Electrolytic anodic coupling

The generation of radicals from carboxylate ions at the anode (Section 2.17.6, p. 115), and their coupling to form new carbon-carbon bonds is illustrated by the synthesis of hexacosane (Expt 5.11). The method has been usefully applied to the preparation of esters of long-chain carboxylic acids, from which of course the free acids may be prepared by hydrolysis. 

Simple anodic coupling by electrolysis in anhydrous methanolic solution (containing a little sodium methoxide) of methyl hydrogen adipate (Expt 5.147) gives dimethyl sebacate methyl hydrogen sebacate (Expt 5.147) in turn yields dimethyl octadecanedioate (Expt 5.131, cognate preparations). 

Electrolysis of a mixture of two carboxylic acids, R -COjH and R2-C02H, leads in addition to the products of normal coupling (R1 — R1 and R2 —R2) to the cross-coupled product (R1 -R2). Similarly if a mixture of a saturated carboxylic acid and a half-ester of an a, co-dicarboxylic acid is electrolysed, there are three main products, viz. a hydrocarbon (6), a mono-ester (7) and a di-ester (8). Normally the three products are readily separable by distillation. Furthermore, by increasing the molar proportion of the monocarboxylic acid, the yield of (7) is improved at the expense of (8). 

The unsaturated ester (9) is also often present in small quantity and arises from the loss of a proton from the intermediate carbocation (11), which is produced when the radical species (10) (which is involved in the coupling reaction) undergoes further anodic oxidation. 

Two alternative syntheses of methyl myristate, and thence myristic acid, are described (Expt 5.131). In Method A hexanoic acid (2mol) is coupled with methyl hydrogen sebacate (1 mol), the products being methyl myristate, decane and dimethyl octadecanedioate. 

---

Hydrocarbons and di-esters, otherwise rather Inaccessible in a pure state, are eonvenimitly prepared by electrolytic (anodic) sjmthesis. Thus simple coupling... 

The anodic coupling of aryl ethers is reviewed in Ref. [180]. Aryl ethers are more selectively coupled than phenols for the following reasons The carbon-oxygen coupling is made impossible and the ortho-coupling and the oxidation to quinones become more difficult. A mixture of triflu-oroacetic acid (TFA) and dichloromethane proved to be the most suitable electrolyte [181]. TFA enhances the radical cation stability and suppresses the nucle-ophilicity of water. Of further advantage is the addition of alumina or trifluo-roacetic anhydride [182]. Table 12 compiles representative examples of the aryl ether coupling. 

While XAS techniques focus on direct characterizations of the host electrode structure, nuclear magnetic resonance (NMR) spectroscopy is used to probe local chemical environments via the interactions of insertion cations that are NMR-active nuclei, for example lithium-6 or -7, within the insertion electrode. As with XAS, NMR techniques are element specific (and nuclear specific) and do not require any long-range structural order in the host material for analysis. Solid-state NMR methods are now routinely employed to characterize the various chemical components of Li ion batteries metal oxide cathodes, Li ion-conducting electrolytes, and carbonaceous anodes.Coupled to controlled electrochemical in-sertion/deinsertion of the NMR-active cations, the... 

A common photoelectrolysis cell structure is that of a semiconductor photoanode and metal cathode, the band diagrams of which are illustrated in Fig. 3.15 together with that of electrolyte redox couples. In Fig. 3.15(a) there is no contact between the semiconductor anode and metal cathode (no equilibrium effects communicated through the electrolyte). As seen in Fig. 3.15(b), contact between the two electrodes (no illumination) results in... 

The purpose of the present chapter is to summarize the electrochemical oxidation of hydrocarbons with special reference to the electrode processes involved. The reader may also find material of interest in Chapters 22 (Electrolytic oxidative coupling), 24 (Anodic substitution and addition), and 32 (Conducting polymers). 

The coupling sites and yields in the anodic coupling of the diarylalkanes (XXII) are strongly dependent on the value of n, the position of substituents in the aryl ring, the choice of the electrolyte-electrode system, the pH of the medium, the current density, and the oxidation potential. 

SACRIFICIAL ANODES - Coupling of a more active metal to a structure resulting in a galvanic current flow through the corroding electrolyte. 

In the above process, anodic coupling of the monomethyl ester of adipic acid takes place. The electrolyte is a 20% aqueous solution of monomethyl adipate, neutralised with sodium hydroxide. The anode is platinum-plated with titanium and the cathode is of steel. 

The t)q)e of supporting electrolyte, and particularly the anion in the case of anodic coupling, is most important. To give an example, tosylate is very helpful for the electrodepositions of polypyrroles (see Section 4.2.3.1) but detrimental to the polymerization of polythiophenes. For the latter the anion must be an innocent type such as perchlorate, tetrafluoborate or hexafluophosphate (see Section 4.2.2.1). 

That is, as shown in Figure 8 electrons can only be exchanged between the semiconductor and the electrolyte if the energy levels of the valence band overlap the occupied energy levels of the electrolyte redox couple (anodic current flow by hole injection), or if the conduction band overlaps the empty energy levels of the electrolyte redox couple fcathodic current flow by electron injection). 

Lockheed Missiles and Space Co., New Cathode-Anode Couples using Nonaqueous Electrolytes, Report No. ASD-TDR-62-837, Contract No. AF-33 (616)-7957 (1962). 

Ethylene glycol can be produced by an electrohydrodimerization of formaldehyde (16). The process has a number of variables necessary for optimum current efficiency including pH, electrolyte, temperature, methanol concentration, electrode materials, and cell design. Other methods include production of valuable oxidized materials at the electrochemical cell s anode simultaneous with formation of glycol at the cathode (17). The compound formed at the anode maybe used for commercial value direcdy, or coupled as an oxidant in a separate process. 

A.sahi Chemical EHD Processes. In the late 1960s, Asahi Chemical Industries in Japan developed an alternative electrolyte system for the electroreductive coupling of acrylonitrile. The catholyte in the Asahi divided cell process consisted of an emulsion of acrylonitrile and electrolysis products in a 10% aqueous solution of tetraethyl ammonium sulfate. The concentration of acrylonitrile in the aqueous phase for the original Monsanto process was 15—20 wt %, but the Asahi process uses only about 2 wt %. Asahi claims simpler separation and purification of the adiponitrile from the catholyte. A cation-exchange membrane is employed with dilute sulfuric acid in the anode compartment. The cathode is lead containing 6% antimony, and the anode is the same alloy but also contains 0.7% silver (45). The current efficiency is of 88—89%, with an adiponitrile selectivity of 91%. This process, started by Asahi in 1971, at Nobeoka City, Japan, is also operated by the RhcJ)ne Poulenc subsidiary, Rhodia, in Bra2il under Hcense from Asahi. 

---