Paneth reaction

The intriguing point is that the actual alkylation step may be the same at the anode and cathode, presumably by alkyl radicals which, in analogy to the Paneth reaction, alkylate the metal. The lifetime of the radical ion, reactivity of the radical ion or the radical towards the metal, stabilization of the radical by adsorption on the electrode surface, stabilization of each of the intermediates by solvation, their build-up in the double layer, the potential applied, all have an important contribution to the outcome. In certain cases the ET takes place catalytically, by a mediator or under the influence of surface effects17. It is therefore important to keep in mind the possible subtle differences between cases described below that otherwise appear similar. 

Lewandowicz A, J Rudzinski, L Tronstad, M Widersten, P Ryberg, O Matsson, P Paneth (2001) Chlorine isotope effects on the haloalkane dehalogenase reaction. J Am Chem Soc 123 4550-4555. 

On the other hand, the presence of a short-lived free radical was confirmed first from an elegant experiment in 1929 by Paneth and Hofeditz [44], although the existence of paramagnetic species was pointed out in the middle of the 19th century by Faraday [45]. When tetramethyllead was thermolyzed, a methyl radical was postulated to be formed as the reaction intermediate (Eq. 2)... 

Secondary isotope effects are small. In fact, most of the secondary deuterium KIEs that have been reported are less than 20% and many of them are only a few per cent. In spite of the small size, the same techniques that are used for other kinetic measurements are usually satisfactory for measuring these KIEs. Both competitive methods where both isotopic compounds are present in the same reaction mixture (Westaway and Ali, 1979) and absolute rate measurements, i.e. the separate determination of the rate constant for the single isotopic species (Fang and Westaway, 1991), are employed (Parkin, 1991). Most competitive methods (Melander and Saunders, 1980e) utilize isotope ratio measurements based on mass spectrometry (Shine et al., 1984) or radioactivity measurements by liquid scintillation (Ando et al., 1984 Axelsson et al., 1991). However, some special methods, which are particularly useful for the accurate determination of secondary KIEs, have been developed. These newer methods, which are based on polarimetry, nmr spectroscopy, chromatographic isotopic separation and liquid scintillation, respectively, are described in this section. The accurate measurement of small heavy-atom KIEs is discussed in a recent review by Paneth (1992). 

Table 10.7 KIEs on the reaction between p-substituted N,N-dimethylanilines and methyl iodide in ethanol at 25 °C (Paneth, P., et al. J. Phys. Org. Chem. 9, 35 (1996)) ...

Table 11.3 Kinetic isotope effects on Zea mays PEP-C catalyzed reaction at pH 7.5 (O Leary, M. H. and Paneth,...

The solution of Eq. (173) poses a rather formidable task in general. Thus the dispersed plug-flow model has not been as extensively studied as the axial-dispersed plug-flow model. Actually, if there are no initial radial gradients in C, the radial terms will be identically zero, and Eq. (173) will reduce to the simpler Eq. (167). Thus for a simple isothermal reactor, the dispersed plug flow model is not useful. Its greatest use is for either nonisothermal reactions with radial temperature gradients or tube wall catalysed reactions. Of course, if the reactants were not introduced uniformly across a plane the model could be used, but this would not be a common practice. Paneth and Herzfeld (P2) have used this model for a first order wall catalysed reaction. The boundary conditions used were the same as those discussed for tracer measurements for radial dispersion coefficients in Section II,C,3,b, except that at the wall. 

This experiment showed that some volatile component was formed in the thermal decomposition of tetramethyllead and that this compound consumed a cold lead mirror with formation of a volatile product. If, instead, a zinc mirror was first deposited and allowed to be consumed by the volatile product from decomposition of tetramethyllead, dimethylzinc could be identified as the product. Paneth concluded that free methyl radical was formed in the thermal reaction and could determine its half-life to be 0.006 seconds under the reaction conditions employed. Also, free ethyl radicals could be formed in... 

Lewandowicz A, Rudzinski J, Tronstad L, Widersten M, Ryberg P, Matsson O, Paneth P (2001) Chlorine Kinetic Isotope Effects on the Haloalkane Dehalogenase Reaction. J Am Chem Soc 123 4550... 

Fang, Y.-R. Gao, Y. Ryberg, P. Eriksson, J. Kolodziejska-Huben, M. Dybala-Defratyka, A. Madhavan, S. Danielsson, R. Paneth, P. Matsson, O. Westaway, K. C. Experimental and theoretical multiple kinetic isotope effects for an Sn2 reaction. An attempt to determine transition-state structure and the ability of theoretical methods to predict experimental... 

In support of the intermediate formation of the acetate radical from N-nitrosoacetanilide is the evolution of carbon dioxide during the reaction. Moreover, metals such as copper, zinc, lead, antimony, and iron are attacked when N-nitrosoacetanilide is allowed to decompose in a non-polar solvent such as carbon disulfide in the presence of the metals, a behavior similar to that of metals in contact with active free radicals in the Paneth test. 

Sicinska D, Truhlar DG, Paneth P (2005) Dependence of transition state structure on substrate the intrinsic C-13 kinetic isotope effect is different for physiological and slow substrates of the ornithine decarboxylase reaction because of different hydrogen bonding structures. J. Am. Chem. Soc. 127 5414-5422... 

The copper methyl is unstable at the temperature of reaction and soon decomposes to metallic copper and free methyl radicals, the latter being identified by their ability to wipe off a mirror of lead on glass (the Paneth test). This decomposition occurs in about 0.003 second... 

Thermal cracking of organic substances is an important reaction in the petroleum industry and has been extensively studied for over seventy years. At least for simple alkanes, the decay is first order in good approximation and therefore was long believed to occur in a single, unimolecular step [21]. However, in the 1930s, Rice and coworkers [22-24] established the presence of free radicals under the conditions of the reaction by means of the Paneth mirror technique [25,26], This observation led Rice and Herzfeld to propose a chain mechanism [22,27,28], Extensive later studies proved the essential features of their mechanism to be correct not only for hydrocarbons, but also for many other types of organic substances. 

---