1.2- Elimination reactions, characteristics orientations

Characteristic features of this mechanism are that (i) the rate of the reaction does not depend on the concentration of the base and the kinetics are first order (in substrate) (ii) the reaction may not be stereospecific (iii) the elimination/substitution ratio is mostly independent of the leaving group (but in solvents of low ionization energy ion pairs are formed and then the ratio depends upon the leaving group) (iv) by-products are formed via rearrangements (v) the reaction is reversible (vi) generally the most stable alkene is formed (Zaitsev orientation see Section 5.1.2.5)... 

Advances in nanostructured conducting materials for DET have resulted in impressive current densities for the ORR, and application of these three-dimensional materials to DET from MCOs other than CueO may provide biocathodes with the characteristics suitable for an implantable EFC. While a DET approach using MCOs can provide for ORR at potentials approaching the thermodynamic reduction potential for oxygen, the current density achievable in this approach still relies upon intimate contact, and correct orientation, ofthe MCO to a conducting surface. Use of a mediator, capable of close interaction with the TI site of the MCOs, and with a redox potential tailored to permit rapid electron transfer to the TI site, can eliminate the requirement for direct contact in the correct orientation between MCO and electrode, and offer the possibility of a three-dimensional biocatalytic reaction layer on electrodes for higher ORR current densities. 

Autocatalysis. The kinetic curves developed by chemists in the U.S.S.R. (Section II,A,l,e) showing the dependence of the rate of gas evolution on time during the Chichibabin reaction revealed an interesting characteristic. It was observed that gas evolution began at a slow rate, followed by a sharp increase. This behavior was interpreted as evidence for the gradual accumulation of some compound in the reaction mixture during the induction period, which later catalyzed the amination process. The compound responsible was assumed to be simply the sodium salt of the aminoheterocyclic product. Indeed, introduction of such a sodium salt prior to the start of amination resulted in a rapid reaction with no observable induction period. The catalysis was theorized to result from a six-membered transition complex (17), which provides the required orientation of proton and hydride ion acceptors for hydrogen elimination. Proton abstraction should take place first, which then positions the transition complex structurally close to the dianionic (T-adduct (II). 

It has always been said in physical organic chemistry that a reaction mechanism is never proven, only conceivable alternatives are eliminated. It can also be said that spectroscopic structure determination doesn t prove a new structure, but only eliminates other conceivable alternatives. Only X-ray crystallography can prove a structure. Spectroscopy provides a means of examining the physical properties of a compound that are affected by its chemical bonding characteristics and relative spatial orientations of the constituent atomic units. Advances in spectroscopic structure determination reflect advances in instrumentation that enhance sensitivity and resolution, and the development of new techniques for extracting maximum spectral information. The combined features of enhanced sensitivity and enhanced resolution enable weak interatomic interactions to be sampled and cleanly observed. 

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