2. preparative inorganic and organometallic electrochemistry advantages:
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DESCRIPTION2. Preparative inorganic and organometallic electrochemistry Advantages: extremely high oxidation and reduction power maintained by controlling the electrode potential ε no contamination derived from ox. /red. agents or by-products selectively driven electrode reaction by adjusting ε - PowerPoint PPT Presentation
2. Preparative inorganic and organometallic electrochemistry
Advantages: extremely high oxidation and reduction power maintained by controlling the electrode potential no contamination derived from ox. /red. agents or by-products selectively driven electrode reaction by adjusting in the case of some substances there are no other synthetic methods (e.g. CrH, layers of numerous metals and alloys)
Disadvantages: relative slowness special equipments
2.1. THEORETICAL BACKGROUND2.1.1. CURRENT EFFICIENCYFaraday law:
where m is the mass of product, M is its molar mass, z is the charge number of the electrode reaction, F is the Faraday constant (96487 C mol-1), I is the current intensity, t is the time of electrolysis
2.1.2. THE CURRENT VS VOLTAGE CURVE (Fig. 1)
2.1.3. THE ROLE OF THE OVERVOLTAGE (Fig. 2, Table 1, Table 2)In aqueous medium it is imperative to push back the undesired gas evolutionPossibilities to avoid it: Use of electrodes with high overvoltage with respect to formation of H2 and O2 E.g. Reduction of V(V)/V(IV) salts to V(III) or V(II):VO2+ + 2 H+ + e- = VO2+ + H2O0 = +1.0 VVO2+ + 2 H+ + e- = V3+ + H2O0 = +0.34 VV3+ + e- = V2+0 = -0.20 V Increase of the reactant concentrations Decrease of the temperature2.2. EXPERIMENTAL CONDITIONS2.2.1. SELECTION OF THE PROPER ELECTRODE
Criteria: inertness/starting material (see also Table 3) overvoltage (in aqueous solutions!) conductivity crystal structure shape2.2.2. SOLVENTS AND SUPPORTING ELECTROLYTESCriteria: conductivity In aqueous medium: mineral acids for acidic, alkaline metal hydroxides for basic, alkaline metal halogenides, sulphates, nitrates, perchlorates for neutral solutionsIn non-aqueous medium: perchlorates of R4N+, Na+ and Li+, furthermore, I-, Br-, PF6- and BF4- salts control of pH (big challenge in non-aqueous medium!) in the case of non-aqueous media there are often no counterpart of the electrochemical reaction in aqueuos medium, more resistant to oxidation/reduction, wider range of (Table 4)
2.2.3. THE ELECTRODE POTENTIALupon changing the reaction belonging to the most positive potential occurs in the cathode and most negative in the anode, resp.reference electrode rquired for precise measurement of (most widely used: in aq. sol. calomel, in acetonitrile Ag/Ag+(0,01M) el.; Table 5, Table 6, Fig. 3)
2.2.4. THE EFFECT OF THE ELECTROLYTE CONCENTRATIONE.g. anodic electrolysis of cold H2SO4 sol. on Pt (Table 7); 3 reactions:2 H2O = O2 + 4 H+ + 4 e-2 SO42- = S2O82- + 2 e-3 H2O = O3 + 6 H+ + 6 e-(at 0C the last one can be neglected)
2.2.5. THE EFFECT OF THE TEMPERATUREresistivity of the cell, changes of the reaction rates (Table 8)
2.2.6. DEPOSITION OF METAL COATINGSInfuenced by: current density (low j: coarse crystals, high j: finely particled crystals, very high j: tree-like sructures + H2 evolution, staining) electrolyte concentration (sufficiently high required) temperature (has effects of different direction and extent) additives (e.g. sugars, camphor, gelatine, glue, casein; their adsorption on the surface may result fine-particled coating) formation of metal complexes (e.g. deposition of Ag from AgNO3 sol.: badly adherent, coarse-particled coating, however, from [Ag(CN)2]- sol.: smooth durable coating)
2.2.7. METHODS OF THE ELECTROLYSIS (Fig. 4) const. U (= IR + , where U is the applied voltage, is the potential difference between the working and auxiliary electrodes, I is the current intensity, R is the cell resistivity); advantageous when only little amount of the electroactive component is deposited const. I (Fig. 5) const. (= potential difference between working and reference electrodes; advantage opposite to the former two: selectivity, disadvantage: sophisticated equipment)2.2.8. CELL DESIGNS (Fig. 6, 7 and 8) undivided compartment (no diaphragma) divided compartment (diaphragma between the working and auxiliary electrodes; usually more advantageous)Interesting case The product of the cathodic reduction of benzene in methyl amine sol. containing LiCl is 1,4-cyclohexadiene in undivided cell cyclohexene in divided cell.
2.3. ELECTROLYTIC SYNTHESES2.3.1. Preparation of gasesH2, O2: yielded in high purity by the electrolysis of KOH sol. of 30% O3: O3/O2 mixture of composition of 12.5% obtained by anodic current of high density from H2SO4 sol. of 1.08 g cm-3 below temperature of -10C; much higher yield from HClO4 sol. Of 40% at -56C Cl2: pure, O2-free gas from HCl of higher conc. than 23% by medium j F2: by the electrolysis of KF/HF GeH4: on Pb cathode from ice-cooled H2SO4 sol. containing GeO2 SbH3: on Pt-Ir cathode at 0C from a H2SO4 sol. (1.7 L of vol., 2 M of conc.) cont. 8 g of Sb and 80 g of tartaric acid PbH4, BiH3: give formation in lower conc. on Pb or Bi cathode from H2SO4 sol.2.3.2. Deposition of metalsIn a fairly pure state (C-, S-, P- and N-free) metals can be obtained from their salt sol.-s by cathodic reduction: most easily the more noble metals, under certain conditions from aq. sol.: Zn, Cd, Ga, In, Tl, Pb, Sb, Ni, Co, Fe, Mn, Cr from aq. citrate sol.-s: Mo, W, Ta and Nb, on mercury cathode (H2 overvoltage! + form. of amalgam) from aq. sol.-s the alkali and alkali earth metals
Non-aqueous solvents more rarely give good results, however, e.g. very pure Li from LiCl sol. of abs. pyridine or acetone, Na, Sr, Cd, Sn, Sb, Bi and As from electrolyte sol. of acetone, La, Nd and Ce from their salts of abs. ethanolic sol. on Hg cathode, and Al from xylenic sol. of its organometallic compounds, resp.can be obtained.
2.3.3. Cathodic reductions without metal depositionSmooth metal surfaces as cathode (H2 overvoltage!) are used (e.g. Hg, Pb, Tl, Zn, Cd and Sn, furthermore their amalgams).Especially applied for the preparation of hydrides, organometallics and compounds bearing metal of low oxidation state (e.g. NH4V(SO4)2.12 H2O, (NH4)2V(SO4)2.6 H2O, CrSO4.5 H2O, Ag2F, K3MoCl6, K3W2Cl9, EuSO4, YbSO4).
2.3.4. Anodic oxidationsUnless the metal of the anode is oxidized, smooth Pt or Pt-Ir (high O2 overvoltage + resistivity to oxidation) and, in sulphuric acid, Pb anodes are preferred.Examples: Cu2O, AgO, (NH4)2S2O8, KClO3, KClO4, KBrO3, Co2(SO4)3.18 H2O, BaFeO4.H2O, Pb(OCOCH3)4, (NH4)2PbCl6.
2.3.5. Electrolysis of molten saltsFrequently applied in industry (e.g. large-scale preparation of Na, K, Be, Mg, Ca and Al), however, in general rarely in the lab due to its uncomfortness and insufficient yields.It has greater importance in the laboratory-scale preparation of pure Li, Ta, Th, U, La and other rare earth metals. The molten electrolyte is usually a halogenide, especially some fluorides (capable of dissolving the oxide well). It is advantageous to carry out the electrolysis at the lowest temperature (e.g. use of eutectic mixture of salt) but above the melting point of the metal.Problems arise if the deposited metal dissolves back into the molten salt or react with the anode gas (e.g. Cl2, CO or fluorine compounds). Solution: efficient separation of the cathode and anode space, use of a guard tube around the anode.The crucible for the electrolysis is usually made of porcelain, glass, quartz, corundum or graphite.
2.3.6. Preparation of organometallic compoundsThe species generated in the course of the electrolysis of organic compounds often result in the formation of organometallic compound when reacting with the electrode metal. In numerous cases, however, new derivatives can be obtained by the electrolysis of organometallics.Cathodic methods (radical mechanism; R is an organic group, M is a metal):R+ + e- RR + M RMAnodic methods (the organometallic often prepared by the anodic oxidation of another one: indirect metallation):R- R + e-R + M RMPossible side-reactions: dimerization of R or cleavage of H atom.Problem: medium has low electric conductivity!E.g. the R2M organometallics (M = Mg, Be, Zn, Cd, Hg) are not only bad conductors in pure state but in their sol. with donor-type solvents (e.g. ethers) as well.Solution: use of salts, metal hydrides and organomet. reagents as additiviesE.g. addition of EtNa to Et2Zn a salt-like adduct forms: NaZnEt3 Na+ + ZnEt3-
18.104.22.168. Cathodic reductions
22.214.171.124.1. Alkali and alkaline earth metal compoundsUsually they are generated in situ in sol. for the preparation of organic compounds.There are more efficient methods than the electrolytic ones. Electrolytic initiation of anionic polymerizations, e.g. Cathodic red. of 1,1-diphenyl ethylene in CaI2/HMPA medium2 Ph2C=CH2 + Ca2+ + 2 e- (Ph2CCH2CH2CPh2)2-Ca2+
126.96.36.199.2. Reduction of onium-type cationsMain application fields: Preparation of organic amalgams via electroreduction of onium ionsR4N+ + e- + Hg R4N/Hg(cf. Na+ + e- + Hg Na/Hg)Organic amalgams bear metallic characters as well.Requirements of the successful synthesis: in gen. T below 0C, solvents being less sensitive to reduction than waterE.g. tetramethyl ammonium amalgam (silverish-white crystalline substance, very reactive, liberates Me3N at room temp.) can be obtained from the red. of abs. ethanolic sol. of Me4N+Cl- on Hg cathode at temp. of. -10C. The analogue S- or P-containing amalgams have similar properties.
Electrolysis of onium ions with the participation of the cathode material as reactantAbove room temp., following the neutralization of the onium cation, an organometallic compound forms by the cleavage of the hetero atom-carbon bond (A = hetero atom, e.g. S, P, I, Sb, N; M = electrode metal, e.g. Hg, Pb; R = organic group)RnA+ + e- + M RM + Rn-1AE.g. Reduction of the aq. sol. of the salt BzMe2S+Tos- on Hg cathode at 90C gives crystalline Bz2Hg in 94% yield (Bz benzyl, Tos tosylate). Electrolysis of organometallic on