Mercaptans, formation

Mercaptan Formation from Hydrogen Sulfide and an Olefin... 

When S( D) atoms react with C2H6, CgHg, iso-CgHio, cyclopropane, cyclobutane, or cyclopentane, only one type of product—the corresponding mercaptan—is detected in photolysis experiments with the mercury arc. Specific search in careful studies has failed to reveal the presence of any additional products in these systems. This fact can be taken as compelling evidence that in the mechanism of mercaptan formation, no free-radical species are involved. The only exception to this pattern is... 

Representative kinetic plots for CO and mercaptan formation, as a function of alkane pressure, are shown in Figures 5 and 6 for ethane and isobutane. All paraffins examined thus far, with the exception of CH4, gave nearly identical plots. From this pressure trend it can be seen that not all of the S( 0) atoms are actually taken up by the paraffin mole-... 

Step (17) is unimportant compared to (3) and can therefore be omitted from the scheme. Steady-state treatment yields the following expression for the rate of mercaptan formation ... 

Rates of CO and Mercaptan Formation in the 2288 A. Phololysis of Carbonyl Sulfide with Added Ethane and Propane as a Function of Reaction Temperature ... 

Fig. 13. Rates of diboryl mercaptan formation as a function of diborane pressure in the COS-B2H6 system. (3) Runs with 900 torr added CO2 ( ) run with 900 torr added CsF. ...

Some experiments have been carried out with ethylene-ethane mixtures as well. The data given in Table VII show the expected trend, in that with increasing ethane pressure, at a fixed ethylene concentration, the ethyl mercaptan yield increases at the expense of the yields of vinyl mercaptan and episulfide. However, while vinyl mercaptan formation seems to be completely suppressible with increasing ethane pressure, this is not the case with the episulfide. This seems clearly to be due to the fact that the two mercaptans form in competing reactions for S( Z)) atoms, while episulfide can form from ( P) atoms, arising by the collisionally induced S( Z>) - S( P) transition. 

The deuterium isotope effect on the rate of vinylic mercaptan formation was examined with ethylene, using the mercury arc and Cd—R sources. A decrease in the order of 8% appeared in the vm/Res ratio, which hardly exceeds the error of the experiments. 

In the overall reaction leading to either episulfide or mercaptan formation [see reactions (28b) and (28c)], the exothermicity is ca. 85 kcal./ mole as mentioned above. Thus, the intermediate biradicals have some 26 kcal./mole of excitation energy which may be suflScient to bring about... 

The rate at which S( Z)) atoms react with ethylene, ethane, and COS are all of the same order of magnitude. Thus some approximate preliminary relative rate constant values are ethyl mercaptan formation 1.0 vinyl mercaptan formation 0.80 abstraction from COS, 2.0 deactivation by CO2, 0.4 and deactivation by COS, 0.06. In addition, preliminary data seem to indicate that the reactivity of sulfur atoms, formed in the photolysis of COS, increases with increasing alkyl substitution on the doubly bonded carbon atoms. However, these rate studies have proven to be more complex than anticipated in that there is an apparent pressure effect on the rate constant values. 

The deuterium isotope effect was examined in the hope that it might provide some clue as to the mechanism of vinylic mercaptan formation. The lack of any significant isotope effect precludes any choice between insertion and isomerization since either mechanism would be consistent with the finding. 

Mitsuhashi, S., 1949. Decomposition of thioether derivatives by bacteria. I. Methyl-mercaptan formation and the properties of the responsible enzyme. Jpn. J. Exp. Med., 20 211-222. 

Pure component studies indicate the rate of mercaptan formation is sufficiently rapid at hydrotreating conditions compared to the saturation step which lead to alkane [8]. The exothermic reversible reaction, which shifts to the left at higher hydrogen sulfide partial pressure, is also dependent on temperature, feedstock type, total sulfiir, partial pressure of hydrogen and alkenes, space velocity and catalyst type. Furthermore the size of the reactor affect the balance between the kinetic sulfur removal and alkene saturation [9]. 

The HDS performance data of the two catalysts are presented in the Table 4. The total product sulfur is shown as mercaptan and other sulfur types for catalyst A and B with SRN and blend naphtha. Figure 1 shows that the blend naphtha is easier to desulfurize and catalyst B is more active. The total sulfur for catalyst A decreased from 72 ppm at 220°C to a minimum of 0.69 ppm at 300°C. However, upon further increase in the temperature up to 350°C, the total sulfur increased as a result of mercaptan formation. Figure 2 shows the total and mercaptan sulfur distribution as a function of temperature for catalyst/4 with blend naphtha. 

The GC/FPD results indicate that the product obtained with catalyst B at 250°C contained twelve sulfur compounds three mercaptans, two sulfides, and seven thiophenes. At temperatures above 280°C, thiophenes were removed almost completely, but mercaptans were still present suggesting the occurrence of recombination reactions. Therefore the hydrotreater has to be operated at the lowest temperature possible to minimize the alkene production leading to mercaptans formation [9]. 

A theoretical study of the interaction of sulfur atoms with ethylene within the framework of the Extended Hiickel MO theory has been reported by HoflFmann and co-workers (19). Potential surface calculations revealed two minima for the 8( 02) + C2H4 system. The higher corresponds to vinyl mercaptan formation via C-H bond insertion, and the lower, lying about 20 kcal below the former, to the least-motion, symmetry-allowed addition of sulfur across the double bond. The two are viewed as competing concerted processes. Similar calculations for the... 

It has been known for more than 50 years that furfiiryl mercaptan is one of the very few critical coffee aroma compounds. Aqueous solutions of ribose and cysteine were reacted in a high temperature/short time reactor and the aromatics generated were separated and identified by GC/MS. This study investigates the effect of pH, time and temperature on fiirfuryl mercaptan formation. At higher temperatures furfuryl mercaptan is the major compound generated. The kinetics describing its formation are covered. 

The effect of pH upon furfiiryl mercaptan formation was investigated by Mottram and Leseigneur (12). They reported the level of Fur-SH increased as the pH was decreased in the cysteine/ribose model system. 

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