Hydrogenation reactions possible mechanism

Reactions of th Halogens with Hydrogen. Two possible mechanisms suggest themselves ... 

Substitution Halogenation. One might expect mechanisms analogous to those for the reactions with hydrogen. Three possible mechanisms, using methane as an example, would be I. 

The differences in rate for the two positions of naphthalene show clearly that an additional-elimination mechanism may be ruled out. On the other hand, the magnitude of the above isotope effect is smaller than would be expected for a reaction involving rate-determining abstraction of hydrogen, so a mechanism involving significant internal return had been proposed, equilibria (239) and (240), p. 266. In this base-catalysed (B-SE2) reaction both k and k 2 must be fast in view of the reaction path symmetry. If diffusion away of the labelled solvent molecule BH is not rapid compared with the return reaction kLt a considerable fraction of ArLi reacts with BH rather than BH, the former possibility leading to no nett isotope effect. Since the diffusion process is unlikely to have an isotope effect then the overall observed effect will be less than that for the step k. ... 

In the past it had been a popular belief that the electrochemical reduction of any inorganic or organic substance involves the primary electrochemical formation of a special, active form of hydrogen in the nascent state (in statu nascendi) and subsequent chemical reaction of this hydrogen with the substrate. However, for many reduction reactions a mechanism of direct electron transfer from the electrode to the substrate could be demonstrated. It is only in individual cases involving electrodes with superior hydrogen adsorption that the mechanism above with an intermediate formation of adsorbed atomic hydrogen is possible. 

A milder procedure involves the reaction of a nitrile with an alkaline solution of hydrogen peroxide.147 The strongly nucleophilic hydrogen peroxide adds to the nitrile and the resulting adduct gives the amide. There are several possible mechanisms for the subsequent decomposition of the peroxycarboximidic adduct.148... 

Fig. 3.4 Schematic drawings illustrating the nucleation of silica precursor and possible mechanisms of reactions leading to silica formation. (A) The precursor hydrolysis. THEOS nucleates on a polysaccharide macromolecule through hydrogen bonds that are formed with hydroxy groups in the biomacromolecule. Per-...

Classically, processes involving surface intermediates were investigated primarily by methods (2) (4) above and in particular by measuring current as a function of concentration of reagents and electrode potential. A familiar example is the hydrogen evolution reaction, which may proceed by one of two possible mechanisms, both of which share a common first step ... 

The same authors studied the CL of 4,4,-[oxalylbis(trifluoromethylsulfo-nyl)imino]to[4-methylmorphilinium trifluoromethane sulfonate] (METQ) with hydrogen peroxide and a fluorophor in the presence of a, p, y, and heptakis 2,6-di-O-methyl P-cyclodextrin [66], The fluorophors studied were rhodamine B (RH B), 8-aniline-l-naphthalene sulfonic acid (ANS), potassium 2-p-toluidinylnaph-thalene-6-sulfonate (TNS), and fluorescein. It was found that TNS, ANS, and fluorescein show CL intensity enhancement in all cyclodextrins, while the CL of rhodamine B is enhanced in a- and y-cyclodextrin and reduced in P-cyclodextrin medium. The enhancement factors were found in the range of 1.4 for rhodamine B in a-cyclodextrin and 300 for TNS in heptakis 2,6-di-O-methyl P-cyclodextrin. The authors conclude that this enhancement could be attributed to increases in reaction rate, excitation efficiency, and fluorescence efficiency of the emitting species. Inclusion of a reaction intermediate and fluorophore in the cyclodextrin cavity is proposed as one possible mechanism for the observed enhancement. 

With the recent development of zeolite catalysts that can efficiently transform methanol into synfuels, homogeneous catalysis of reaction (2) has suddenly grown in importance. Unfortunately, aside from the reports of Bradley (6), Bathke and Feder (]), and the work of Pruett (8) at Union Carbide (largely unpublished), very little is known about the homogeneous catalytic hydrogenation of CO to methanol. Two possible mechanisms for methanol formation are suggested by literature discussions of Fischer-Tropsch catalysis (9-10). These are shown in Schemes 1 and 2. 

The more precise formulation of the transition complex of n complex substitution reactions makes it possible to write a reaction scheme [Eq. (17)] showing the possible interconnection of a number of hitherto unrelated hydrogenation and exchange mechanisms. 

A possible mechanism for the P-alkylation of secondary alcohols with primary alcohols catalyzed by a 1/base system is illustrated in Scheme 5.28. The first step of the reaction involves oxidation of the primary and secondary alcohols to aldehydes and ketones, accompanied by the transitory generation of a hydrido iridium species. A base-mediated cross-aldol condensation then occurs to give an a,P-unsaturated ketone. Finally, successive transfer hydrogenation of the C=C and C=0 double bonds of the a,P-unsaturated ketone by the hydrido iridium species occurs to give the product. 

A possible mechanism for the alkylation of arylacetonitriles is shown in Scheme 5.30. The reaction would proceed via successive hydrogen transfer and Knoevena-gel condensation as follows ... 

The nitrophenyl radical can react with the iodide ion and solvent, methanol, as well. Transference of hydrogen radical from methyl alcohol to nitrophenyl radical gives rise to nitrobenzene and formaldehyde (CHjOH —> CH2O). Though carefully sought among the products of the reaction, 3-iodonitro-benzene and 4-nitroanisole were lacking. This completely rejects another possible mechanism of the reaction, cine-substitution, which involves the formation of dehydrobenzene as described earlier. 

Thus, there are four possible mechanisms for the hydrogen evolution in an acid solution (1) CT is RDS, CD fast (2) CD is RDS, CT is fast (3) CT is RDS, ED is fast and (4) ED is RDS, CT is fast. Different paths and different mechanisms have different Tafel slopes. Readers are referred to Refs. 11, 15, 21-23, and 26 for determination of the reaction mechanisms. 

It was found that only deuterated furanone was obtained if water had been replaced with deuterated water (D2O). This observation corroborated the fact that water was the hydrogen source in this reaction. Two possible mechanisms were proposed. The first one is described as follows (Scheme 18, path a) acetylene 119 can react with bimetallic species to afford /r, -acetylene complex 122, which reacts further with CO to give /tjTj -furanone... 

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