Aromatic rings hydrogenation rate

In the case of 4,6-DMDBT, it was possible to determine the rate constants for direct extraction of sulfur from the fully saturated sulfur-containing ring system (k0l) and for the secondary hydrogenation of the tetrahydro-dibenzothiophene intermediate (fcHs2)- As might be expected, the rate constants for direct sulfur extraction follow a clear trend in which A Dq < < d2 The reverse trend is observed in the aromatic ring hydrogenation rates, /cHs, > kHS, and kHPl > kw , which is consistent with the literature (see Fig. 10) (5, 35). 

There are different methods to cleave benzyl ether bonds. The most common one is hydrogenolysis with palladium on carbon or platinum as catalysts under H2 atmosphere. The standard solvents are ethanol or ethyl acetate. Pd is the preferred and milder one, because the use of Pt at any rate results in aromatic ring hydrogenation. Also a number of methods have been developed in which hydrogen is generated in situ, e. g. from cyclo-hexene, -hexadiene or formic acid (see Chapter 7). 

However, other results indicate that the P/Al catalyst has very low activity for C —N or C —C bond breaking 74, 81). A detailed kinetics study by Jian and Prins 81) showed that phosphorus addition decreases the C—N bond cleavage (rate constants k[ and k) in the reaction networks Figs. 33a and 33b) and the subsequent alkene hydrogenation (rate constant k() reactions in piperidine and DHQ HDN (see Table XIV). On the other hand, the presence of phosphorus increases aromatic ring hydrogenation of OPA... 

Reaction (61) would now be expected to be an equilibrium. The RO2 radicals involved in these types of reactions are likely to have a considerably enhanced stability as a result of the delocalized character of the aromatic rings. The rate of oxidation of p-xylene was found, by Barnard and Ibbison [216], to be very similar to that of toluene. The slow oxidation of all of the xylene isomers and of toluene were found to be autocatalytic [36, 215], and a mechanism through formaldehyde oxidation was proposed, since it was found to be one of the intermediate products. Hydrogen peroxide must also be formed from the predominantly HO2 propagation, and this is likely to be an important contributor to the autocatalysis. 

The hds of thiophen, benzothiophen, and dibenzothiophen and their methyl-substituted derivatives were compared in pulse experiments (623-723 K) over a sulphided CoO(5.6)-Mo03(11.2)/Al203 catalyst. Reactivities at 1 atm pressure were roughly the same but at hi er pressures, reactivity decreased with the number of rings. For benzothiophen, methyl substituents did not affect the reactivities, but for dibenzothiophen methyl substituents in the 4- or 4 - and 6- positions caused a decrease of desulphurization rate. Aromatic ring hydrogenation was not a prerequisite of C-S scission, which was the slow step. 

Addition of gold to the parent nickel catalyst slightly decreases the rate of the C=C bond hydrogenation while it drastically decreases the rate of the aromatic ring hydrogenation, leading consequently to a great improvement of selectivity. 

The results are consistent with the rate-determining step being addition of the aryl radical to the aromatic ring, Eq. (9). Support for this mechanism is derived from the results of three other studies (a) When A -nitrosoacetanilide is decomposed in pyridine, the benzene formed by abstraction of hydrogen from pyridine by phenyl radical accounts for only 1 part in 120 of the reaction leading to phenyl-pyridines. (b) 9,9, 10,lCK-Tetrahydro-10,10 -diphenyl-9,9 -bianthryl is formed in the reaction between phenyl radicals and anthracene, probably by the addition mechanism in Eq. (11). Adducts are also formed in the reactions of benzyl radicals with anthracene- and acridine. ... 

It is clear from the results that there is no kinetic isotope effect when deuterium is substituted for hydrogen in various positions in hydrazobenzene and 1,1 -hydrazonaphthalene. This means that the final removal of hydrogen ions from the aromatic rings (which is assisted either by the solvent or anionic base) in a positively charged intermediate or in a concerted process, is not rate-determining (cf. most electrophilic aromatic substitution reactions47). The product distribution... 

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