Partial positive charges

Partial positive charge Partial negative charge... 

Because of its aza-aromatic character the thiazole ring is little inclined to react with w-acceptors. Comparing thiazole and pyridine, it appears that a positive charge (partial or total) is less stabilized... 

The Claisen condensation is initiated by deprotonation of an ester molecule by sodium ethanolate to give a carbanion that is stabilized, mostly by resonance, as an enolate. This carbanion makes a nucleophilic attack at the partially positively charged carbon atom of the e.ster group, leading to the formation of a C-C bond and the elimination ofan ethanolate ion, This Claisen condensation only proceeds in strongly basic conditions with a pH of about 14. 

More recent developments are based on the finding, that the d-orbitals of silicon, sulfur, phosphorus and certain transition metals may also stabilize a negative charge on a carbon atom. This is probably caused by a partial transfer of electron density from the carbanion into empty low-energy d-orbitals of the hetero atom ( backbonding ) or by the formation of ylides , in which a positively charged onium centre is adjacent to the carbanion and stabilization occurs by ylene formation. 

Alkyl halides and sulfonates are the most frequently used alkylating acceptor synthons. The carbonyl group is used as the classical a -synthon. O-Silylated hemithioacetals (T.H. Chan, 1976) and fomic acid orthoesters are examples for less common a -synthons. In most synthetic reactions carbon atoms with a partial positive charge (= positively polarized carbon) are involved. More reactive, "free carbocations as occurring in Friedel-Crafts type alkylations and acylations are of comparably limited synthetic value, because they tend to react non-selectively. 

The symbols and indicate partial positive and partial negative charge respectively)... 

Recall that the carbon atom of carbon dioxide bears a partial positive charge because of the electron attracting power of its attached oxygens When hydroxide ion (the Lewis base) bonds to this positively polarized carbon a pair of electrons in the carbon-oxygen double bond leaves carbon to become an unshared pair of oxygen... 

Carbon-oxygen and carbon-halogen bonds are polar covalent bonds and carbon bears a partial positive charge in alcohols ( " C—0 ) and in alkyl halides ( " C—X ) Alcohols and alkyl halides are polar molecules The dipole moments of methanol and chloromethane are very similar to each other and to water... 

Electron delocalization m allylic carbocations can be indicated using a dashed line to show the sharing of a pair of rr electrons by the three carbons The structural formula IS completed by placing a positive charge above the dashed line or by adding partial pos itive charges to the carbons at the end of the allylic system... 

Turning now to electrophilic aromatic substitution in (trifluoromethyl)benzene we con sider the electronic properties of a trifluoromethyl group Because of their high elec tronegativity the three fluorine atoms polarize the electron distribution m their ct bonds to carbon so that carbon bears a partial positive charge... 

Carbon is more electronegative than metals and carbon-metal bonds are polarized so that carbon bears a partial to complete negative charge and the metal bears a partial to complete positive charge... 

Alkyl groups under nonacidic conditions sterically deflect nucleophiles from C, but under acidic conditions this steric effect is to some extent offset by an electronic one the protonated oxirane opens by transition states (Scheme 40) which are even more 5Nl-like than the borderline Sn2 one of the unprotonated oxirane. Thus electronic factors favor cleavage at the more substituted carbon, which can better support a partial positive charge the steric factor is still operative, however, and even under acidic conditions the major product usually results from Cp attack. 

As will be indicated when the mechanism is discussed in more detail, discrete carbocations may not be formed in all cases. An unsymmetrical alkene will nevertheless follow Markownikoff s rule, because the partial positive charge that develops will be located preferentially at the carbon that is better able to accommodate the electron deficiency, that is, the more substituted one. 

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