Sulfides, Thiols, and Disulfides

Since the electronegativity of sulfur is considerably less than that of oxygen, sulfur causes a correspondingly smaller chemical shift. Examples of thiols, sulfides, and disulfides are given in Table 4.18. 

Carbon-13 NMR spectrometry permits direct observation of carbon-containing functional groups the shift ranges for these are given in Appendix C. With the exception of CH=0, the presence of these groups could not be directly ascertained by H NMR. 

TABLE 4.17 Chemical Shift of Acyclic and Alicyclic Amines (in CDCl3, ppm from TMS) 

TABLE 4.18 Shift Position of Thiols, Sulfides, and Disulfides (ppm from TMS) 

1 Ketones and Aldehydes The R2C=0 and the RCH=0 carbon atoms absorb in a characteristic region. Acetone absorbs at 203.3 ppm, and acetaldehyde at 199.3 ppm. Alkyl substitution on the a-carbon causes a shift to the left of the C=0 absorption of 2-3 ppm until steric effects supervene. Replacement of the CH3 of acetone or acetaldehyde by a phenyl group causes a shift to the right of the C=0 absorption (acetophenone, 195.7 ppm benzaldehyde. 190.7 ppm) similarly, a,/3-unsaturation causes shifts to the right (acrolein, 192.1 ppm, compared with propionaldehyde, 201.5 ppm). Presumably, charge delocalization by the benzene ring or the double bond makes the carbonyl carbon less electron deficient. 

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Irradiation of aliphatic thiols, sulfides, and disulfides with a mercury lamp produces gaseous products identified by the mass spectrograph. Thiols are the least stable to light, with the formation of hydrogen95-129 as the main product. Sulfides and disulfides yield, as the predominant products, saturated hydrocarbons of structures corresponding to the smallest alkyl radical attached to sulfur. Haines et al.95-97 have offered a mechanism to explain the predominant production of hydrogen during photolysis of thiols. 

White et al. (65) positively identified a variety of thiophenes in a Bevier seam coal extract. Using low-voltage high-resolution mass spectrometry they were able to observe low concentrations of compounds in the extract that had molecular formulae consistent with those of some thiols, sulfides and disulfides. It should be emphasized however, that no thiol, sulfide or disulfide was positively identified in the extract. 

The types of organosulfur compounds found in petroleum feedstocks are shown in Table Of them, the alkyl and aryl thiols (RSH), sulfides (RSR ), and disulfides (RSSR ) are the most rapidly desulfurized (equation 1). It is the broad class of thiophenes, stabilized by their aromatic character, that are most difficult to desulfurize and require relatively severe temperatures ( 400 °C) and H2 pressures ( 100 atm). Thus, it is the HDS of thiophenes that has been of greatest interest to inorganic and organometallic chemists. There are fewer model studies of the HDS of thiols, sulfides, and disulfides. Organometallic aspects of HDS and HDN have been summarized most comprehensively in a recent book by Sanchez-Delgado. A briefer overview has also been published. More specific reviews are cited in later sections of this chapter. 

Upon hydrotreating under standard conditions, a good proportion of these species can be removed with relative ease, such as the thiols, sulfides, and disulfides, so they do not represent a major problem with today s technology, but others are more refractory, like the thiophenes, benzothiophenes, and specially the dibenzothiophenes, as a result of their aromatic character. 

Sulfur is assimilated and used by several organisms in different ways. As a characteristic part of enzymes and structure proteins, it plays an important part in, for example, biological redox systems, blood coagulation, and the natural detoxihcation of many organisms and is essential for their development. On the other hand, many organic sulfur compounds such as thiols, sulfides, and disulfides are a risk to human health and the environment. 

Tenax (2,6-diphenyl- -phenylenoxid) has been used by several authors. " " " Tenax is thermally very stable (320°C), but has a relatively low enrichment capacity. " It is recommended for the analysis of compounds with more than six C atoms. It has also been proved suitable for compounds of lower molecular weights. Tangerman " enriched H2S, COS, CS2, thiols, sulfides, and disulfides from air at — 196°C on Tenax. After gas chromatographic separation, he detected the compounds by flame photometric detector (FPD) in the lower ng/1 range. 

The monolayers are not perfectly ordered intermolecular voids are present and, therefore, no efficient inhibition of electron transfer across such layers was seen, as reported by Kaifer et al. [36]. There have been several approaches to the provision of anchoring groups of thiol, sulfide, and disulfide functionalities with which CyDs would be deposited on the gold surface as a better ordered monolayer. In the case of j5-CyD, full substitution of the seven primary hydroxyl groups by thiol- or sulfide-containing groups results in submonolayers with relatively large defect sites [36]. 

Hydrodesulfurization [HDS, Eq. (1)] is the process by which sulfur is removed from fossil materials upon treatment with a high pressure of H2 (3.5-17 MPa) at high temperature (300-425 °C) in the presence of heterogenexius catalysts, generally transition metal sulfides (Mo, W, Co, Ni) supported on alumina [1]. About 90% of the sulfur in fossil materials is contained in thiophenic molecules, which comprise an enormous variety of substituted thiophenes, and benzo[b]thiophenes, di-benzo[b,d]thiophenes as well as other fused-ring thiophenes, most of which are generally less easily desulfurized over heterogeneous catalysts than any other sulfur compound in petroleum feedstocks (e.g., thiols, sulfide, and disulfides). 

Bentenitis, N., N. R. Cox, and P. E. Smith. 2009. A Kirkwood-Buff derived force field for thiols, sulfides, and disulfides. Journal of Physical Chemistry B. 113, 12306. 

The sulfur compounds are very similar in their characteristics to hydrocarbons, for instance vapor pressures, and this is why they cannot be isolated by distillation process. Sulfur containing compovmds present varying reactivity towards HDS. Reactivity is dependent upon the local environment of the sulfur atom in the molecule, and the overall shape of the molecule. HDS is highly efficient at removing thiols, sulfides, and disulfides, but is less effective for aromatic thiophenes and thiophene derivatives, especially refractory compovmds, those containing functional groups that hinder the sulfur atoms. Sterically hindered DBTs that are substituted in the 4,6-positions such as 4,6-dimethyldibenzothiophene represent a major challenge for researchers in HDS (147). 

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