-1-thiol and hydrogen atoms at 133, 266, 532, 2660, and 5320 Pa, using two types of fast-flow discharge reactors, have been studied. Butane and but-l-ene were the main products. Pressure dependence indicated decomposition through vibrationally activated species. 

In many cases, rate constants for the fragmentation reactions are not available, but the reactions can be deduced to be quite fast because intermediate radicals are not trapped efficiently by reactive hydrogen atom donors. 

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