A Metallic Polymer

 

Conductive polymers such as polyacetylene and polyaniline that have been studied for several decades are semiconductors , having typical electrical conductivities of only a few reciprocal ohm-cm at room temperature . (The references are listed at the end of this web page . Underlined blue hyperlinks can be clicked when online to download the PDF or HTML file , which will open in a new window) . Chiang et al. observed that the conductivity of halogen-doped polyacetylene decreased with decreasing temperature . This direct relationship of temperature and electrical conductivity is an excellent diagnostic of semiconductor behaviour of a crystalline solid . True metals , on the other hand , always exhibit an inverse relationship between their temperature and electrical conductivity . As they warm up , their conductivity decreases ; as they are cooled down , it rises . The following are graphs of the temperature-conductivity relationship for gray tin , a semiconductor (like silicon and germanium ; all three have the diamond crystal structure) , and for gold , a true metal :

In an earlier work I referred to semiconductors as pseudometals , and what are broadly classified in the scientific literature as "metallic solids" I have called true metals . In my general classification , metallic solids are comprised of both true metals and pseudometals . The true metals have an inverse temperature-conductivity relationship , while that of the pseudometals is direct in nature . Clearly , pseudometals can never become superconductors , because as they are cooled down their conductivity eventually vanishes . On the other hand , true metals might become superconductors under the right conditions . As they are cooled down toward Absolute Zero , their conductivities increase , often in a spectacular manner . Twenty-nine elementary true metals become superconducting near Absolute Zero , but many more don't . For example , three of the most metallic substances we know of , copper , silver , and gold , never become superconducting , even very close to Absolute Zero .

In my earlier work mentioned above , I have studied and discussed the nature of the metallic bond , together with numerous examples of varied metallic solids , in considerable detail . Briefly , in all metallic solids the metallic bond - or conduction band , if you prefer - is comprised of a crystal orbital (XO) spanning the entire dimensions of the solid structure ; there is only a single metallic bond in the crystal , unlike the vast number of covalent bonds in covalent solids . The XO is comprised of all the atomic orbitals (AOs) overlapping and blending together throughout the solid , again contrasting with the countless separate molecular orbitals (MOs) in covalent solids . The arrangement of the atoms' valence electrons in the XO is governed by quantum rules , those of the Pauli principle and the Fermi-Dirac distribution .

What distinguishes the pseudometals (semiconductors) and the true metals is the nature of the orbital overlapping and topography . In pseudometals we always find nodes - band gaps , if you prefer - periodically intersecting the XO around the atomic kernels . In true metals there are no nodes (or nodal planes or nodal surfaces , as they are sometimes called) , along at least one major crystal axis . If we look at the valence electron distribution in the elementary true metals , we see that there is often one valence electron formally assigned to an outer s orbital . In the case where there are two s electrons , "leakage" can occur into an adjacent empty AO . In other cases leakage can occur into or out of s AOs from p and d AOs . A crystal-wide overlapping of the partially-filled s AOs creates the sigma XO as the metallic bond in the solid .

The presence of nodes in the XO obliges the mobile electrons in it (those above the Fermi level) to tunnel through them , as they are pulled through the crystal by the potential difference (voltage , which causes the elecrons to "flow" in conductors) . However , electron tunneling requires energy , which the electrons obtain from their environment : externally provided heat and/or light . As more heat is provided , the temperature rises and more electrons tunnel through the nodes . The material's conductivity increases . Removing heat - cooling - reduces the tunneling , and so reduces the solid's conductivity .

In true metals , there are no nodes , and thus no electron tunneling is required . As the temperature of a true metal increases , its conduction electrons (above the Fermi level) are scattered more and more by the vibrating atomic kernels (phonons) and impurity atoms , which knock more and more of them backward . The conductivity of the metal decreases with rising temperature . Conversely , its conductivity increases as it is cooled down .

The s AOs are spherical , and when they overlap continuously in a solid to form a sigma XO metallic bond , there are no nodes in the resultant crystal-wide XO . As a result , metallic solids - the elementary metals , and others - with such XOs are always true metals , with the typical inverse temperature-conductivity relationship shown above for gold . An orbital analysis of pseudometals (semiconductors , like doped silicon and germanium , gallium arsenide , etc.) shows that there are always nodes around the atomic kernels , created by the sigma MOs that form their bonding skeletons . In pseudometals , the metallic bond (conduction band , XO) is in the covalent bond skeletons , which always have nodes .

If we examine the true metals , we see that in all cases the metallic bond is in a nodeless sigma XO that surrounds either the atomic kernels (for example , in the alkali metals , their inert gas cores) , or that is in a higher energy frontier atomic orbital surrounding the lower energy covalent bond skeleton of the molecule . The frontier AOs can then overlap continuously in the crystalline solid to form a nodeless XO . In an earlier work devoted to molecular metals , I suggested that in the case of many molecular metals in which sulfur atoms were present the metallic bond XO was formed around the molecules when aromatization of the underlying pi MO structure , requiring only five of the sulfurs' six valence electrons (3s2 3p4) , "popped" (promoted) the extra sixth valence electron into a frontier orbital . In some cases this could be an antibonding molecular orbital (ABMO) , or it could be the sulfurs' 4s sigma AOs . Since ABMOs are highly nodal in nature , we wouldn't expect a true metal to result from their use in forming an XO . On the other hand , a true metal would result if the sulfur 4s sigma AOs were used to form the nodeless sigma XO . Such an electronic situation does seem to be happening spontaneously in the case of the fascinating inorganic metallic polymer , poly(sulfur nitride) , [polythiazyl , (SN)x] , which is a true metal and a superconductor (Tc = 0.26 K) .

Based on my hypothesis concerning the sulfur 4s AOs , I designed a number of novel molecules containing sulfur atoms , in which reproportionation of sulfur II and IV valences into the "unnatural" sulfur III valence would result in the "popping" of the sixth extra valence electrons into the sulfurs' 4s AOs . The new molecules in their crystalline bulk state should then exhibit a true metal behaviour , including an inverse temperature-conductivity relationship .

For example , the compound bis[(phenylthio)methylidene]sulfurane was one such possible molecular metal candidate :

Its electronic structure , resulting in a metallic condition , is shown stepwise as follows :

Note that the neutral molecule would likely be a stable , "ordinary" organic compound (and having sulfide sulfurs , would undoubtedly be quite a stinky one , too !) . To activate its metallic bond we could treat it with one equivalent of a one-electron oxidizer , such as a nitrosonium salt , NO+ X- , where X- is BF4- , PF6- , AsF6- , ClO4- , etc. . This would create an electron resonance which would reproportionate the two sulfur valences , II and IV , to the unnatural sulfur (III) . With promotion of two sulfur valence electrons into their 4s AOs , the remaining five electrons on each sulfur can be distributed in a trigonal planar sp2 hybridized AO , as in the pyridine molecule , for example . We now have in the molecule the strong , skeletal structure of the sigma covalent bonds , over which is a smooth , continuous pi MO (including over the three sulfur atoms) ; and above them are the two promoted "sixth" electrons in the 4s sigma AOs .

In the crystalline solid these 4s AOs should be able to overlap continuously to form the sigma XO , which is half-filled with valence electrons . The sulfur-containing molecule would in fact be a form of metallic sulfur , with the surrounding organic molcule being electronically inert . Bis[(phenylthio)methylidene]sulfurane is particularly interesting because the two promoted "sixth" electrons might form a single Cooper pair per molecule . In the hypothetical sulfurane the 4s electron pair is in an intermolecular mixed-valent compound , in which resonance causes them to "hop" from atom to atom . This might assist in the pairing process which generates Cooper pairs . In other words , the sulfurane salt compound could be superconducting at an unusually high temperature .

On the other hand , it might not . Research with a class of molecular metal salts based on TMTSF (tetramethyltetraselenofulvalene) revealed that the metallic nature of the compound was highly dependent on the spectator anion accompanying the molecule in the compound , whose formula was (TMTSF)2+ X-, where X = PF6- or ClO4- . The TMTSF molecules formed a stack structure in the crystalline solid , in which it is assumed that the seleniums' 5s AOs , with the "extra" promoted electrons , overlap along the columns to form the metallic bond in them :

Under ambient pressure the perchlorate compound was a true metal , and became superconducting at 1.2 K . By contrast , TMTSF hexafluorophosphate was a pseudometal , never becoming superconducting at ambient pressure . However , when it is compressed to 12,000 atmospheres , it too becomes a true metal and a superconductor , at around 1 K :

A possible explanation for these observations is that the selenium 5s AOs are "stunted" or "dwarfs" because they are partially diverted into forming hybrid AOs and ABMOs (unlike the frontier orbitals in the metal elements) . They are thus physically smaller than metal atom AOs , and don't extend outward very far . The molecules containing them , and sulfur 4s AOs , must either pack together closely in the crystal in order for them to overlap enough for XOs to form , or else their molecules must be forcibly compressed together by the researcher to accomplish this , and to observe a metallic state in the compound . When the molecules are too far apart , the frontier orbitals don't quite overlap , yet some conduction electrons can tunnel across the molecular gaps under a suitable applied potential difference . The molecular gaps are thus acting as orbital nodes in the XO , making the compound a pseudometal .

In a similar manner the conductive polymers such as doped polyacetylene and polyaniline are semiconducting pseudometals . Although the XO is in their pi MOs along the polymer spines , and such pi MOs are nodeless along their major axes , the interchain distance in the uncompressed solids seems to be just a little too great , and the interchain gaps are acting as orbital nodes in the pi XO .

A similar situation might occur with both the small molecule compound I have discussed above , bis[(phenylthio)methylidene]sulfurane , and with its polymeric analog , which is the main focus of this web page .

The proposed synthesis route to the hypothetical sulfurane was based on a unusual application of the Wittig reaction , which in organic synthesis is generally employed in the condensation of halides with aldehydes or ketones to produce olefins . In this fluorine analogue of the Wittig reaction , two equivalents of a Wittig ylid would be condensed with one equivalent of sulfur tetrafluoride , to produce the sulfur (IV) sulfurane . If the ylid contains sulfur (II) - as in a sulfide (thioether) function - the resulting molecule will contain both sulfur (II) and sulfur (IV) . Further , if the two types of sulfur atoms can be linked via resonance in a pi MO , then their two valences can be reproportionated to produce the unnatural sulfur (III) valence . When sulfur (III) is in a pi MO system , it will use five of its six valence electrons in the sigma and pi bonding . Since there is no room for the sixth valence electron in the covalent bonding orbitals , it will be "popped" (promoted) into the sulfur 4s frontier orbital . These half-occupied sulfur 4s orbitals , if they can overlap continuously throughout the crystalline solid , will form the sigma XO metallic bond in it . This was the synthesis strategy behind the proposed route to bis[(phenylthio)methylidene]sulfurane :

Wittig ylids react with sulfur dioxide to form the sulfine functionality . It might be possible to use the inexpensive sulfur dioxide (one equivalent) with two equivalents of the ylid to produce the desired sulfurane . Another well-known sulfur (IV) precursor that might be tried in this reaction is thionyl chloride , SOCl2 , b.p. 79 ║C . Sulfur tetrafluoride is known to be an extremely reactive substance , sometimes violently so . Clearly it is a highly reactive electrophile , and so it should combine rapidly and completely with the nucleophilic Wittig ylid . Sulfur dioxide and thionyl chloride are also very reactive electrophiles , although not to the same extent as sulfur tetrafluoride .

A polymeric version of bis[(phenylthio)methylidene]sulfurane might be synthesized by substituting the corresponding difunctional aromatic thiol , 1,4-benzenedithiol , for the thiophenol starting material in the small molecule compound , then proceeding with the Wittig-based method :

Unlike phenol , thiophenol can undergo methylolation at its sulfur rather than on its aromatic ring , as in the preparation of chloromethyl phenyl sulfide from thiophenol , formaldehyde , and HCl (phenol , by contrast , readily forms phenol-formaldehyde polymers with methylene bridges between the aromatic rings) . Such a reaction could be applied to 1,4-benzenedithiol , to produce the benzene-1,4-bis(methylenetriphenylphosphonium chloride) salt . This latter intermediate would then be treated with two equivalents of a strong base in an inert hydrocarbon solvent (lithium hydride in benzene is indicated in the above sketch) to generate the bis-ylid . Then , one equivalent of sulfur tetrafluoride gas [caution : very reactive , toxic compound !] would be bubbled into the ylid solution or suspension . The polymer should form rapidly , along with the by-products lithium chloride and difluorotriphenylphosphorane (the latter is a colourless , crystalline compound , m.p. 136-140 ║C) .

An interesting challenge in this project would be the preparation of the thiol starting material , 1,4-benzenedithiol . It is commercially available , but is very expensive ($100 per gram , roughly) . Although it has been known since at least 1914 , there are few satisfactory syntheses of the dithiol . The following are two possible routes to it , based on well-known procedures for preparing thiols :

Conventional wisdom states that unactivated halogen atoms on aromatic rings are very difficult to replace in nucleophilic substitution reactions . For example , the chlorine atom in chlorobenzene is for all intents and purposes inert to nucleophilic attack . On the other hand , a halogen atom on an aromatic ring that is activated by an electron-withdrawing group ortho or para to it (that is , its carbon-halogen bond is made more labile) can usually be displaced by a nucleophile under mild conditions . This would be the case , for example , with the chlorine in 4-nitro-1-chlorobenzene .

However , a literature search into this topic revealed that nucleophilic displacement of unactivated halogen atoms in aromatic molecules can indeed occur if a strongly polar solvent is used as the reaction medium . This implies that the nucleophilic displacement occurs by an ionic mechanism , and that the ionized aromatic intermediate is stabilized in a strongly polar environment . The following two examples are relevant to the present quest for a suitable synthesis of 1,4-benzenedithiol :

Good indicators of the polarity of an organic molecule are provided by its dielectric constant and dipole moment ; the higher these physical values are for a solvent , the more polar it is . In the following table data is gathered together for thirteen common organic solvents and water (for comparison) :

Propylene carbonate (PC) , generally underappreciated by organic chemists as a solvent , emerges from this list as a formidable polar reaction medium , probably superior to those used in the aromatic nucleophilic displacement reactions sketched above (N-methyl-2-pyrrolidone and N.N-dimethylacetamide) . PC , a cyclic carbonate ester, is a relatively inexpensive industrial chemical :

It can dissolve inorganic salts to a certain extent , and so has been used as a solvent in many electrochemical studies . With PC as a highly polar solvent , the inexpensive para-dichlorobenzene (which , like napthalene , is found in the familiar mothballs , and can be purchased cheaply in retail stores by the general public) can now be used as a starting material in the synthesis of 1,4-benzenedithiol :

In the first example , I have recommended the use of hydrated sodium sulfhydride as the sulfide source . This chemical is inexpensive (eg. Aldrich) but would have to be "assayed" for sulfur content before use so that the correct stoichiometry could be calculated . The sulfhydride anion , HS- , is a fairly strong nucleophile , and should be able to displace the chlorines from the para-dichlorobenzene starting material . The by-product sodium chloride is innocuous , but is only slightly soluble in PC . The hot reaction mixture would have to be stirred mechanically , so as to avoid any possible "bumping" of the solvent caused by the suspended NaCl crystals .

Jasinski noted in his review of PCs electrochemical applications that there are limitations on the thermal stability of this solvent :

"Although the solvent [PC] boils at +241 ║C , thermal decomposition can occur at and above +150 ║C" (p. 254) .

Researchers attempting these nucleophilic displacement reactions with para-dichlorobenzene in heated PC should keep this precaution in mind .

In the second example , I have suggested trying to react thiourea with para-dichlorobenzene in PC to prepare the bis-S-thiouronium chloride salt . Many alkyl halides are known to react with thiourea to produce their S-thiouronium halide salts , which can then be hydrolyzed in aqueous alkai to the corresponding thiol (mercaptan) . In fact , this reaction has been recommended as a method of preparing a crystalline derivative of the halide as a proof of its identification . Normally the technique wouldn't work with unactivated aryl halides , but perhaps by using PC as the reaction solvent the bis-S-thiouronium chloride salt of para-dichlorobenzene could actually be prepared . Its alkaline hydrolysis would then afford the desired 1,4-benzenedithiol in a rather neat procedure .

Assuming a supply of 1,4-benzenedithiol can be obtained , synthesis of the polymer could proceed along the route outlined above . Because of electron resonance in the conjugated pi XO along the polymer spine , including the sulfur atoms , we would expect the doped material to be a reasonably good electrical conductor . We may see an enhancement of this conductivity into the metal range , well above that of semiconductors , from a formation of the sigma XO metallic bond in the overlapping 4s frontier orbitals above the sulfur atoms . This would permit the polymer to exhibit an indirect temperature-conductivity relationship , characteristic of true metals . Finally , when cooled sufficiently , the polysulfurane might also become superconducting , which would make it the first organic polymer to do so [poly(sulfur nitride) , an inorganic polymer , is also superconducting near Absolute Zero] . The realization of these properties in an organic conductive polymer would make it a tantalizing objective for synthesis and study by polymer and solid state chemists .

 

References , Notes , and Further Reading

 

polyacetylene : Useful review articles on the subject of conductive polymers are provided by Wikipedia , the online encyclopedia . See the web pages , http://en.wikipedia.org/wiki/Organic_electronics and http://en.wikipedia.org/wiki/Conductive_polymers .

An excellent review article on the development of conductive polymers , Twenty-five Years of Conductive Polymers, by Nina Hall (Royal Society of Chemistry , UK) , can be downloaded for free (PDF, 2911 KB) , from http://www.rsc.org/delivery/_ArticleLinking/DisplayArticleForFree.cfm?doi=b210718j&JournalCode=CC

C.B. Duke and H.W. Gibson , Polymers , Conductive, Kirk-Othmer Encyclopedia of Chemical Technology , 3rd edition , vol. 18 , pp. 755-793 ; M. Grayson and D. Eckroth (eds.) , John Wiley , New York , 1982 .

R.B. Kaner and A.G. MacDiarmid , Plastics That Conduct Electricity, Scientific American 258 (2) , pp. 106-111 (February , 1988) . Photo of polyacetylene on p. 111 .

polyaniline : J. Stejskal and R.G. Gilbert , “Polyaniline . Preparation of a Conducting Polymer”, Pure Appl. Chem. 74 (5) , pp. 857-867 (2002) . This definitive report on the preparation and properties of polyaniline (green Emeraldine hydrochloride) can be downloaded for free (PDF , 206 KB) from http://www.iupac.org/publications/pac/2002/7405/7405x0857.html .

Chiang : C.K. Chiang et al. , “Conducting Polymers : Halogen Doped Polyacetylene”, J. Chem. Phys. 69 (11) , pp. 5098-5104 (1978) . “In general , we find that the conductivity of halogen doped polyacetylene decreases with decreasing temperature” (p. 5101) .

gray tin : A.W. Ewald and E.E. Kohnke, “Measurements of Electrical Conductivity and Magnetoresistance of Gray Tin Filaments”, Phys. Rev. 97 (3) , pp. 607-613 (1955) ; see Figure 2 , p. 609 for the graph of the electrical conductivity of gray tin over a range of temperatures .

gold : D.R. Lide (ed.) , CRC Handbook of Chemistry and Physics , 82nd edition , CRC Press , Boca Raton , FL , 2001 ; data for the electrical conductivity of gold over a range of temperatures are listed on p. 12-45 .

Fermi-Dirac : A.R. Mackintosh , “The Fermi Surface of Metals”, Scientific American , 209 (1) , pp. 110-120 (July , 1963) .

polysulfur : A.G. MacDiarmid et al. , “Synthesis and Selected Properties of  Polymeric Sulfur Nitride (Polythiazyl) , (SN)x”, Ch. 6 , pp. 63-72 in Inorganic Compounds with Unusual Properties , R.B. King (ed.) , Adv. Chem. Series 150 , American Chemical Society , Washington , D.C. , 1976 ; M.M. Labes , P. Love , and L.F. Nichols , “Polysulfur Nitride – A Metallic , Superconducting Polymer”, Chem. Rev. 79 (1) , pp. 1-15 (1979) ; R.T. Oakley , “Cyclic and Heterocyclic Thiazenes”, Prog. Inorg.Chem. 36 , pp. 299-391 , S.J. Lippard (ed.) , John Wiley , New York , 1988 ; G.B. Street and W.D. Gill , “The Chemistry and Physics of Polythiazyl , (SN)x , and Polythiazyl Halides”, pp. 301-326 in Molecular Metals , W.E. Hatfield (ed.) , Plenum Press , New York , (1979) .

TMTSF : K. Bechgaard and D. JÚrome , “Organic Superconductors”, Scientific American 247 (1) , pp. 52-61 (July , 1982) ; graph on p. 55 ; see also , K. Bechgaard and D. JÚrome , “Superconducting Organic Solids”, Chemtech 15 (11) , pp. 682-685 (November , 1985) ; Figure 3 , p. 684 .

Wittig : A. Maercker , “The Wittig Reaction”, Organic Reactions , Vol. 14 , Ch. 3 , pp. 270-490 , A.C. Cope (ed.) , John Wiley , New York , 1965 ; in Wikipedia , the online encyclopedia : “Wittig reaction”, at http://en.wikipedia.org/wiki/Wittig_reaction .

sulfur tetrafluoride : supplied by (for example) Air Products & Chemicals , at http://www.airproducts.com/Products/Chemicals/Fluorination/sulfur.htm  , and Matheson-Tri-Gas at https://www.mathesontrigas.com/pdfs/products/Sulfur-Tetrafluoride-Pure-Gas.pdf .

sulfine : B. Zwanenburg et al. , “Preparation of Sulfines by a Wittig Alkylidenation of Sulfur Dioxide”, Tet. Lett. (9) , pp. 807-810 (1978) .

methylolation : H.E. Zaugg and W.B. Martin , “a-Amidoalkylations at Carbon”, Organic Reactions , Vol. 14 , Ch. 2 , pp. 52-269 , A.C. Cope (ed.) , John Wiley , New York , 1965 ; C.M. McLeod and G.M. Robinson , “Pseudo-bases . III Dialkylaminomethyl Alkyl Ethers and Sulfides”, J. Chem. Soc. 119 , pp. 1470-1476 (1921) ; T.D. Stewart and W.E. Bradley , “The Mechanism of Hydrolysis of Dialkylaminoethyl Ethers”, J. Amer. Chem. Soc. 54 (11) , pp. 4172-4183 (1932) .

chloromethyl phenyl sulfide : D. Enders , S. von Berg , and B. Jandeleit , “Diethyl [(Phenylsulfonyl) Methyl] Phosphonate”, Org. Synth. Coll. Vol. 10 , p.289 (2004) . This synthesis procedure can be downloaded for free (PDF, 164 KB) from : http://www.orgsyn.org/orgsyn/pdfs/V78P0169.pdf .

difluorotriphenylphosphorane : W.C. Smith , “Chemistry of Sulfur Tetrafluoride . VIII . Synthesis of Phenylfluorophosphoranes and Phenylarsenic(V) Fluorides”, J. Amer. Chem. Soc. 82 (23) , pp. 6176-6177 (1960) .

satisfactory : A. Ferretti , “1,2-Dimercaptobenzene”, Org. Synth. Coll. Vol. 5 , p. 419 (1973) . A similar technique was used to prepare 1,4-benzenedithiol . This synthesis procedure can be downloaded for free (PDF, 138 KB) from : http://www.orgsyn.org/orgsyn/pdfs/CV5P0419.pdf .

activated : J.D. Roberts and M. Caserio , Basic Principles of Organic Chemistry , W.A. Benjamin , New York , 1965 , p. 844 ; J. March , Advanced Organic Chemistry , Reactions , Mechanisms , and Structure , 4th edition , John Wiley , New York , 1992 , pp. 649-651 .

industrial : W.J. Peppel , “Preparation and Properties of the Alkylene Carbonates”, Ind. Eng. Chem. 50 (5) , pp. 767-770 (1958) . Several physical properties of PC tabulated in this review : m.p. – 49.2 ║C ; b.p. 241.7 ║C ; s.g. 1.2057 ; dielectric constant , 69.0 . Peppel notes that salts dissolved in PC can catalyze its decomposition at elevated temperatures . For example , ethylene carbonate with 1% dissolved LiCl heated at 175 ║C for 2 hours gave an estimated yield of 65% of ethylene oxide .

electrochemical : R. Jasinski , “Electrochemistry and Application of Propylene Carbonate”, Adv. Electrochemistry Electrochem. Eng.  8 , pp. 253-335 , P. Delahay and C.W. Tobias (eds.) , Wiley Interscience , New York , 1971 . See Table XIII , p. 279 .

assayed : chemical analysis for sulfur , as sulfide . For example , precipitation as a highly insoluble sulfide , such as silver sulfide (argentite , Ag2S , solubility in water 0.00014 g/L at 20 ║C) , by treating an aqueous solution of the commercial sodium sulfhydride hydrate with a solution of silver nitrate or silver fluoride , then filtering , washing , drying , and weighing as Ag2S . Actually , I was unable to find a suitable gravimetric analysis for soluble sulfide (there are procedures for insoluble sulfide , as in minerals such as pyrites) . The standard analysis for soluble sulfide seems to be the iodimetric titration method : J. Bassett et al. , Vogel's Textbook of Quantitative Inorganic Analysis , 4th edition , Longman , London (UK) , 1978 ; pp. 384-385 .

nucleophile : J. March (see above , under activated) , Table 10.9 , “Nucleophilicities of Some Common Reagents”, p. 351 . Sulfhydride anion , HS- , is as strong a nucleophile as cyanide and iodide anions .

recommended : R.L. Shriner et al. , The Systematic Identification of Organic Compounds , 7th edition , John Wiley , New York , 1998 ; pp. 365 , 368-369 .

 

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