Replacement of halogen by nitrogen

M ercuryj nitrogen d i oxi d e I oxygen Preferential replacement of halogen by oxygen Carboxylic acid fluorides from polyhalides... 

As noted earlier, the replacement of halogen by an amino group (a substitution reaction at carbon and at nitrogen [ ]) is generally not useful for the preparation of primary (RNH2) or secondary (R2NH) amines since the alkylamines are stronger bases than ammonia. However, the substitution reaction is useful for tertiary (R3N) amines, for example, the A-methylation of pyridine with methyl iodide (Equation 10.48) and the formation of other quaternary ammonium salts (cf. the Menschutkin reaction. Table 7.7, item h). 

In general, in Part II we apply the same pattern of analysis to the numerous published vibrational spectra derived from the adsorption of alkynes, alkanes, and aromatic hydrocarbons. In addition, we summarize recently obtained spectroscopic results characterizing hydrocarbon species obtained by thermal, photochemical, or electron-bombardment dissociation of halogen- or nitrogen-substituted alkanes on single-crystal metal surfaces. The hydrocarbon surface species so obtained are normally as anticipated from the replacement of the heteroatoms by surface metal atoms. The... 

The photocyclizations of halogenated A-benzyl-/ -phenethylamines576 are examples of reactions in which two aryl rings are connected by a chain of four atoms, one of which is a nitrogen atom. In the cases reported, one phenyl ring has a bromine atom and the other an iodine atom at the ortho position. As expected, products were formed via initial rupture of the carbon-iodine bond and these products still contained the bromine atom. In addition, however, some unexpected cyclization products were encountered, containing iodine instead of bromine. The formation of these products was ascribed to replacement of bromine by iodine in the intermediate cyclohexadienyl radicals. 

By contrast, the nitrogen atom makes pyridines more reactive towards nucleophilic substitution, particularly at the 2- and 4-positions, by lowering the LUMO energy of the 7t system of pyridine. You can see this effect in action in the ease of replacement of halogens in these positions by nucleophiles. 

The reverse order of replacement of halogens, i.e. preferential substitution of chlorine rather than of bromine by hydrogen in bromochloro compounds, is possible if chlorine is activated by its position in the molecule. For example, 5-bromo-6-chloro-2,4-dimethylpyrimidine, in which chlorine is rendered labile by its a-position to one of the ring nitrogens, is reduced to 5-bromo-2,4-dimethylpyrimidine by refluxing with p-toluenesulfonylhydrazine in chloroform (equation 28). ... 

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