Reversal of polarity

IJmpolung - reversal of polarity AJdrichimica Acta 1981, 14, 73 ACHE 1979, 18,239. 

Another cause of trouble is overheatingi especially during the early life of the tank. Above 70 C a reversal of polarity may take place,. the zinc becom-... 

The reversal of polarity of Zn and Fe which can occur in certain circumstances is discussed in Section 1.3. 

Strategy III Reversal of Polarity, Cyclisation Reactions, Summary of Strategy... 

This reversal of polarity at an atom, which is referred to as an iimpolung, cannot be effected directly on RCHO itself. The anion... 

Fig. Id represents the ionic changes and reversal of polarity of the membrane when the nerve is stimulated. Na+ ions enter the membrane ahead of the electrical charge and K+ ions pass out at the peak of the potential reversal.1 Fig. le shows how the ionic interchange is related to the action potential (or magnitude of polarity change). It must be stressed that the actual percentage changes of concentration are very small indeed. The exact nature of the restoration of the original concentration of ions is not completely known. Obviously a source of energy is required, and this is considered to be derived from the metabolism of the cell.

The benzoin reaction dates back to 1832 when Wohler and Liebig reported that cyanide catalyzes the formation of benzoin 6 from benzaldehyde 5, a seminal example in which the normal mode of polarity of a functional group was reversed (Eq. 1) [26], This reversal of polarity, subsequently termed Umpolung [27], effectively changes an electrophilic aldehyde into a nucleophilic acyl anion equivalent. 

Amiloride and triamterene-Am or 6e and triamterene not only inhibit sodium reabsorption induced by aldosterone, but they also inhibit basal sodium reabsorption. They are not aldosterone antagonists, but act directly on the renal distal tubule, cortical collecting tubule and collecting duct. They induce a reversal of polarity of the transtubular electrical-potential difference and inhibit active transport of sodium and potassium. Amiloride may inhibit sodium, potassium-ATPase. 

The use of neutral nucleophiles, such as ammonia and hydrazine, causes a reversal of polarity of one of the transitory dipoles, and these displacement reactions are therefore more favorable than with azide. On the other hand, the greater basicity of these reagents are more likely to cause elimination. 

Umpolung The reversal of polarity of the carbonyl carbon atom is termed umpolung (German for polarity reversal). Normally the carbonyl carbon atom of an aldehyde (or a ketone) is partially positive i.e., electrophilic and therefore it reacts with nucleophiles. When the aldehyde is converted to a dithiane by reaction with 1,3-propanedithiol and reacted with butyl lithium the same carbon now becomes negatively charged to react with electrophiles. This reversed polarity of the carbonyl carbon is termed umpolung which increases the versatility of the carbonyl group in synthesis. The sulphur atoms stabilize... 

Fig. 3. Formation of living ends by electrolysis followed by their destruction on reversal of polarity. Net charge transferred from development of first color...

Research began as an investigation of electrically induced ionic adsorption on porous "inert" electrodes. Electrode pairs based on carbon have been developed which will demineralize saline water at low voltage, and can be regenerated upon reversal of polarity. Various carbon electrodes have been conveniently classified into cation- and anion-responsive types. As received carbons are normally cation-responsive, but anion-responsive types have been made by chemical treatment. Laboratory demineralization cells based on this principle have been constructed and operated. Owing primarily to the low cost of basic construction materials, the process shows great promise for the economical conversion of saline waters. 

In particular, 1,3-dithiane prepared from dimethoxymethane (methylal) and pro pane-1,3-dithiol in the presence of boron trifluoride-etherate,237 and 2-alkyl-1,3-dithianes prepared similarly from aldehydes,2383 are important acyl anion equivalents. These and other uses are discussed in Sections 5.7.5, p. 596, and 6.6.1, p. 909. A wide-ranging review of the reversal of polarity of the carbonyl group through the formation of these sulphur-containing reagents has emphasised their value in organic synthesis.2388... 

To de-acidify orange juice it was also suggested using an ED stack composed of anionic membranes only and two compartments. In this way, only the anions, mainly citrate, were removed from the juice and replaced by the hydroxyl ions supplied by the KOH solution flowing in the adjacent compartments. Periodic reversal of polarity helped removal of the colloidal material deposited onto the membrane surfaces. Voss (1986) suggested two alternative ED processes. One was a three-compartment stack, obtained by arranging a cationic membrane and two anionic ones in sequence... 

Reversible hydrogen electrosorption is another kind of pseudofaradic effect which might be also observed with nanoporous carbons in aqueous medium. In this case, nascent hydrogen formed by water reduction under negative polarization of carbon is adsorbed in the material reversal of polarization provokes its desorption by oxidation [39],... 

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