Organometallic Chemistry of Mercury
A. J. Canty University of Tasmonia Hobort, Tosmania, Australia
A n u n d e r g r a d u a t e experiment
The presence of organomercury compounds in the environment is well established (1, 2), and it is in this form that mercury is most toxic. The compounds have been added directly to the environment, e.g., methylmercuric dicyandiamide (Panogenm) as a fungicide and phenylmercurie acetate as a slimicide, or formed by microbiological methylation (1-3) of inorganic mercury. The volatility and high toxicity of methylmercury compounds render undergraduate experiments with them rather hazardous. However, arylmercurials which are nonvolatile solids and less toxic, can be readily synthesized and may be used to illustrate important features of the chemistry of organomercury compounds. In addition this experiment involves important characterization techniques and chemical reactions of current interest, e.g., the use of thin-layer chromatography, infrared spectroscopy, and conductivity data to estahlish the identity and nature of compounds; and metallation reactions, coordination chemistry of organometallic compounds, and redistribution reactions. Benzene is mercurated by mercuric acetate, and the reaction is most readily carried out in a high boiling solvent (4).
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.
Rather than isolate phenylmercuric acetate, portions of the reaction solution may be treated with CaXz (X = CI or Br) to precipitate C6H5HgX (X = C1 or Br) via a simple metathesis reaction C,H,HgOCOCH,
+
l/XaX,
-
C,H,HgXI
+ 1/2Ca(OCOCH,h
(X = Cl or Br) Characterization of C6HsHgX These compounds can be characterized by conductance and nmr and ir spectroscopy. Acetone solutions are nonconducting, and thus eliminate the ionic formulation csH~Hg+Cl-.Nmr and ir spectroscopy establish the presence of a phenyl group, and students may be referred to a complete vibrational assignment of monosubstituted hen-
750 1 Journal of Chemical Education
Mercury-Halogen Stretching Frequenciesa Compund
p(Hp.X) (X = CI or Br)
-
Nujol mulls, s = stmng, br = bmsd, m medium, sh = shoulder. Fmm referenee (13). 6 From reference (8. 9). *Previously reported (13) 2798 for the higher frequency band. The intensity of the lower band partly detivee fmm ligand ahsorption (13).
zenes (5) for comparison (6). Far infrared spectra (to -200 cm-l) allow assignment of u(Hg-CI) and u(Hg-Br) by comparison of the spectra of CeHsHgCl and CsHsHgBr. Organomercury compounds present in the environment can he detected by thin-layer chromatography. Some of the published procedures involve preliminary formation of a dithizone derivative. Dithizone solutions of CsHzHgX ( X = CI or Br) and Hg2+ are easily prepared and can be eluted on either Silica Gel or alumina (7). Coordination Chemistry of Phenylmercuric Halides Some interesting coordination chemistry of C6HsHgX (X = C1 or Br) can he studied with the chelate 1,lO-phenanthroline (phen) (8,9).
and
Identification of the different products is an interesting exercise for students. Infrared spectra of A and B indicate the presence of 1,lO-phenanthroline and the presence of a phenyl group in A. For complex A, conductance measurements in acetone eliminate ionic formulations, e.g., PhHgphen+CI- or PhzHgC12-Hg(phen)22+.Complex B is insufficiently soluble for conductance measurements. Far
infrared spectra of A indicate a lowering of v(Hg-C1) as expected (10) for an increase in coordination number. Students may subsequently prepare the complexes HgXzphen (X = C1 or Br) and compare spectral properties. Since CsHJHgX (X = C1 or Br) does not react with 2,2'-hipyridyl, which is known to he a poorer donor ligand than 1,lO-phenanthmline, students may be asked to discuss preliminary complex formation
as a requirement for disproportionation of organomercury compounds in the presence of chelating agents (9, 11,12J. Experimental
Phenyirnercuric Halides These may he prepared according t o Kobe and Lueth (4) with modification to allow use of readily available lahoratory glassware. Fourteen grams of mercuric oxide is dissolved in 40 ml of glacial acetic acid, and this solution is transferred to a heated (use of electrical heating tape is satisfactory) dropping funnel. The solution is added dropwise over 1 hr to a refluxing mixture of 60 ml benzene, 15 ml of glacial acetic acid, and 2M) ml of nitrobenzene. The dropping funnel is then rinsed with about 5 ml hot glacial acetic acid, and the resulting solution refluxed for a n additional hour. The solution is allowed to cool and is then filtered. The filtrate is divided into two equal portions, and each portion treated with CaClz (5 g) or CaBrz (9 g) in 50 ml of 95% ethanol. The immediate white precipitate is collected and recrystallized from 95% ethanol (X = CI, mp 251°C; X = Br, mp 276T).
color indicates an excess of reagent. A few drops of CsHsHg(dithizone) and Hg(dithizone)z are added to a thin-layer plate with either Silica Gel or alumina as the stationary phase, and the plates developed in hexane-acetone (19:l). The RF value of CeHsHg(dithizone) is about four times that of Hg(dithizone)z. Preparation of Complexes CeHsHgX (X = C1 or Br) react readily with an equivalent amount of 1,lO-phenanthmline monohydrate in henzene to give CeHsHgCl(phen) (mp 159'C) or HgBrAphen) (8, 9). The complexes HgXz(phen) ( X = C1 or Br) are nrecinitated on mixine- eauimolar amounts of mercuric . halide and ligand in ethanol or methanol (13) (mp > 360°C (X = C1) and 355-360°C (X = Br)). ~ a n t i o n a i n o t e Although . arylmercury compounds are nonvolatile solids the use of a fume hood is recommended. Accidents or chemical spills are then confined to a small area and can he more easily rectified. In order to prevent large amounts of mercury compounds entering the environment via waste water and solvents, restriction of the experiment to a few students in each class is recommended. Products should he stored and, as they are nonvolatile and stable, no special precautions are required. Acknowledgment
The author wishes to thank the Commonwealth Development Bank for a Postdoctoral Fellowship. Literature Cited
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,
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.
Thin-Layer Chromatography (7)
(5) Whiffen, D.H.. J. Chem Soe. 1350(1956). (8) Croon, J . H.S.,Spsrfroehim. Aeto, ZdA.W(l968). (7) See. e.p.,Tatton. J. O'G. and Wagsteffe. P. J.. J. Chmmalogr. 44.284 (1969). (8) Deamn. G.B..andCanty,A. J..lnorg. Nucl. Chpm Left.. 4,125l1968). (9) Csnty, A. J..andDeacon,G.B.,Avsf. J. Chern.. 21.1757 (1968). (101 Clark. R. J . H., Rer. Chsm Pmpr, 26, 269 (1955): Speclrochim. Acfo. 21, 955
Dithizone solutions of CsHsHgX (X = C1 or Br) and HgX2 are formed by adding a solution of 4% dithizone in chloroform to a solution of a few milligrams of the mercury compound in 1-2 ml chloroform until a slight green
(19651. (11) Jenaen. F.R..Rickbom,B.,andMiller. J . J.. J A m e r . Chem, Soc.. 88.34011956). (12) A large scale application of a redistribution reaction using cmrdinatins polymem has h e n recently de~cribed.Wade, R. C.. and Seyferth, D.,J Orpanometol. Chem.. 22.265 (1970). (131 Coatca. G.E.. sodRidley,D.,J. Chem Soc.. 166(1961).
Volume 51. Number 11. November 1974 1 751
