Nucleophiles s. a. C-Hydrogen

N-nitrosourethans 17, 342 urethans 17, 342 Nitrous acid esters s. Alkyl nitrite Nitryl chloride 16,520 1 Nonanol as solvent 16, 728 Nuclei s. Heterocyclics, Isocyclics, Rings Nucleophiles (s. a. C-Hydrogen, active)... 

Pyridine compounds in which phosphorus is directly attached to a ring carbon are relatively rare. Phosphorus nucleophiles are not able to replace ring hydrogen atoms in pyridines and pyridine 1-oxides. Some time ago it was found that pyridine yields a zwitterion (170) when it is heated under reflux in the presence of tetraphosphorus decasulfide (Scheme 105) (68MI20500). Recently, a product that contains a C—P bond was isolated after extended heating under the same conditions followed by treatment with hydrochloric acid (81 JCR(S)285). However, it is uncertain whether free pyridine undergoes reaction in this case. Attack by phosphorus nucleophiles on salts is well established. Af-Methylacridinium methosulfate affords a stable isolable 9,10-dihydro adduct (171) that readily forms a... 

The reaction begins with an attat on the electrophile. HBr. by the eleC trons of the nucleophilic w bond. Two electrons from the s bond form a new T bond between the entering hydrogen and an alkene carbon, as shown the curved arrow at the top of Figure 6.10. The carbocation intemiediats that results is itself an electrophile, which can accept an electron pair fruni nucleophilic Br ion to form a C Br bond and yield a -neutral addition product. 

R2C—6. In the second resonance structure, C is electron-deficient, so carbonyl compounds are good electrophiles. Carbonyl groups with a-hydrogen atoms are relatively acidic compounds, because the carbanions produced upon removal of the H are stabilized by resonance with the carbonyl group 0=CR—CR2 O— CR=CR2. The enolate anions that are thereby obtained are nucleophilic at the a-carbon and on O. Under basic conditions, then, carbonyl compounds are electrophilic at the carbonyl C and nucleophilic at the a C s (if they have H s attached). All the chemistry of carbonyl compounds is dominated by this dichotomy. 

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