Xn:Harmful;5465-13-4Relevant articles and documents
Inverted potentials in two-electron processes in organic electrochemistry
Evans, Dennis H.,Hu, Kai
, p. 3983 - 3990 (1996)
Many molecules can accept or lose electrons in two sequential one-electron steps. Normally, gain or loss of the second electron occurs less readily than the first, which gives rise to two separate one-electron processes detected by voltammetry. In this instance, the intermediate (one-electron product) is stable with respect to disproportionation. There are cases known, however, in which the gain or loss of the second electron occurs more easily than the first, leading to a single two-electron voltammetric process. Here, the standard potentials are inverted with respect to their normal order and the one-electron intermediate is unstable with respect to disproportionation. Semiempirical molecular orbital calculations (AMI) have been used to compute disproportionation energies for a variety of aromatic hydrocarbons and the results were found to be remarkably similar to those calculated for charging spheres in vacuum. Experimental values of the disproportionation Gibbs energies in solution, calculated from the difference in potential for cases which show normal ordering, have been used to develop an empirical relation for the attenuation of the disproportionation energy on going from vacuum to solution. This relationship was then used to predict and/or rationalize cases where inversion or compression of potentials has been observed for hydrocarbons in the solution phase. A similar approach was used for other classes of molecules. Here, only a single model compound with normal ordering of potentials was used to predict the effect of solvation on the disproportionation energies for structurally related species. In general, the approach is quite successful in predicting and/or rationalizing the occurrence of inversion of potentials. The reduction of 3,6-dinitrodurene was predicted to occur with inversion and this was verified by cyclic voltammetric studies.
Electrophilic Aromatic Substitution. Part 27. Chemical Selectivities Disguised by Mass Diffusion. Part 6. The Kinetics of Nitration in Aqueous Sulphuric Acid of Durene (1,2,4,5-Tetramethylbenzene). Nitrodurene, and Nitroprehnitene (Nitro-1,2,3,4-tetramethylbenzene). A Comparison ...
Manglik, Ajay K.,Moodie, Roy B.,Schofield, Kenneth,Dedeoglu, Erol,Dutly, Andreas,Rys, Paul
, p. 1358 - 1366 (2007/10/02)
Durene (1,2,4,5-tetramethylbenzene) is nitrated in sulphuric acid at the encounter rate.Nitrations of nitrodurene and nitroprehnitene (nitro-1,2,3,4-tetramethylbenzene) are complicated by the formation of nitrous acid, presumably as a consequence of ipso-attack, and subsequent undefinied reactions of this.When an efficient nitrous acide trap is present the complications are removed and the kinetics become straightforward.Although nitrobenzene is 108 times less reactive than benzene in nitration, nitroprehnitene and nitrodurene are only 41 and 20 times less reactive than their respective parent hydrocarbons.These reduced relative reactiviities are a consequence of the fact that prehnitene and durene react at the encounter rate.The low relative reactivity of durene and 3-nitrodurene leads to the formation of some 3,6-dinitrodurene in the nitration of durene, even under the most favourable circumstances, and if mixing is inefficient the dinitro-compound may be the main product.By measuring the yield of 3-nitrodurene, as it varies with acidity, it is possible to determine the amount formed by direct attack at C-3 as distinct from that formed by ipso-attack followed by rearrangement.As a consequence the ratio of positional selectivity between C-3 and C-1 is shown to be 1 : 3.6.Thus, positional selestivity does not disappear.The intrinsic rate constants for nitronium ion nitrations in sulphuric acid of a number of methylnitrobenzenes show an excellent linear correlation with those for nitrations with nitronium hexafluorophosphate in nitromethane deduced by application of a theoretical mixing-reaction model.Differences between the two systems are not large, but appear to be in the direction showing the electrophile in sulphuric acid to be rather more reactive and more selective than in the organic solvent.
