Hydrogenation reactivities, comparison

Fig. 3.2 (a) Evolution of the XRD patterns of 2Mg-Fe mixture reactively milled sequentially for various times under 1MP2 mode in 880 kPa of hydrogen. For comparison the XRD pattern of the mixture milled continuously for 270 h is also shown, (b) Morphology of 2Mg-Fe mixture reactively milled for 270 h in a continuous manner... 

In accordance with these early findings, a recent detailed study of the perfluorination of neopentane by Adcock [36] found the order of hydrogen reactivity to be CH3 > CH2F > CHF2 by a comparison of statistical and actual yields of the hydrofluorocarbon products obtained upon polyfluorination. Thus, the hydrogen abstraction step 2 a (Table 1) becomes less favourable as the C-H bond becomes increasingly electron poor and, consequently, less reactive towards highly electrophilic fluorine radicals. 

Peroxynitrite is a nonspecific oxidant that reacts with all classes of biomolecules depleting low-molecular-weight antioxidants, initiating lipid peroxidation, damaging nucleic acids and proteins. Its reactions are much slower than those of the hydroxyl radical but are faster than those of hydrogen peroxide. Comparison of peroxynitrite reactivity with various amino acid residues of human serum albumin have shown that cysteine, methionine, and tryptophan are the most reactive... 

Catalytic activities of (111) and (110) oriented PdgNi92 and PdsoCuso alloys for the 1,3-butadiene hydrogenation, in comparison with pure Pd (at RT, 10 torrs hydrogen, Ph2 / puc = 10). Ni is about ten times less reactive than Pd pure Cu presents no measurable activity for this reaction. 

Figure 1 presents a comparison of the measured and calculated core length hydrogen reactivity worths (nor-mialized to PERT-ID calculation near the center of the Cpre) as a function of relative core radius. Table I summarizes the calculated and measured reactivity worths of carbon, uranium, and polyethylene. The agree-meht between the calculations and the experiment is good. 

Ketones oxidize about as readily as the parent hydrocarbons or even a bit faster (32). Although the reactivities of hydrogens on carbons adjacent to carbonyl groups are perhaps doubled, the effect is small because one methylene group is missing in comparison to the parent hydrocarbon. Ketones oxidize less readily than similar primary or secondary alcohols (35). 

Another experiment of the competition type involves the comparison of the reactivity of different atoms in the same molecule. For example, gas-phase chlorination of butane can lead to 1- or 2-chlorobutane. The relative reactivity k /k of the primary and secondaiy hydrogens is the sort of information that helps to characterize the details of the reaction process. 

Any discussion on the data reported in Table XIV suffers from two relevant shortcomings, the lack of a comparison of the substituent effects with respect to the effect of the hydrogen atom and the lack of information on substituents of widely different types. For example, it would be of interest to know the exact position of the hydrogen atom in Goi s reactivity sequences... 

If, for the purpose of comparison of substrate reactivities, we use the method of competitive reactions we are faced with the problem of whether the reactivities in a certain series of reactants (i.e. selectivities) should be characterized by the ratio of their rates measured separately [relations (12) and (13)], or whether they should be expressed by the rates measured during simultaneous transformation of two compounds which thus compete in adsorption for the free surface of the catalyst [relations (14) and (15)]. How these two definitions of reactivity may differ from one another will be shown later by the example of competitive hydrogenation of alkylphenols (Section IV.E, p. 42). This may also be demonstrated by the classical example of hydrogenation of aromatic hydrocarbons on Raney nickel (48). In this case, the constants obtained by separate measurements of reaction rates for individual compounds lead to the reactivity order which is different from the order found on the basis of factor S, determined by the method of competitive reactions (Table II). Other examples of the change of reactivity, which may even result in the selective reaction of a strongly adsorbed reactant in competitive reactions (49, 50) have already been discussed (see p. 12). 

In the discussion of electrophilic aromatic substitution (Chapter 11) equal attention was paid to the effect of substrate structure on reactivity (activation or deactivation) and on orientation. The question of orientation was important because in a typical substitution there are four or five hydrogens that could serve as leaving groups. This type of question is much less important for aromatic nucleophilic substitution, since in most cases there is only one potential leaving group in a molecule. Therefore attention is largely focused on the reactivity of one molecule compared with another and not on the comparison of the reactivity of different positions within the same molecule. 

Wherever possible, we have sought a direct comparison of the reactivities of structurally related Crni and q-II alkyls with ethylene. For example, after having established the catalytic activity of complexes of the type [( Cri (L)2R] (see above), we showed that the isostructural neutral compounds Cp Crn(L)2R did not polymerize ethylene instead facile P-hydrogen elimination was observed. [3) This difference in reactivity was not due to the charge of the complexes. Thus, we have subsequently shown that neutral Cr J alkyls are also active polymerization catalysts. For example, Cp Cr I(THF)Bz2 and even anionic Li[Cp Cr H(Bz)3] (Bz = benzyl) polymerized ethylene at ambient temperature and pressure, while the structurally related CpCrD(bipy)Bz proved inert.[5]... 

At this point let us briefly consider the relationship between the carbonyl triplet state and another system capable of hydrogen atom abstraction alkoxy radicals. A comparison of the differences and/or similarities between the reactivity of the carbonyl triplet and that of an alkoxy radical should indicate whether the triplet state behaves as a normal ground state radical or if electronic excitation imparts unique properties leading to reactions not characteristic of ground state radicals. 

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