Electron equivalence

This is a very popular approach to ring synthesis and in most cases depends on reaction between a carbanion, or its electronic equivalent, and an appropriate substrate. It is convenient to discuss these syntheses sequentially according as to whether the C2 or the C2X unit fulfils the role of the nucleophile. 

In order for a substitution to occur, a n-complex must be formed. The term a-complex is used to describe an intermediate in which the carbon at the site of substitution is bonded to both the electrophile and the hydrogen that is displaced. As the term implies, a a bond is formed at the site of substitution. The intermediate is a cyclohexadienyl cation. Its fundamental structural characteristics can be described in simple MO terms. The a-complex is a four-7t-electron delocalized system that is electronically equivalent to a pentadienyl cation (Fig. 10.1). There is no longer cyclic conjugation. The LUMO has nodes at C-2 and C-4 of the pentadienyl structure, and these positions correspond to the positions meta to the site of substitution on the aromatic ring. As a result, the positive chargex)f the cation is located at the positions ortho and para to the site of substitution. 

Mild and reversible reduction of 1 12 and 2 18 heteropoly-molybdates and -tungstates produces characteristic and very intense blue colours ( heteropoly blues ) which find application in the quantitative determinations of Si, Ge, P and As, and commercially as dyes and pigments. The reductions are most commonly of 2 electron equivalents but may be of 1 and up to 6 electron equivalents. Many of the reduced anions can be isolated as solid salts in which the unreduced structure remains essentially unchanged and... 

B The fourth possibility arises in chiral molecules, such as (R)-2-butanol. The two — CH2- hydrogens at C3 are neither homotopic nor enantiotopic. Since replacement of a hydrogen at C3 would form a second chirality center, different diastereomers (Section 9.6) would result depending on whether the pro-R or pro-S hydrogen were replaced. Such hydrogens, whose replacement by X leads to different diastereomers, are said to be diastereotopic. Diastereotopic hydrogens are neither chemically nor electronically equivalent. They are completely different and would likely show different NMR absorptions. 

The design of a AA-size alkaline manganese dioxide cell is shown in Fig. 1 (Sec. 3.1). Primary and secondary alkaline batteries are constructed in the same way and can be manufactured on essentially the same machinery. The separator material, electrode formulation, and the Mn02 Zn balance are different. Rechargeable cells are zinc-limited to prevent a discharge beyond the first electron-equivalent of the MnOz reduction. The electrolyte is 7-9 mol L KOH. The electrode reactions are ... 

The positive hole requires further explanation. jr+ is the positive electronic equivalent of the electron, e-, in solids and they annihilate each other upon reaction ... 

If existence of a persulphide or other potentially electron accepting sulphur group is confirmed, this might explain why redox titration experiments have shown the number of electron equivalents which the xanthine oxidase molecule can accept to be greater than is required for reduction of the three non-protein components (58, 91). Certainly, this interpretation seems more probable than the original suggestion (58, 91) that the molybdenum can be reduced to lower oxidation states than Mo(IV) by some substrates. 

Owing to these nonelectrodic reactions, the apparent number of electron equivalents involved in the electrooxidation of Ph2SnH2 at —0.27 V is < 2. Reduction of the chloro-hydride Ph2SnHCl yields Ph2SnH2 as the main product ... 

The number of electrons exchanged in a redox reaction with an organic matter as electron donor is, therefore, determined only by the change in the oxidation level for carbon. The unit for this exchange (electron equivalent, e-eq) is ... 

The electron equivalent (e - eq) number for a redox process where organic matter is the electron donor is, therefore, the following ... 

The microbial activity of wastewater under anoxic conditions is lower compared with aerobic conditions (Abdul-Talib et al., 2001). This is important to consider, because a low nitrate uptake rate (NUR) compared with the oxygen uptake rate (OUR) in units of electron equivalents means a reduced transformation rate of the most biodegradable fractions of the organic matter. As mentioned under the point on injection of air, this may have implications in terms of treatment. Furthermore, a relatively low NUR value also has operational advantages because of a reduced demand for nitrate to suppress sulfide formation. 

The anion [Osg(CO)18p has an octahedral arrangement of metal atoms of approximately Oh symmetry, and is crystallographically very similar to the [HRus(CO)w]- ion. This collection of structural data on electron-equivalent systems emphasizes some of the dangers in trying to predict the structure of complexes solely on the basis of electron counting procedures (220). 

From Eq. (a) it is evident that each molecule of FeS04, upon oxidation, happens to lose one electron. Hence, one mole of FeS04 loses 6.02 x 1023 electrons which is equivalent to 1 Faraday or 96,500 C. Thus, in electrochemical determination of equivalence point the quantity of electricity is almost identical with that required to reduce 1 mole of Ce(S04)2. It follows from here that 1 mole of FeS04 and 1 mole of Ce(S04)2 are chemical equivalents. In other words, 1 g of H, acting as a reducing agent, loses electrons equivalent to 96,500 C. 

Fig. 8. Plots of the corrected electron equivalents, n = (C-Cb)/FN, passed during btdk electrolysis at controlled potential versus time. (I) 3.37 mM (NH4)AuCl4, 50 mM Na2S04 buffer, pH ca. 2, 0.4 M NaCl, = 320 mV ...

If we treat a and -spin electrons equivalently by choosing V, = Vp, so that... 

Fermi level to or hypothetical Fermi level of the metal ion transfer equilibrium i.e. the Fermi level of hypothetical electrons equivalent to the metal ion level in the ion transfer equilibrium. 

Apart from the further refinements of the BO approach, there has been a continuing interest in theoretically describing molecular systems with a method that treats the motions of both nuclei and electrons equivalently. This type of methodology has to entirely depart from the PES concept. It is particularly interesting how this type of approach describes the conventional notions of the molecular and electronic stmctures. In particular, the concept of chemical bonding, which at the BO level is an electronic phenomenon, has to be described in an approach departing from the BO approximation, as an effect derived from collective dynamical behavior of both electrons and nuclei. 

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