Tafel rearrangement

Reduction of p-ketoesters in aqueous ethanolic sulphuric acid leads to removal of both functional groups and the formation of a hydrocarbon. This reaction which was discovered in 1907 [117] and recognised in 1912 [118] as involving a skeletal rearrangement is now termed the Tafel rearrangement. Conversions of the type 26 into 27 occur in 30 - 60 % yields [118,119] and the hydrocarbon is easily separated... 

The Tafel rearrangement only occurs in acid medium. Simultaneous reduction of both carbonyl groups leads to interaction and formation of a cyclopropane. Acid catalysed cyclopropane ring opening follows to yield an a-diketone 28 which undergoes the electrochemical Clemmensen reduction step to the hydrocarbon. Side products include the two monoketones derived by partial deoxygenation of the a-diketone and the secondary alcohols from reduction of these raonoketones. Separate experiments show that the a-diketone 28 can be reduced to the hydrocarbon. 

TAFEL REARRANGEMENT. Rearrangement of the carbon skeleton of substituted acetoacetic esters to hydrocarbons with the same number of carbon atoms by electrolytic reduction to a lead cathode in alcoholic sulfuric acid. 

Intermediate formation of a cyclopropyl ring has also been suggested in electrochemical reductions although to a lesser extent. 2-Alkyl-substituted acetoacetic esters undergo the Tafel rearrangement upon reduction at a lead cathode in an acidic medium with complete reduction of the functional groups Analysis of the reduction products... 

The Tafel rearrangement only occurs under acid conditions. A satisfactory reaction mechanism was established in 1976 [122]. In acid medium it is proposed that reduction of carbonyl groups is followed by reaction of intermediates to form a hydroxycyclopropa-none. Acid-catalyzed ring opening then yields a a-diketone XXIX. This last intermediate is... 

Tafel rearrangement. Rearrangement of the carbon skeleton of substituted acetoacetic esters to... 

The electrochemical reduction of ethyl a-butylacetoacetate yields products the nature of which implies that the Tafel rearrangement proceeds via a cyclopropane intermediate. A theoretical study of the rearrangement (378)- (380) indicates that (379) could be on the energy surface. Further studies on the interconversion... 

Swem Oxidation Synthol Process Tafel Rearrangement... 

Tafel-type rearrangement was found to occur in the cathodic reduction of p-diketones. This is in agreement with the involvement of cyclopropanediol intermediates in this reduction ... 

The rate-controlling step was found to be that represented by Eq. (30) when the Tafel relation holds (low polarization region) or by Eq. (27) for the limiting current (high polarization region) (241). The subsequent steps to (30) must conform to the experimental result of nondetection of HOJ ion in the electrolyte. The rearrangement of adsorbed HO2 on the electrode surface with formation of OH" in a manner such as represented by Eq. (31) has been proposed (240) ... 

The graphical construction performed in Case Study 3.1 may also be carried out in Case Study 3.2. Below is the derivation for the corrosion rate, J<.orr- Rearranging Eq. (3.28), the Tafel kinetics for the anodic and the cathodic processes can be represented as ... 

The anodic and cathodic Tafel slopes are b and be- Assigning a positive sign for anodic Tafel slope, b, and negative sign for the cathodic Tafel slope, b, and rearranging Eqs. (5.10) and (5.11), one obtains ... 

Another approach to obtain 4on- and Econ is to use the Tafel extrapolation method. Here a sufficiently large polarization from Ecorr is used so that one or the other of the terms in Equation 15.6 is negligible. For a positive (or anodic) polarization when (E—Ecorr) > 0> the second term in Equation 15.6 is negligible (recall that Be is negative) and the equation may then be rearranged to give an expression relating the anodic polarization rjd to the anodic current (ij ... 

The second term of the expression (C dEldt) approaches 0 at low voltage scan rates dEldt. Steady state is desirable, since the goal is to obtain at E, Note that t app becomes approximately equal to either or at large Ti, where tp = (E-E ). At very large anodic or cathodic overpotentials, Eq 19 can be rearranged in the form of the Tafel expression [1-3,5,7,18,19]... 

Therefore, with high losses at a given electrode, one of the branches can be neglected, leaving the Tafel kinetics model. This can be rearranged to provide an explicit expression for as a function of cell current ... 

---