Aliphatic Organosulfur Compounds

Polarization of double bonds due to the electron releasing effect of thioalkyl groups in thioenol ethers is much weaker than that reported for enol ethers (Table 4.26), as can be verified for the (E) and (Z) isomers of methyl propenyl sulfide in Table 4.40 [326, 327]. 

Thioacetal and thioketal carbon shieldings (45-65 ppm) [328, 329] also reflect the heavy atom influence when compared with acetal and ketal carbon shifts (100 + 5 ppm). 

A linear relation (eq. (4.11)) permits prediction of thiocarbonyl shifts from the C = O shift values of the parent carbonyl compounds [330]. 

20 6 P H3C-CM71 3 30 2 P H3C-C 195 2 Thioacetic acid 392 p H3C-C2341 Dithioacetic acid 

In contrast to N,N-dimethylaminoacrolein, restricted rotation of the thioaldehyde group is observed in the 13C NMR spectrum of N,N-dimethylaminothioacrolein [336]. The C-2 carbon shift of the thioaldehyde (118.5 ppm) also indicates that electron release by the dimethylamino group is attenuated when compared with the aldehyde (101.3 ppm). 

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Table 4.40. 13C Chemical Shifts of Selected Aliphatic Organosulfur Compounds (i>c in ppm) [322-327]. 

The types of organosulfur compound present in petroleum feedstocks are alkyl and aryl thiols (RSH), thioethers (RSR ), disulfides (RSSR ), and thiophenic compounds (Fig. 1). The ease with which sulfur is abstracted depends very much on the nature of the sulfur-containing molecule aliphatic compounds (thiols, thioethers) are usually desulfurized much more easily than heteroaromatic (e.g. thiophenes, benzothiophenes, dibenzothiophenes). Among the latter, reactivity decreases in the order thiophene > benzothiophene > dibenzothiophene. The presence of aliphatic substituent groups can sometimes alter reactivity. The sterically hindered compound 4,6-dimethyldibenzothiophene is, for example, very difficult to desulfurize. 

Deep desulfurization of diesel fuels is particularly challenging due to the difficulty of reduce aromatic sulfur compounds, particularly 4,6-dialkyldibenzothiophenes, using conventional hydrodesulfurization processes (HDS). The HDS process is normally only effective for removing organosulfur compounds of aliphatic and alicyclic types. The aromatic sulfur molecules including thiophenes, dibenzothiophenes (DBT), and their alkylated derivatives are very difficult to convert to H2S through HDS. 

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