Hydrogen Sulfide1


[7783-06-4]  · H2S  · Hydrogen Sulfide  · (MW 34.09)

(nucleophile and precursor of Na2S, NaSH, K2S, and KSH for the synthesis of thiols,2 thioethers,3 thiocarbonyl compounds,4 and heterocyclic compounds; reducing agent, particularly for the conversion (via S2-, HS-, S22-) of aromatic nitro compounds to amines5)

Physical Data: colorless gas; bp -60.75 °C; mp -85.60 °C; d (0 °C) 1.539 g L-1 (more dense than air); dipole moment (C6H6) 0.85-0.97 D. pKa1 (aq) 6.88, pKa2 (aq) 14.15; E0(H2S -> S + 2H+ + 2e-) -0.14 V.

Solubility: sol H2O (0.12 mol L-1 at 20.1 °C); sol polar and nonpolar organic solvents.

Form Supplied in: liquefied gas in steel cylinders.

Handling, Storage, and Precautions: extremely toxic, malodorous gas; use of a well-ventilated hood is a necessity; a scrubber flask containing 20% aqueous NaOH prevents loss of H2S to the atmosphere. The sense of smell is paralyzed at concentrations of 150-250 ppm and death may ensue at concentrations above 300 ppm. An irritant to the eyes and mucous membranes; highly flammable; ignites spontaneously in air at around 250 °C; mixtures of H2S (4.5-45%) and air are explosive; corrosive (especially in aqueous solutions) to many metals; anhydrous H2S is unreactive to stainless steel at ambient temperatures.

Formation of Na2S, NaSH, K2S, and KSH.

Passage of H2S into excess aq NaOH or KOH yields Sodium Sulfide, and K2S, which can be used in situ.6 Sodium Hydrogen Sulfide and Potassium Hydrogen Sulfide are produced in ethanolic base saturated with H2S;7 addition of, for example, excess KOEt yields K2S.7a In the formation of KSH, H2S is passed into an ethanolic KOH solution until the mixture does not test alkaline with phenolphthalein.7c Solutions of hydrogen sulfide salts are in equilibrium with the sulfide and H2S: 2NaSH &ibond; Na2S + H2S.


Thiols or their salts are obtained by displacement reactions2 by SH- or S2- (derived from H2S) on primary or secondary halides or sulfonate esters, on oxiranes and aziridines, on quaternary ammonium salts, and on activated aromatic halides (eq 1).8 The displacement of chloride ion shown in eq 1 takes precedence over the addition reaction of H2S to the nitrile group (see below). Halonitrobenzenes may undergo both substitution of halogen and reduction of the nitro group.2a,6,9 High temperatures are required for unactivated aryl halides (eq 2).10 Elimination and thioether formation are side reactions with aliphatic substrates. To minimize thioether formation, an excess of H2S is employed (see the above equilibrium).

Benzyl thiols can be obtained directly from the alcohol by the action of H2S in the presence of Octacarbonyldicobalt (eq 3).11a Direct reaction between H2S and alcohols normally requires high temperatures, high pressures, and special catalysts; this method is not generally suited for a laboratory preparation.2b The reaction of a-aryl-a-amino nitriles with H2S under mild conditions gives high yields of thiols uncontaminated with a-amino thioamides (eq 4).11b

Thiolacids and their derivatives are obtained from the acid chloride or anhydride (eqs 5 and 6).7c,12 Potassium thiotosylate, useful in the synthesis of thiotosylates, is obtained in a similar way from tosyl chloride (eq 7).13 The solution must be saturated with H2S. gem-Dithiols are obtained by treatment of ketones with H2S and an organic base (or the preformed enamine may be used) (eq 8),14 and best yields are obtained with cyclic ketones. Use of the ketimine or enamine gives satisfactory yields (53-83%) of gem-dithiols of acyclic ketones.14b Addition of H2S to propionaldehyde in the presence of Chlorotrimethylsilane results in the silyloxythiol (eq 9).15 A Michael-type of addition of H2S to a,b-unsaturated systems provides b-mercapto derivatives that are readily desulfurized, thus providing a method for the selective reduction of the carbon-carbon double bond (eq 10).16 Attempted reduction of this double bond (eq 10) by borohydride, hydride, and silane reagents was unsuccessful.

Thioethers and Disulfides.

Conversion of H2S to sulfide ion, followed by treatment with a primary or secondary halide, sulfonate ester, quaternary ammonium salt, or an oxirane, aziridine, or activated aryl halide gives a good yield of symmetrical thioether via a thiolate intermediate.3 Base-catalyzed addition of H2S to electrophilic alkenes yields first the thiol which couples with excess alkene to give the thioether (eq 11).17a Disulfides are obtained via dithiol intermediates from addition of H2S to aldehydes, ketones, and imines.17b

Thiocarbonyl Compounds.

Thioaldehydes, thioketones, thioesters, thioamides, and related compounds may be prepared by addition of H2S (or SH-, S2-) to a C=X or a C=C-X bond where X is a group that is replaceable by sulfur.4 In the total synthesis of chlorophyll, an important thioaldehyde intermediate was obtained by addition of H2S to an imine salt (eq 12).18 Related syntheses of thioaldehydes involve additions to Vilsmeier intermediates (iminium salts, etc).19 The more common thioketones are prepared by addition of H2S to ketones typically in the presence of HCl;7b,14f,20 gem-dithiols14 may be intermediates and they can be thermolyzed to thioketones. While aromatic thioketones are readily available (eq 13),20a,b their enethiolizable aliphatic analogs are often contaminated by the enethiol. Trimerization to 1,3,5-trithianes is a common side reaction. Methods for preparation of aliphatic thioketones (stable for 1 month at -15 °C) free from enethiols, and their conversion to pure enethiols free of the thioketone tautomer, have been reported (eq 14).21 Aliphatic thioketones also are obtained in good yields (52-75%) by treatment of ketone anils with H2S followed by removal of aniline from the initial adduct with Benzoic Anhydride.22 Thioketones are obtained by addition of H2S to imines and iminium salts.23

Derivatives of thiocarboxylic acids4c,24 are obtained from H2S and orthoesters (eq 15),25 carboxylic acid ester enolates (eq 16),26 and imino esters.27 A method for reduction of a tertiary alcohol to an alkane involves its conversion to a thioformate ester followed by treatment with Tri-n-butylstannane (eq 17).27b Addition of H2S to thioimino esters provides derivatives of dithiocarboxylic acids (eq 18).4d,28 The thioamide function4e,29 is obtained by addition of gaseous or liquid H2S to nitriles (eq 19),17b,30 ynamines,31 various imines (eq 20),32 and to orthoesters or chloroform in the presence of an amine (eq 21).32f,33 Thioureas, thiobiurets, and thio heterocycles,32d,34 thioimides (eq 22),32b,35 and thioacylhydrazines36 are obtained by similar reactions.

Heterocyclic Compounds.

The following types of sulfur-containing heterocyclic compounds obtained from hydrogen sulfide are listed according to ring size and the number of sulfur and other heteroatoms: 3 (1 S),37 4 (1 S),38a,b 4 (2 S),39 5 (1 S),40a-d 5 (2 S),41 5 (1 S, 1 N),42 5 (1 S, 2 N),43 6 (1 S),44 6 (3 S),45 8 (2 S).46 The first synthesis of a cyclopenta[b]thiophene by a keto alkyne cyclization is shown in eq 23.40c Hydrogen sulfide plays a key role (possibly by reducing elemental chlorine) in the suppression of chlorinated byproducts in the synthesis of a-methylthioindoles (eq 24).47


Hydrogen sulfide reduces nitro groups (eq 25),5,9,48 nitroso groups,49 azido groups,50 the S=O group (eq 26),51 the chlorosulfonyl group,52 sulfur-sulfur,53 sulfur-nitrogen,54 nitrogen-nitrogen,55 carbon-nitrogen,11b,56 and carbon-mercury57 bonds, the C-I and C-O bonds of some a-iodo and a-alkoxy ketones,58 osmate esters,59 and 1,2,3-tricarbonyl compounds60a and their hydrates60b-d (the 2-carbonyl functionality is reduced to the alcohol and dimers (eq 27)60d may be formed). Benzil is reduced quantitatively either to deoxybenzoin or benzoin, depending on conditions.14c The carbonyl group of aldehydes and ketones undergoes nucleophilic reduction with H2S and a reducing thiol to give good yields of thiols via thiocarbonyl derivatives (eq 28).61 The reduction of the azido group in 4-azidobutyryl derivatives of alcohols provides a mild method for deprotection of the hydroxy function (eq 29).50a Reduction of the azido group by elemental hydrogen instead of H2S in the deprotection scheme may be accompanied by the reduction of other functional groups (eg alkene).


Workup procedures of reactions that use salts of heavy metals (Hg, Pb, Ag, Cu) involve precipitation of the metal as its sulfide by H2S.62

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Donald C. Dittmer

Syracuse University, NY, USA

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