Lead bromide, electrolysis

Barba and coworkers [84-91] have published a number of papers dealing with novel syntheses based on the reduction of phenacyl bromides. Electrolysis of phenacyl bromide at a mercury cathode leads to an intermediate, which reacts to give 2,4-diphenylfuran [84]. However, when a proton donor (CH3OH) is present, the reduction of phenacyl bromide yields acetophenone and 2-bromo-l,3-diphenyl-3,4-epoxy-butan-l-one. Interestingly, if phenacyl bromide is slowly introduced into an electrolysis cell so that the unreduced starting material is maintained at a low concentration, the products are different [85] (Scheme 7). 

A demonstration of the electrolysis of a molten ionic binary compound con be carried out by the teacher in a fume cupboard. Suitable compounds include lead bromide (PbBr2< melting temperature 373 °C) or lead iodide (Pblj, melting temperature 402 C). 

On the last page, you saw that molten lead bromide decomposes when it conducts electricity. Decomposition caused by electricity is called electrolysis, and molten lead bromide is an electrolyte. 

The electrolysis Once the switch is closed, bromine vapour starts to bubble out of the molten lead bromide, around the anode. After some time a bead of molten lead forms below the cathode. The electrical energy from the battery has caused a chemical change ... 

Why the molten lead bromide conducts During the electrolysis, each lead ion takes two electrons from the cathode, as shown on the right. At the same time two bromide ions each give an electron to the anode. The effect is the same as if two electrons flowed through the liquid from the cathode to the anode. In other words, the lead bromide is acting as a conductor of electricity. 

For the electrolysis of molten lead bromide, draw diagrams to show ... 

The electrolysis of lead bromide can be investigated using the following apparatus. 

The electrochemical analysis allowed the determination of kinetic constants for this reaction46. Thus, in the presence of bromobenzene, the rate constant for the oxidative addition was found to be equal to about 70 M 1 s 1. The a-arylnickel complexes are unstable, except those obtained from o-tolyl or mesityl bromide as starting substrates. In these particular cases, the arylnickel complexes can be prepared by electrolysis from an ArBr/NiBr2(bpy) equimolar ratio. However, the exhaustive electrolysis of an aromatic iodide in the presence of ZnBr2, in DMF and at —1.4 V/SCE, leads to the corresponding arylzinc compound but the yield remains low (<20%). Indeed, the aryl iodide is mainly converted to ArH according to, very likely, a radical process (Scheme 11). 

Electrolysis is very important in industry. To help you to understand what is happening in the process shown in the photographs, we will first consider the electrolysis of lead(u) bromide. 

The break-up (decomposition) of lead(n) bromide into its constituent elements by the passage of an electric current is called electrolysis. 

Other interesting electrolytic systems have been developed based on electrolysis of triethylsulfonium bromide in acetonitrile with a lead cathode H8,ii9)j and electrolyses of aqueous solutions of sodium tetra-alkylboron compounds 245,337) Altogether, the breadth and depth of the research effort that has been devoted to the various electrolytic methods for the synthesis of tetraalkyllead compounds is remarkable. However, the successful development of a commercial process has not been accomplished, and indeed, it is probably less likely today than it was several years ago. 

As expected, fast addition of 2.2 equiv of Sml2 to sulfone 46 provides exclusively glycal 48 by elimination (rf an anomeric or-ganosamarium. In the presence of 16 equiv of HMPA, dimers 47 become the major products. This behavior of anomeric sulfones is unique in that, under the same circumstances, phenyl sulfone 50 or bromide 49 only leads to elimination. Previously, dimerization of anomeric radicals was only observed by photolysis (or electrolysis ) of the corresponding bromides or phenyl selenides. For example, irradiation of bromide 51 in benzene at 50 °C in the presence of 1 equiv of hexamethylditin yields 32% of dimers 53 (Fig. 28). ... 

A hydrotrope is a compound that helps other compounds normally not very soluble in water to dissolve in water.) The one used was (2.26). Pyrolysis of the carbamate (obtained in 90-96% yield) would lead to the isocyanate. This type of reaction can also be carried out in 95% yield using TV-bromosuccinimide and sodium methoxide in methanol or with bromine and sodium hydroxide.67 This suggests the possibility of generating the bromine or iodine during the reaction by electrolysis of sodium bromide or sodium iodide so that no waste salts result. The starting amides can be made by heating the ammonium salts of the acids. This... 

The replacement of OH on carbon adjacent to phosphorus by Br or I using allyl bromide or Mel in the presence of carbonyldiimidazole, has been reported. The electrolysis of diisopropyl (trichloromethyl)phosphonate in DMF containing the alkyl halide RX with a carbon cathode and magnesium anode affords diisopropyl (l,l-dichloroalkyl)phosphonates (132) interestingly, the corresponding diethyl esters may be reduced further. A similar reaction leading to (l-chloro-l-cycloalkyl)phosphonic diesters (133) has already been referred to. ... 

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