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Synthetic and mechanistic aspects of the electrochemical formation of polimeric alkyl-mercury compounds
Eiecrrochimica
Acra.
1977, Vol. 22. pp, 225-228.
Pergamon RN.
Printed in tireat Britain
SYNTHETIC AND MECHANISTIC ASPECTS OF THE ELECTROCHEMICAL FORMATlON OF POLIMERIC ALKYL-MERCURY COMPOUNDS* L. A. AVACA, E. R. GONZALEZ and N. R. SmAr_uoTTot Instituto de Fisica e Quimica de SHo Carlos, Universidade de SHo Paulo, C.P. 369, 13560~SBo Carlos (SP), Brazil (Received
in jinal
form
12 JuZy 1976)
Abstract-The electrochemical reduction on mercury of a series of a, o-dibromoalkanes was studied at low cathodic potentials in aprotic media. i,4-dibromobutane and 1,5dibromopentane yielded solid
alkyl-mercury compounds, the structures of which were established as being polymeric and of the general type R-Hg-(R-Hgh-R (R = alkyl). Proper adjustment of the electrochemical parameters allowed very high current efficiencies for the syntheses. The results obtained for 1,Cdibromobutane. combined with those already existing in the literature, permitted the formulation of a general mechanism for the process.
INTRODUCTION Alkyl-mercury compounds have found several industrial and pharmacological applications[l, 21. Compounds of the type RzHg (R = alkyl) can be synthesized electrochemically by reduction of the corre sponding alkyl halides on mercury[3,4] or, under certain conditions, from the .!x,,w-dihalides[& 61. Studying the reduction of 1,4-dibromobutane on mercury Brown and Gonz#ez[S] reported that, in addition to the formation of dibutylmercury, solid butyl-mercury compounds were formed at low cathodic potentials. In general terms this reduction process can lead to a variety of products depending on the working conditions[5-81. The lack of a general mechanism and, in some cases, of proper control of the electrochemical parameters led to apparently conflicting results in the literature and also prevented the possibility of establishing the conditions for the selective synthesis of a given product. In this work the experimental conditions FOT the electrochemical synthesis of some solid alkyl-mercury compounds are described. A critical analysis of existing data is made in order to establish a relationship between electrode potential and product distribution. EXPERIMEFTAL Controlled potential electrolysis were carried out in divided cells designed to maintain the cathode at a uniform potential. A mercury pool (1.2 x 10-j m2) served as the cathode. The counter electrode was a carbon disk (4.1 x 1O-4 m2) enclosed in a concentric compartment separated by a medium porosity glass sinter. The reference electrode consisted of a silver wire immersed in a 0.01 M solution of AgN03 made * Presented in Dart at the 11 Latin-American Conference in Electrochemistq, October, 1974, Rio de Janeiro, Brazil. 7 Present address: Faculdade de Filosofia, Ci&cias e Letras de Ribeirk Preto, S.P., Brazil.
E.h22/3-A
up in 0.1 M solution of tetraethylammonium bromide (TEAB) in the solvent used for the catholyte. This
complex silver/silver bromide electrode will be referred to as Ag/AgBr. It was joined to the main compartment through a side arm and separated by the usual Luggin capillary and a liquid junction formed in a closed tap. The catholyte solutions were deaerated by a stream of pure nitrogen which also served to stir the catholyte during the runs. Cells were rigorously cleaned, rinsed and dried before use. Mercury was purified by the usual double distillation. The solvents, N,N-dimethylformamide (DMF) and acetonitrile (AN), were purchased from Merck (zur Synthese) and used without further purification. The water content of the solvents was considered acceptable for the purposes of this work. TEAB was synthesized from triethylamine (C. Erba) and ethyl bromide (C. Erba), recrystallized from ethanol and dried at 80°C under vacuum. Tetrabutylammonium perchlorate (TBAP) was synthesized and purified in a similar way[9]. Lithium perchlorate was prepared from lithium carbonate (Merck) and Analar perchloric acid, recrystallized and dried at 160°C under vacuum. Due to the difficulty of complete removal of water from this salt, solutions were dried with 4 8, molecular sieves (Merck). The substrates, 1,3-dibromopropane (lJ-DBP), 1,4-dibromobutane (l,cDBB) and 1,5_dibromopentane (l$DBP), were used as received from BDH after checking their purity by gas-liquid chromatography and refractive index measurements. The electrolyses were potentiostatically controlled by means of a Beckman Electroscan TM 30. Currents were graphically recorded as a function of time during the preparative runs. Elemental analyses were made in a Perkin-Elmer 240 instrument. Mass spectra were obtained using a Finnigan 10155/h spectrometer. Mercury was detected and measured by means of a Hillger-Watts H-l 170 Atomic Absorpti& Spectrometer. X-Ray diffractograms were obtained in a Debye-Scherrer type camera. 225
L. A. AVACA, E. R. GONZ~Z
226
A standard procedure was adopted for all preparative electrolyses. A current-potential curve was first recorded in a 0.1 M solution of the supporting electrolyte and then in the same solution made 0.3 M in the corresponding substrate. The electrolysis potential was then fixed at the foot of the reduction wave such as the residual current could always be neglected in comparison with the total current which varied between 2 and 30 A m-‘. Except for one of the substrates, electrolysis yieldeci a white insoluble solid which was separated, weighted and analysed. Since the purpose of this work was the synthesis and characterization of these solids which under proper conditions could be obtained in yields near lW?, no attempt was made to identify other products of the reaction. RESULTS
Table 1 shows a selection of experimental results from a number of preparative electrolyses of l,CDBB, 1,5-DBP and 1,3-DBP on mercury. The electrochemical parameters (el?trode potential, supporting electrolyte and solvent) were changed to maximize the yield of solids and to gain information on the mechanistic pathways of the reaction. Although only one typical result for 1,3-DBP is presented, the reduction of this compound was actually studied under different conditions failing to give any solid. The two solid compounds obtained as the final product of the electrolyses described in Table 1 did not show a definitive melting point but a gradual transition to the liquid state between 50 and 100°C. Routine solubility tests, using polar, non-polar, and mixed solvent systems, gave negative results in both cases. This fact prevented the use of colligative properties for direct molecular weight determination. Analyses of the solids showed the presence of carbon, hydrogen and mercury while nitrogen and bromine were absent, Percentual compositions were determined by elemental analysis for C and H, * The authors thank Dr. C. H. Williams from University of Campinas, Brazil, for running the mass spectra and helping with their interpretation. Table 1. Preparative Substrate (0.3 M)
Solvent
Electrolyte (0.1 M)
1 2 3 4 5
IA-DBB
DMF
TEAB
7
l,%DBP
Expt
6 : 10 11 12 13
1,3-DBP
AN DMF
* For details of the calculations
Table 2. Analysis of the solids Solid from
C H Hg
Solid from
Method
1,4-DBB (%)
1,5-DBP (“/
Elemental analysis Atomic absorption
18.50 3.40 78.00
22.36 4.06
Element
73.30
whereas Hg was independently measured by atomic absorption spectroscopy. The results for each of the solids are collected in Table 2. The mass spectra* of the solid obtained from 1,CDBB presented several clusters with the typical isotopic distribution for mercury. The base peak was m/e 56 (C,H,). The highest molecular weight fragment ion was m/e 572 (CI,H,,Hg2). The other organo-mercury ions present were m/e 258 (C4HpHg), m/e 314 (CsH16Hg) and m/e 516 (CsH16Hg,). For the solid obtained from 1,5-DBP a similar picture was observed, with the base peak being m/e 70 (C5H10) and the highest fragment m/e 614 (CISHl,,Hg2). Other peaks appeared at m/e 272 (CSHlOHg), m/e 342 (CJ%,Hg) and m/e 54.4 (ClOHZOHg& X-Ray diffractograms of both the solids showed them as being polycrystalline. This fact prevented the possibility of gaining direct additional information from this technique.
DISCUSSION
Synthetic
aspects
As can be seen from 1,3-DBP product,
Table 1, the reduction of on mercury failed to produce the expected while 1,CDBB and lJ-DBP gave solid com-
pounds which were very similar in their properties and, in both cases, clearly appearing to have an organometallic nature. Thus, the analyses showed the exclusive presence of C, H and Hg with percentage compositions corresponding to a relation alkyl: Hg very near 1 for both the solids, while the mass spectra presented several clusters of alkyl-Hg fragrnedts which for low values of m/e were sin@ to those electrolyses on mercury Potential V vs Ag/AgBr
Charge (C)
Wt. Solid (mg)
Yield* (%)
-
TEAB TEAB
1.20 1.40 1.50 1.30 - 1.20 - 1.30 - 1.30
39.6 176.5 580.1 64.8 34.9 59.1 34.9
52.0 116.0 164.1 33.7 28.2 59.4 43.0
98.4 49.4 21.2 39.0 60.6 75.4 86.8
TBAP LiClO, TEAB TEAB
- 1.50 1.60 - 1.50 - 1.60 -1.60 - 1.10
445.0 88.7 67.8 33.1 77.5 53.1
188.4 92.9 23.9 25.4 90.1
74.5 30.1 25.1 54.6 82.7
TBAP
AN DMF
AND N. R. STRADIOTTO
LICIO,
see Discussion section.
Electrochemical
formation
of polymeric
reported for the corresponding RzHg compound [lo]. In addition, the ions observed for higher values of m/e suggest fragments with a general formula -R-Hg-R-Hg-R(R = C,+Hs or CsH,,). A simple structure, identical with that corresponding to the heaviest fragment for each case, would lead to percentage compositions in appreciable disagreement with the experimental values. Therefore, and in accordance with a previous suggestion[SJ, it must be concluded that the most probable .structure for the solids synthesized will be R-Hg--(R-H&-R. Detailed calculations of the compositions as a function of n show that this number should be at least. 10 to reproduce, within the experimental error, the values presented in Table 2. On this basis, thd coulomb yields presented in Table 1 were calculated assuming 2 electrons per -R-Hg- unity formed. In addition to the linear polymeric structure presented other possibilities could be considered. Thus, a cyclic arrangement of the type (RHg), with n = 1, 2, . . . , efc will also give the experimental composition but only for n > 3 will it produce fragments as observed in the mass spectra. However, simple probability considerations will favour the formation of a linear structure. On the other hand, all other alkyl mercury compounds reported as the product of electrochemical syntheses are Hg(I1) derivatives, so it seems improbable that linear structures of the type R-Hg,-(K-K-Hg,),-R, with the metal appearing as Hg(I), could have been formed in this case. From a quantitative point of view, the results of the preparative electrolyses, which were highly reproducible, show that with proper adjustment of the electrochemical parameters very high yields of the polymeric alkyl-mercury compounds can be obtained. This is a much better situation than that normally encountered in electrosynthetic work.
Mechanistic
aspects
In order to understand the observed influence of the electrochemical parameters on the yields of solid organo-mercurials, a detailed knowledge of the mechanism of reduction of cc,o-dibromoalkanes on mercury is necessary. One way of gaining such knowledge will be by means
of a critical
Table 3. Examples
Electrolyte
Bu.,NClO, Me,NClO, Bu,NF,B Et+NF4B Et,NBr Et,NBr-
Potential V vs see
analysis
of existing
of preparative
obs. N.O. 98% 21%
electrode potential. Cell voltage: (b) Results reported as coulomb yields. (ohs) Qualitatively observed product.
(N.O.) Product reported as
data to find a possible correlation between product distribution and the experimental conditions. Table 3 shows the results of the present work integrated with those already reported in the literature for the reduction on mercury of 1,4-DBB in DMF solutions. For comparison purposes all potentials have been referred to a common standard. Except for the unconfirmed report about extensive formation of cyclobutane[7], each of the other products presented in Table 3 were obtained by more than one independent work under similar conditions. Therefore, the results in Table 3 can be taken as evidence for the existence of a definite correlation between the electrode potential and main product formed. As the potential changes from very low to high cathodic values, the preferentially formed product shifts from the left (solid organo-mercurial) to the right (n-butane) of the table. This fact should not be taken as meaning a stepwise mechanism since by their nature the products formed at low cathodic potentials (solid organo-mercurial and bromobutane) must originate from quite different intermediates. In accordance with the accepted mechanism for the reduction of the carbon-halogen bond[ll], the first electron transfer will produce a monohalide radical which will be stabilized by adsorption on the mercury surface[12]. Further reduction of this radical will lead to either a diradical, again stabilized by adsorp tion[5], or to an anion which will be rapidly prt>tonated to give bromobutane. Taking into account the established structure of the solid organo-mercurial it must be concluded that it is formed from the adsorbed diradicals in a kinetic step. As the potential is made more cathodic, ie -2.lOV us see, less diradicals will be generated (see below) and those formed could suffer, prior to the kinetic step, a further reduction to an anion radical leading eventually to the formation of KzHg or to the hydrocarbon (RH,). These products can also result from direct reduction of bromobutane. In both cases a kinetic step involving an adsorbed monoradical and mercury will be present. As before, the kinetic step will compete with an electron transfer step leading to the -hydrocarbon. Scheme 1 summarizes the different stages of the mechanism proposed for the reaction.
Bromobutane BrRH
Dibutyl mercury RzHg
obs.
(a) Uncontrolled
not observed.
227
compounds
electrolysis for 1,4-DBB in DMF on mercury cathodes
Solid organomercurial R-Hg-(R-H&-R
high” -2.13 - 2.45 -2.50 high -2.10 - 2.50 -1.70 ~ 2.00
alkyl-mercury
26% 8%
3&50 V.
93% N.O. N.O. 24%
Cyclobutane
n-butane RHZ
Ref.
25%
75%
7 8
N.O. T-Z. N.O.
obs. N.O. 45% 7% 3%
6 5b this workb
228
L. A. AVACA, E. R. GONZALEZ ANDN. R. STRADIOTTO R-Hg-(R-Hg),-R
Scheme 1. General mechanism for the reduction of 1,CDBB
As the results in Table I indicate, the solid organomercurial is the only product formed at very low cathodic potentials. Mechanistically this means that under those conditions the electron transfer step o, in Scheme 1, is the preferred path for the reduction of the radical formed in step o. At more cathodic potentials, keeping all other conditions constant, the yield of solid falls considerably indicating the growing importance of step 0. This is confirmed by the results obtained using different supporting electrolytes (Table 1). Thus, changing the cation present in solution from Et,N+ to Lit will shift the potential required for the formation of anionic species to less negative values due to the formation of stronger contact ion-pairs between BrRand the corresponding cation[l3]. Consequently, the yield of solid will diminish. Whereas the same final effect will be observed if the conditions for the stabilization by adsorption of the diradicals are perturbed by the presence of Bu4N+ which is more strongly adsorbed than EtdN+ [14]. The fact that all these considerations apply equally well to the results obtained for 1,5-QBP suggests that the mechanism in Scheme 1 could be applicable to the reduction of cc,w-dibromoalkanes (o 2 4) in general. The behaviour of higher members of the series as well as the particular case of lJ-DBP are at present under investigation. Acknowledgements-The authors thank Dr. 0. R. Brown from Newcastle University, England, for helpful comments on the manuscript. They also thank the Funda$o de
on mercury.
Amparo 5r Pesquisa do Estado de SIo Paul0 (FAPESP), Brazil, for financial support. REFERENCES 1. J. H. Harwood, Industrial metallic Compounds, p. 60. 2. M. D. Morris, In Advances try, Vol. 7 (Edited by A. York (1974).
Applications of the OrganoReinhold, N.Y. (1963). in Electroanalytical
Chemis-
J. Bard). M. Dekker,
New
3. A. P. Tomilov and I. N. Brago, In Progress
in Electro(Edited by A. N. Frumkin and A. B. Ershler). Plenum, London (1971). 4. 0, R. Brown, H. R. .mirsk and 3. Thornton, Electrochim. Acta 16, 495 (1971). 5. 0. R. Brown and E. R. Gontilez, J. electroanal. Chem. 43, 215 (1973). 6. 1. Casanova and H. R. Rogers, J. Am. them. Sot. %, chemistry
of Organic
Compmds
1942 (1974).
:: 9. 10. 11.
M. R. Rifi, J. Am. them. Sot. 89, 4442 (1967). J. A. Dougherty and A. J. Diefenderfer, J. electroanul. Chem. 21, 531 (1969). H. 0. House, E. Feng and N. P. Peet, J. org. Chem. 36,237i (1971). W. F. Bryant and T. H. KinstIe, J. organometal. Chem. 24, 573 (1970). M. Stackelberg and W. Stracke, Z. Elektrochem. 53, 118 (1949).
12. 0. R. Brown and K. Taylor, J. electroanal. Chem. 50, 211 (1974). 13. L. A. Avaca and A. Bewick, J. elecctroanal. Chem. 41, 405 (1973). 14. J. Piro, R. Bennes and E. Bou Karam, 1. electroanal. Chem. 57, 399 (1974).
Acra.
1977, Vol. 22. pp, 225-228.
Pergamon RN.
Printed in tireat Britain
SYNTHETIC AND MECHANISTIC ASPECTS OF THE ELECTROCHEMICAL FORMATlON OF POLIMERIC ALKYL-MERCURY COMPOUNDS* L. A. AVACA, E. R. GONZALEZ and N. R. SmAr_uoTTot Instituto de Fisica e Quimica de SHo Carlos, Universidade de SHo Paulo, C.P. 369, 13560~SBo Carlos (SP), Brazil (Received
in jinal
form
12 JuZy 1976)
Abstract-The electrochemical reduction on mercury of a series of a, o-dibromoalkanes was studied at low cathodic potentials in aprotic media. i,4-dibromobutane and 1,5dibromopentane yielded solid
alkyl-mercury compounds, the structures of which were established as being polymeric and of the general type R-Hg-(R-Hgh-R (R = alkyl). Proper adjustment of the electrochemical parameters allowed very high current efficiencies for the syntheses. The results obtained for 1,Cdibromobutane. combined with those already existing in the literature, permitted the formulation of a general mechanism for the process.
INTRODUCTION Alkyl-mercury compounds have found several industrial and pharmacological applications[l, 21. Compounds of the type RzHg (R = alkyl) can be synthesized electrochemically by reduction of the corre sponding alkyl halides on mercury[3,4] or, under certain conditions, from the .!x,,w-dihalides[& 61. Studying the reduction of 1,4-dibromobutane on mercury Brown and Gonz#ez[S] reported that, in addition to the formation of dibutylmercury, solid butyl-mercury compounds were formed at low cathodic potentials. In general terms this reduction process can lead to a variety of products depending on the working conditions[5-81. The lack of a general mechanism and, in some cases, of proper control of the electrochemical parameters led to apparently conflicting results in the literature and also prevented the possibility of establishing the conditions for the selective synthesis of a given product. In this work the experimental conditions FOT the electrochemical synthesis of some solid alkyl-mercury compounds are described. A critical analysis of existing data is made in order to establish a relationship between electrode potential and product distribution. EXPERIMEFTAL Controlled potential electrolysis were carried out in divided cells designed to maintain the cathode at a uniform potential. A mercury pool (1.2 x 10-j m2) served as the cathode. The counter electrode was a carbon disk (4.1 x 1O-4 m2) enclosed in a concentric compartment separated by a medium porosity glass sinter. The reference electrode consisted of a silver wire immersed in a 0.01 M solution of AgN03 made * Presented in Dart at the 11 Latin-American Conference in Electrochemistq, October, 1974, Rio de Janeiro, Brazil. 7 Present address: Faculdade de Filosofia, Ci&cias e Letras de Ribeirk Preto, S.P., Brazil.
E.h22/3-A
up in 0.1 M solution of tetraethylammonium bromide (TEAB) in the solvent used for the catholyte. This
complex silver/silver bromide electrode will be referred to as Ag/AgBr. It was joined to the main compartment through a side arm and separated by the usual Luggin capillary and a liquid junction formed in a closed tap. The catholyte solutions were deaerated by a stream of pure nitrogen which also served to stir the catholyte during the runs. Cells were rigorously cleaned, rinsed and dried before use. Mercury was purified by the usual double distillation. The solvents, N,N-dimethylformamide (DMF) and acetonitrile (AN), were purchased from Merck (zur Synthese) and used without further purification. The water content of the solvents was considered acceptable for the purposes of this work. TEAB was synthesized from triethylamine (C. Erba) and ethyl bromide (C. Erba), recrystallized from ethanol and dried at 80°C under vacuum. Tetrabutylammonium perchlorate (TBAP) was synthesized and purified in a similar way[9]. Lithium perchlorate was prepared from lithium carbonate (Merck) and Analar perchloric acid, recrystallized and dried at 160°C under vacuum. Due to the difficulty of complete removal of water from this salt, solutions were dried with 4 8, molecular sieves (Merck). The substrates, 1,3-dibromopropane (lJ-DBP), 1,4-dibromobutane (l,cDBB) and 1,5_dibromopentane (l$DBP), were used as received from BDH after checking their purity by gas-liquid chromatography and refractive index measurements. The electrolyses were potentiostatically controlled by means of a Beckman Electroscan TM 30. Currents were graphically recorded as a function of time during the preparative runs. Elemental analyses were made in a Perkin-Elmer 240 instrument. Mass spectra were obtained using a Finnigan 10155/h spectrometer. Mercury was detected and measured by means of a Hillger-Watts H-l 170 Atomic Absorpti& Spectrometer. X-Ray diffractograms were obtained in a Debye-Scherrer type camera. 225
L. A. AVACA, E. R. GONZ~Z
226
A standard procedure was adopted for all preparative electrolyses. A current-potential curve was first recorded in a 0.1 M solution of the supporting electrolyte and then in the same solution made 0.3 M in the corresponding substrate. The electrolysis potential was then fixed at the foot of the reduction wave such as the residual current could always be neglected in comparison with the total current which varied between 2 and 30 A m-‘. Except for one of the substrates, electrolysis yieldeci a white insoluble solid which was separated, weighted and analysed. Since the purpose of this work was the synthesis and characterization of these solids which under proper conditions could be obtained in yields near lW?, no attempt was made to identify other products of the reaction. RESULTS
Table 1 shows a selection of experimental results from a number of preparative electrolyses of l,CDBB, 1,5-DBP and 1,3-DBP on mercury. The electrochemical parameters (el?trode potential, supporting electrolyte and solvent) were changed to maximize the yield of solids and to gain information on the mechanistic pathways of the reaction. Although only one typical result for 1,3-DBP is presented, the reduction of this compound was actually studied under different conditions failing to give any solid. The two solid compounds obtained as the final product of the electrolyses described in Table 1 did not show a definitive melting point but a gradual transition to the liquid state between 50 and 100°C. Routine solubility tests, using polar, non-polar, and mixed solvent systems, gave negative results in both cases. This fact prevented the use of colligative properties for direct molecular weight determination. Analyses of the solids showed the presence of carbon, hydrogen and mercury while nitrogen and bromine were absent, Percentual compositions were determined by elemental analysis for C and H, * The authors thank Dr. C. H. Williams from University of Campinas, Brazil, for running the mass spectra and helping with their interpretation. Table 1. Preparative Substrate (0.3 M)
Solvent
Electrolyte (0.1 M)
1 2 3 4 5
IA-DBB
DMF
TEAB
7
l,%DBP
Expt
6 : 10 11 12 13
1,3-DBP
AN DMF
* For details of the calculations
Table 2. Analysis of the solids Solid from
C H Hg
Solid from
Method
1,4-DBB (%)
1,5-DBP (“/
Elemental analysis Atomic absorption
18.50 3.40 78.00
22.36 4.06
Element
73.30
whereas Hg was independently measured by atomic absorption spectroscopy. The results for each of the solids are collected in Table 2. The mass spectra* of the solid obtained from 1,CDBB presented several clusters with the typical isotopic distribution for mercury. The base peak was m/e 56 (C,H,). The highest molecular weight fragment ion was m/e 572 (CI,H,,Hg2). The other organo-mercury ions present were m/e 258 (C4HpHg), m/e 314 (CsH16Hg) and m/e 516 (CsH16Hg,). For the solid obtained from 1,5-DBP a similar picture was observed, with the base peak being m/e 70 (C5H10) and the highest fragment m/e 614 (CISHl,,Hg2). Other peaks appeared at m/e 272 (CSHlOHg), m/e 342 (CJ%,Hg) and m/e 54.4 (ClOHZOHg& X-Ray diffractograms of both the solids showed them as being polycrystalline. This fact prevented the possibility of gaining direct additional information from this technique.
DISCUSSION
Synthetic
aspects
As can be seen from 1,3-DBP product,
Table 1, the reduction of on mercury failed to produce the expected while 1,CDBB and lJ-DBP gave solid com-
pounds which were very similar in their properties and, in both cases, clearly appearing to have an organometallic nature. Thus, the analyses showed the exclusive presence of C, H and Hg with percentage compositions corresponding to a relation alkyl: Hg very near 1 for both the solids, while the mass spectra presented several clusters of alkyl-Hg fragrnedts which for low values of m/e were sin@ to those electrolyses on mercury Potential V vs Ag/AgBr
Charge (C)
Wt. Solid (mg)
Yield* (%)
-
TEAB TEAB
1.20 1.40 1.50 1.30 - 1.20 - 1.30 - 1.30
39.6 176.5 580.1 64.8 34.9 59.1 34.9
52.0 116.0 164.1 33.7 28.2 59.4 43.0
98.4 49.4 21.2 39.0 60.6 75.4 86.8
TBAP LiClO, TEAB TEAB
- 1.50 1.60 - 1.50 - 1.60 -1.60 - 1.10
445.0 88.7 67.8 33.1 77.5 53.1
188.4 92.9 23.9 25.4 90.1
74.5 30.1 25.1 54.6 82.7
TBAP
AN DMF
AND N. R. STRADIOTTO
LICIO,
see Discussion section.
Electrochemical
formation
of polymeric
reported for the corresponding RzHg compound [lo]. In addition, the ions observed for higher values of m/e suggest fragments with a general formula -R-Hg-R-Hg-R(R = C,+Hs or CsH,,). A simple structure, identical with that corresponding to the heaviest fragment for each case, would lead to percentage compositions in appreciable disagreement with the experimental values. Therefore, and in accordance with a previous suggestion[SJ, it must be concluded that the most probable .structure for the solids synthesized will be R-Hg--(R-H&-R. Detailed calculations of the compositions as a function of n show that this number should be at least. 10 to reproduce, within the experimental error, the values presented in Table 2. On this basis, thd coulomb yields presented in Table 1 were calculated assuming 2 electrons per -R-Hg- unity formed. In addition to the linear polymeric structure presented other possibilities could be considered. Thus, a cyclic arrangement of the type (RHg), with n = 1, 2, . . . , efc will also give the experimental composition but only for n > 3 will it produce fragments as observed in the mass spectra. However, simple probability considerations will favour the formation of a linear structure. On the other hand, all other alkyl mercury compounds reported as the product of electrochemical syntheses are Hg(I1) derivatives, so it seems improbable that linear structures of the type R-Hg,-(K-K-Hg,),-R, with the metal appearing as Hg(I), could have been formed in this case. From a quantitative point of view, the results of the preparative electrolyses, which were highly reproducible, show that with proper adjustment of the electrochemical parameters very high yields of the polymeric alkyl-mercury compounds can be obtained. This is a much better situation than that normally encountered in electrosynthetic work.
Mechanistic
aspects
In order to understand the observed influence of the electrochemical parameters on the yields of solid organo-mercurials, a detailed knowledge of the mechanism of reduction of cc,o-dibromoalkanes on mercury is necessary. One way of gaining such knowledge will be by means
of a critical
Table 3. Examples
Electrolyte
Bu.,NClO, Me,NClO, Bu,NF,B Et+NF4B Et,NBr Et,NBr-
Potential V vs see
analysis
of existing
of preparative
obs. N.O. 98% 21%
electrode potential. Cell voltage: (b) Results reported as coulomb yields. (ohs) Qualitatively observed product.
(N.O.) Product reported as
data to find a possible correlation between product distribution and the experimental conditions. Table 3 shows the results of the present work integrated with those already reported in the literature for the reduction on mercury of 1,4-DBB in DMF solutions. For comparison purposes all potentials have been referred to a common standard. Except for the unconfirmed report about extensive formation of cyclobutane[7], each of the other products presented in Table 3 were obtained by more than one independent work under similar conditions. Therefore, the results in Table 3 can be taken as evidence for the existence of a definite correlation between the electrode potential and main product formed. As the potential changes from very low to high cathodic values, the preferentially formed product shifts from the left (solid organo-mercurial) to the right (n-butane) of the table. This fact should not be taken as meaning a stepwise mechanism since by their nature the products formed at low cathodic potentials (solid organo-mercurial and bromobutane) must originate from quite different intermediates. In accordance with the accepted mechanism for the reduction of the carbon-halogen bond[ll], the first electron transfer will produce a monohalide radical which will be stabilized by adsorption on the mercury surface[12]. Further reduction of this radical will lead to either a diradical, again stabilized by adsorp tion[5], or to an anion which will be rapidly prt>tonated to give bromobutane. Taking into account the established structure of the solid organo-mercurial it must be concluded that it is formed from the adsorbed diradicals in a kinetic step. As the potential is made more cathodic, ie -2.lOV us see, less diradicals will be generated (see below) and those formed could suffer, prior to the kinetic step, a further reduction to an anion radical leading eventually to the formation of KzHg or to the hydrocarbon (RH,). These products can also result from direct reduction of bromobutane. In both cases a kinetic step involving an adsorbed monoradical and mercury will be present. As before, the kinetic step will compete with an electron transfer step leading to the -hydrocarbon. Scheme 1 summarizes the different stages of the mechanism proposed for the reaction.
Bromobutane BrRH
Dibutyl mercury RzHg
obs.
(a) Uncontrolled
not observed.
227
compounds
electrolysis for 1,4-DBB in DMF on mercury cathodes
Solid organomercurial R-Hg-(R-H&-R
high” -2.13 - 2.45 -2.50 high -2.10 - 2.50 -1.70 ~ 2.00
alkyl-mercury
26% 8%
3&50 V.
93% N.O. N.O. 24%
Cyclobutane
n-butane RHZ
Ref.
25%
75%
7 8
N.O. T-Z. N.O.
obs. N.O. 45% 7% 3%
6 5b this workb
228
L. A. AVACA, E. R. GONZALEZ ANDN. R. STRADIOTTO R-Hg-(R-Hg),-R
Scheme 1. General mechanism for the reduction of 1,CDBB
As the results in Table I indicate, the solid organomercurial is the only product formed at very low cathodic potentials. Mechanistically this means that under those conditions the electron transfer step o, in Scheme 1, is the preferred path for the reduction of the radical formed in step o. At more cathodic potentials, keeping all other conditions constant, the yield of solid falls considerably indicating the growing importance of step 0. This is confirmed by the results obtained using different supporting electrolytes (Table 1). Thus, changing the cation present in solution from Et,N+ to Lit will shift the potential required for the formation of anionic species to less negative values due to the formation of stronger contact ion-pairs between BrRand the corresponding cation[l3]. Consequently, the yield of solid will diminish. Whereas the same final effect will be observed if the conditions for the stabilization by adsorption of the diradicals are perturbed by the presence of Bu4N+ which is more strongly adsorbed than EtdN+ [14]. The fact that all these considerations apply equally well to the results obtained for 1,5-QBP suggests that the mechanism in Scheme 1 could be applicable to the reduction of cc,w-dibromoalkanes (o 2 4) in general. The behaviour of higher members of the series as well as the particular case of lJ-DBP are at present under investigation. Acknowledgements-The authors thank Dr. 0. R. Brown from Newcastle University, England, for helpful comments on the manuscript. They also thank the Funda$o de
on mercury.
Amparo 5r Pesquisa do Estado de SIo Paul0 (FAPESP), Brazil, for financial support. REFERENCES 1. J. H. Harwood, Industrial metallic Compounds, p. 60. 2. M. D. Morris, In Advances try, Vol. 7 (Edited by A. York (1974).
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M. R. Rifi, J. Am. them. Sot. 89, 4442 (1967). J. A. Dougherty and A. J. Diefenderfer, J. electroanul. Chem. 21, 531 (1969). H. 0. House, E. Feng and N. P. Peet, J. org. Chem. 36,237i (1971). W. F. Bryant and T. H. KinstIe, J. organometal. Chem. 24, 573 (1970). M. Stackelberg and W. Stracke, Z. Elektrochem. 53, 118 (1949).
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