Thiols and Sulfides

Thiols, sometimes called mercaptcms, are sulfur analogs of alcohols. They are named by the same system used for alcohols, with the suffix -thiol used in place of -ol. The -SH group itself is referred to as a rnercapto group. 

The most striking characteristic of thiols is their appalling odor. Skunk scent, for instance, is caused primarily by the simple thiols 3-methyl-1-butanethiol and 2-butene-l-thiol. Volatile thiols such as ethanethiol are also added to natural gas and liquefied propane to serve as an easily detectable warning in case of leaks. 

Thiols are usually prepared from alkyl halides by Snj2 displacement with a sulfur nucleophile such as hydrosulfide anion, SH. 

The reaction often works poorly unless an excess of the nucleophile is used because the product thiol can undergo a second S 2 reaction with alkyl halide to give a sulfide as a by-product. To circumvent this problem, thiourea, (NH2J2C=S, is often used as the nucleophile in the preparation of a thiol from an alkyl halide. The reaction occurs by displacement of the halide ion to yield an intermediate alkyl isothiourea salt, which is hydrolyzed by subsequent reaction with aqueous base. 

Thiols can be oxidized by Br2 or I2 to yield disulfides (RSSR )- The reaction is easily reversed, and a disulfide can be reduced back to a thiol by treatment with zinc and acid. 

Sulfides are named by following the same rules used for ethers, wit i sulfide used in place of ether for simple compounds and alkylthio used in place of alkoxy for more complex substances. 

Dimethyl sulfide Methyl phenyl sulfide 3-(MethylthiA)cyclohexene 

Sulfur analogs of alcohols and ethers are thiols or mercaptanes and sulfides, respectively. According to the nomenclature rules, thiols are named following the same rules as for alcohols, the only difference being that the suffix -ol is replaced with -thiol. Most of these substances can be recognized by their unpleasant odor. Ethanethiol is used as an additive to natural gas since its odor serves as an indicator of a gas leak in home installations. Pure natural gas is odorless. Some animals such as skunks release an unpleasant odor which originates from thiols shown in the next scheme. However, some thiols have a pleasant odor, for instance the components of the aroma of coffee. 

Sulfur is directly below oxygen in the periodic table (in the same column), and therefore, many oxygen-containing compounds have sulfur analogs. Sulfur analogs of alcohols contain an SH group in place of an OH group and are called thiols. The nomenclature of thiols is similar to that of alcohols, but the suffix of the name is thiol instead of ol  

Notice that the e is kept before the suffix thiol. When another functional group is present in the compound, the SH group is named as a substituent and is called a mercapto group  

The name mercapto is derived from the fact that thiols were once called mercaptans. This terminology was abandoned by lUPAC several decades ago, but old habits die hard, and many chemists still refer to thiols as mercaptans. The term is derived from the Ladn mercurium captans (capturing mercury) and describes the abiUty of thiols to form complexes with mercury as well as other metals. This abihty is put to good use by the drug called dimercaprol, which is used to treat mercury and lead poisoning. 

Thiols are most notorious for their pungent, unpleasant odors. Skunks use thiols as a defense mechanism to ward off predators by spraying a mixture that delivers a mighty stench. Methanethiol is added to natural gas so that gas leaks can be easily detected. If you have ever smelled a gas leak, you were smelling the methanethiol (CH3SH) in the natural gas, as natural gas is odorless. Surprisingly, scientists who have worked with thiols report that the nasty odor actually becomes pleasant after prolonged exposure. The author of this textbook can attest to this fact. 

Thiols can be prepared via an Sn2 reaction between sodium hydrosulfide (NaSH) and a suitable alkyl halide for example  

The importance of crown ethers derives from their ability to sequester specific metal cations in the center of the polyether cavity. 18-Crown-6, for example, binds strongly with potassium ion. As a result, a solution of 18-crown-6 in a nonpolar organic solvent will dissolve many potassium salts. Potassiirm permanganate, KMn04, dissolves in toluene in the presence of 18-crown-6, for instance, and the resulting solution is a valuable reagent for oxidizing alkenes. 

The effect of using a crown ether to dissolve an inorganic salt in a hydrocarbon or ether solvent is similar to the effect of dissolving the salt in a polar aprotic solvent such as DMSO, DMF, or HMPA (Section 11.3). In both cases, the metal cation is strongly solvated, leaving the anion bare. Thus, the Sn2 reactivity of an anion is tremendously enhanced in the presence of a crown ether. 

15-Crown-5 and 12-crown-4 ethers complex Na and Li , respectively. Make nrwdelsof these crown ethers, and compare the sizes of the cavities. 

C-Sst 710-570 Weak, broad, of no practical significance. In Raman, strong 

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The effect of pH and the piC of the thiol has been discussed. This reaction is not of great synthetic interest, primarily because it yields a mixture of products, but it is of commercial consequence. It is also appHcable ia polysulfide synthesis, where the presence of small amounts of thiols can cause significant problems for the stabiUty of the polysulfide (51). A similar reaction between thiols and sulfides has also been described (52). In this instance, the process is heterogenous and acid-cataly2ed. 

This chapter finishes the coverage of functional groups with C-O and C—S single bonds that was begun in Chapter 17. We ll focus primarily on ethers and take only a brief look at thiols and sulfides before going on to an extensive coverage of compounds with C=0 bonds in Chapters 19 through 23. 

Thiols and sulfides are occasionally prepared by treatment of Grignard reagents with sulfur.Analogous reactions are known for selenium and tellurium compounds. 

The described results indicate that there is not evident influence of a kind of nickel matrix on the dehydrosulfurization of thiols and sulfides. In order to obtain active catalysts for these reactions, the Ni-cation exchange procedure insted of the impregnation should be applied. 

Be(Pc) and Mg(Pc) are obtained by the condensation reaction of phthalonitrile in the presence of metallic Be and Mg respectively.197,198 They form very stable complexes with water. Mg(Pc)(OH2) is square pyramidal and the magnesium atom is displaced out of the N4 plane by 0.5 A with an Mg—distance of 2.04 A. They lose the water on sublimation. Amines, alcohols, ketones, thiols and sulfides also coordinate to Mg(Pc). Ca(Pc) and Ba(Pc) are obtained from phthalonitrile and the metal oxide.197,199... 

Sulfur as Heteroatom. Thiols and sulfides are protonated on sulfur in superacid media and give mono- and dialkylsulfonium ions, respectively.136 Thio-carboxylic acids, 5-alkyl esters, thioesters, dithioesters, and thiocarbonates in similar media also form stable protonated ions541,647 such as cations 348-353. 

Alternatively, if reduction of sulfate in associated waters occurred, the active sulfur (H2S) would have preferentially reacted with the saturates producing thiols and sulfides. This mechanism, too, should result in an increased ratio of aliphatic sulfur compounds over aromatic in the biodegraded oil. 

Common low-molecular-mass thiols and sulfides. All are liquids at room temperature, except for methanethiol, which boils at 5.9°C. 

Substitution of alkyl and aryl groups for H on H2S yields thiols and sulfides (thioethers). Structural formulas of examples of these compounds are shown in Figure 17.2. 

Homogenous desulfurization of thiols and sulfides can be achieved by stoichiometric use of Yb(btsa)2(THF)2 as expressed in Eq. (27) [291]. The desulfurization reaction is assumed to proceed via one electron reduction of the substrate and its extent to be dependent on the stability of the radical intermediates. 

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