Electron-accepting

Amines are powerful nucleophiles which react under neutral or slightly basic conditions with several electron-accepting carbon reagents. The reaction of alkyl halides with amines is useful for the preparation of tertiary amines or quaternary ammonium salts. The conversion of primary amines into secondary amines is usually not feasible since the secondary amine tends towards further alkylation. 

For ionic reactivity two cases must be considered depending on the electron demand the thiazole ring may either be electron donating or electron accepting. 

As with pyridine acids, the presence of a positive center or an electron accepting group facilitates decarboxylation that must proceed by way of the anion. 

A group of aminoxanthenes, ie, pyra2oloxanthenes, is used as color formers ia pressure or heat-sensitive imaging papers (43). These compounds are colorless, but, upon contact with acidic electron-accepting material, are converted to resonance forms that are lightly colored. An example is stmcture [58294-05-6] (35), which forms upon the condensation of A[,A/-diethyl-y -aminophenol with phthalic anhydride, followed by addition of 6-hydroxyinda2ole ia 80% sulfuric acid (44). 

Phosphoms pentafluoride behaves as a Lewis acid showing electron-accepting properties. It forms complexes, generally in a ratio of 1 1 with Lewis bases, with amines, ethers, nitriles, sulfoxides, and other bases. These complexes are frequently less stable than the similar BF complexes, probably owing to stearic factors. Because it is a strong acceptor, PF is an excellent catalyst especially in ionic polymeri2ations. Phosphoms pentafluoride is also used as a source of phosphoms for ion implantation (qv) in semiconductors (qv) (26). 

Acid Halides (Lewis Acids). AH metal haUde-type Lewis catalysts, generally known as Friedel-Crafts catalysts, have an electron-deficient central metal atom capable of electron acceptance from the basic reagents. The most frequendy used are aluminum chloride and bromide, followed by... 

Weak to moderate chemiluminescence has been reported from a large number of other Hquid-phase oxidation reactions (1,128,136). The Hst includes reactions of carbenes with oxygen (137), phenanthrene quinone with oxygen in alkaline ethanol (138), coumarin derivatives with hydrogen peroxide in acetic acid (139), nitriles with alkaline hydrogen peroxide (140), and reactions that produce electron-accepting radicals such as HO in the presence of carbonate ions (141). In the latter, exemplified by the reaction of h on(II) with H2O2 and KHCO, the carbonate radical anion is probably a key intermediate and may account for many observations of weak chemiluminescence in oxidation reactions. 

In solutions, the concentration of available chlorine in the form of hypochlorite or hypochlorous acid is called free-available chlorine. The available chlorine in the form of undissociated A/-chloro compounds is called combined-available chlorine. Several analytical methods can be used to distinguish between free- and combined-available chlorine (8). Bleaches that do not form hypochlorite in solution like chlorine dioxide and nonchlorine bleaches can be characterized by thek equivalent available chlorine content. This can be calculated from equation 5 by substituting the number of electrons accepted divided by two for the number of active chlorine atoms. It can also be measured by iodomettic titration. 

The use of an electron-accepting counter ion leads to a photochromic system that is highly reversible under an inert atmosphere. An anion that has been used successfully is tetra-bis[3,5-di(trifluoromethyl)phenyl]borate anion [79250-20-9], C22H22BF 24 ( )-... 

Acidic Heterocycles. A similar classification is made for the acidic electron-accepting terminal groups used in dipolar (merocyanine) chromophores. The unsymmetrical dyes again incorporate the -dimethylarninophenyl group, coimected to the acidic group (Fig. 3) by one or three methine carbon atoms as in the merocyanine(9), n = 0 [23517-90-0]-, n = 1 [42906-02-5]-, n = 2 [66037-49-8]-, n = 3 [66037-48-7]. 

The reversed polarity of the double bond is induced by a n electron-accepting substituent A (A = C=0, C=N, NO2) the carbon and proton in the p-position are deshielded (-A/effect, larger shifts). These substituents have analogous effects on the C atoms of aromatic and heteroaromatic rings. An electron donor D (see above) attached to the benzene ring deshields the (substituted) a-C atom (-/ effect). In contrast, in the ortho and para positions (or comparable positions in heteroaromatic rings) it causes a shielding +M effect, smaller H and C shifts), whereas the meta positions remain almost unaffected. 

An electron-accepting substituent A (see above) induces the reverse deshielding in ortho and para positions (-M effect, larger //and shifts ), again with no significant effect on meta positions. 

Extent of bond lengthening increases with electron-accepting capacity of OR. 

For cyclopentanone, cyclohexanone, and cycloheptanone, the K values for addition are 48, 1000, and 8 M , respectively. For aromatic aldehydes, the equilibria are affected by the electronic nature of the aryl substituent. Electron donors disfavor addition by stabilizing the aldehyde whereas electron-accepting substituents have the opposite effect. 

Peifluorinated nitriles are highly electrophihc compounds because of the adjacent electron-accepting substituent Therefore, the addition of amines, alcohols, and mer-captans forming amidines, imidates, and thiobnidates is well-documented [63]. 

It is known that the ability of nitrotolane to cyclize depends on electronic factors (69MI2) hence l,3-dimethyl-4-nitro-5-phenylethynylpyrazole, whose acetylene group is in the most electron-accepting position of the pyrazole ring, i.e., favorable for nucleophilic addition, was introduced into the reaction of cyclization. Thus,... 

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