organosulfur compound

organosulfur compound, a subclass of organic substances that contain sulfur and that are known for their varied occurrence and unusual properties. They are found in diverse locations, including in interstellar space, inside hot acidic volcanoes, and deep within the oceans. Organosulfur compounds occur in the bodies of all living creatures in the form of certain essential amino acids (such as cysteine, cystine, and methionine, which are components of proteins), of the tripeptide glutathione, and of enzymes, coenzymes, vitamins, and hormones.

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Typical organisms contain 2 percent sulfur dry weight. Coenzyme A (CoA), biotin, thiamin chloride (vitamin B₁), α-lipoic acid, insulin, oxytocin, sulfated polysaccharides, and the nitrogen-fixing nitrogenase enzymes are but a few examples of important natural sulfur-containing compounds. Certain simple organosulfur compounds, such as thiols, are repugnant to humans and most higher animals even at extraordinarily low concentrations; they are used as defensive secretions by a variety of animal species and figure in unpleasant odors associated with polluted air and water, particularly that resulting from the use of sulfur-rich fossil fuels. However, related types of organosulfur compounds found in such foods as garlic, onion, chive, leek, broccoli, cabbage, radish, asparagus, portobello mushroom, mustard, truffle, coffee, and pineapple are sources of olfactory and gustatory delight.

Mustard gas, or bis(β-chloroethyl) sulfide, (ClCH₂CH₂)₂S, is a potent chemical warfare agent, whereas other sulfur compounds such as sulfanilamide (a sulfa drug), penicillin, and cephalosporin are valued antibiotics. Synthetic organosulfur compounds include polysulfones, inert polymers used in astronauts’ transparent face shields; polythiophenes, materials possessing the metal-like ability to conduct electricity; agricultural chemicals, insecticides, and organic solvents, such as dimethyl sulfoxide, CH₃S(=O)CH₃, and carbon disulfide, CS₂; dyes; lubricating oil constituents; food additives; and substances used to make rayon. In chemical research, organosulfur compounds are valued reagents widely used for synthesizing new compounds. A global sulfur cycle exists that interconverts natural organosulfur compounds with either inorganic sulfide or sulfate ions. Sulfide or sulfate ions can also be formed in nature from elemental sulfur.

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The sulfur atom

Differences between the chemistry of sulfur compounds and that of other common heteroatomic organic compounds (i.e., organic compounds containing elements other than carbon [C] and hydrogen [H], such as those of oxygen [O] and nitrogen [N]), are primarily due to the fact that sulfur is a member of the third period of elements, employing 3s, 3p, and sometimes 3d orbitals, which are significantly larger than the more compact 2s and 2p orbitals of second-period elements such as oxygen and nitrogen. The larger orbital size means that the outer valence electrons are more loosely held, being further removed from the influence of the positive nuclear charge. Such loosely held electrons are said to be more polarizable, allowing them to engage in bonding interactions with electrophilic partners more easily and earlier during the course of a reaction than in the case of lighter elements, where bonding interactions require close approach of partner atoms. In addition, in protic hydrogen-bonding solvents such as water and alcohols, sulfur is more weakly solvated than lighter heteroatoms.

In these solvents, heavier heteroatoms such as sulfur show enhanced nucleophilicity compared with lighter heteroatoms because of their higher polarizability combined with decreased solvation (the solvation shell must be disrupted in reaching the transition state), despite the fact that stronger bonds are formed by the lighter heteroatoms. Thus, divalent sulfur compounds, such as thiols (containing an ―SH group) and sulfides (containing an ―S― group), readily bind to heavy metal ions such as silver (Ag), mercury (Hg), lead (Pb), and cadmium (Cd). Indeed, another name for thiol is mercaptan (from Latin mercurium captans, meaning “seizing mercury”), reflecting the use of thiols in treating mercury poisoning. Interactions between divalent sulfur and the metal ions iron (Fe), molybdenum (Mo), zinc (Zn), and copper (Cu) are crucial in metalloenzymes—for example, cytochrome C, in which the sulfur of methionine is coordinated to the iron in heme; the iron-sulfur proteins, in which cysteine sulfur is bound to iron; and molybdenum-containing enzymes, some of which involve dithiolate (two-sulfur) cofactors.

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It is useful to compare features of the compounds of sulfur (electron distribution 1s²2s²2p⁶3s²3p⁴) with those of oxygen, which lies directly above sulfur in the periodic table (electron distribution 1s²2s²2p⁴), and with those of the heavier member of the chalcogen family, selenium (electron distribution 1s²2s²2p⁶3s²3p⁶4s²3d¹⁰4p⁴), which lies directly below sulfur. There are structural similarities, for example, between alcohols (R―OH), thiols (R―SH), and selenols (R―SeH), between hydroperoxides (R―OOH), sulfenic acids (R―SOH), and selenenic acids (R―SeOH), between ethers (R―O―R), sulfides (R―S―R), and selenides (R―Se―R), between ketones (R―C(=O)―R), thioketones (R―C(=S)―R), and selenoketones (R―C(=Se)―R), between peroxides (R―OO―R), disulfides (R―SS―R), and diselenides (R―SeSe―R), and between oxonium (R₃O⁺), sulfonium (R₃S⁺), and selenonium salts (R₃Se⁺), where R represents a general carbon group—e.g., the methyl group, CH₃, or the ethyl group, C₂H₅.

There are significant differences in the properties of these groups of related compounds. For example, thiols are somewhat stronger acids than the corresponding alcohols because the S―H bond is weaker than the O―H bond and because the larger sulfur atom better disperses the resulting negative charge as compared with oxygen. For the same reasons, selenols are even stronger acids than thiols. At the same time, SH hydrogen bonding is much weaker than OH hydrogen bonding, with the consequence that thiols are more volatile and have lower boiling points than the corresponding alcohols—for example, 6 °C (43 °F) for methanethiol compared with 66 °C (151 °F) for methanol. Compared with alcohols and ethers, low-molecular-weight thiols and selenols as well as sulfides and selenides have highly unpleasant and obnoxious odors, although the perception of the odor as unpleasant or pleasant can sometimes vary with the concentration of the particular compound. Disulfides and diselenides are far more stable than peroxides, and sulfonium and selenonium salts are much less reactive than oxonium salts; at the same time, simple thiocarbonyl (C=S) and selenocarbonyl (C=Se) compounds are much more reactive than simple carbonyl (C=O) compounds. In the case of the homologues of carbonyl compounds, the difference in reactivity is attributed to the poorer match in the size of the orbitals of the carbon and sulfur double bond (carbon 2p and sulfur 3p) or the carbon and selenium double bond (carbon 2p and selenium 4p) compared with the similar 2p orbitals used for the double bond between carbon and oxygen.

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Both sulfur and selenium also have the ability to form compounds in which atoms of these elements have higher valences; these compounds have no counterpart in oxygen chemistry. In the case of sulfur, some examples are sulfoxides (R₂S=O, typically written R₂SO), sulfones (R₂S(=O)₂, typically written R₂SO₂), sulfonic acids (RSO₃H), and oxosulfonium salts (R₃S⁺=O). Analogs of the above sulfur compounds also exist for selenium. These higher-valence compounds of sulfur (or selenium) are stabilized through bonding involving 3d (or 4d) orbitals, not available to oxygen, as well as other factors associated with the larger size of sulfur and selenium as compared with oxygen. The longer, weaker bonds and higher degree of polarizability of selenium compared with sulfur lead to differences in properties and reactions of compounds of these two elements.

Analysis of organosulfur compounds

In addition to routine methods of analysis that can be used with all classes of organic compounds (see analysis), certain procedures reflect specific characteristics of sulfur. In a mass spectrometer, organosulfur compounds frequently produce strong molecular ions in which the charge is located predominantly on sulfur. The presence of sulfur is indicated by the occurrence of sulfur-34 (³⁴S) isotope peaks, 4.4 percent of the abundance of ³²S. Organically bound sulfur in the form of the natural isotope ³³S can be directly examined by nuclear magnetic resonance (NMR) spectroscopy, although the low natural abundance (0.76 percent) and small magnetic and nuclear quadrupole moments make analysis more difficult than for protons (¹H) or carbon-13 (¹³C). Levels of organosulfur compounds in crude petroleum as low as 10 parts per billion or less may have a detrimental effect on metallic catalysis or may cause unpleasant odors. These very low sulfur levels are detected using gas chromatographs with sulfur chemiluminescence or atomic-emission detectors with high sensitivity to detect sulfur compounds in the presence of other compounds.

Organic compounds of bivalent sulfur