Addition to Nitrogen

In reactions that involve bond formation using the lone pair of electrons on the ring nitrogen, such as protonation and quatemisation, pyridines behave just like tertiary aliphatic or aromatic amines. When a pyridine reacts as a base or a nucleophile it forms a pyridinium , cation in which the aromatic sextet is retained and the nitrogen acquires a formal positive charge. 

Pyridines form crystalline, frequently hygroscopic, salts with most protic acids. Pyridine itself, with p/faH 

2 in water, is a much weaker base than saturated aliphatic amines which have p/faH values mostly between 9 and 11. Since the gas-phase proton affinity of pyridine is actually very similar to those of aliphatic amines, the observed solution values reflect relatively strong solvation of aliphatic ammonium cations this difference may in turn be related to the mesomerically delocalised charge in pyridinium ions and the consequent reduced requirement for external stabilisation via solvation. 

Electron-releasing substituents generally increase the basic strength 2-methyl- (pK 5.97), 3-methyl (5.68) and 4-methylpyridine (6.02) illustrate this. The basicities of pyridines carrying groups that can interact mesomerically as well as inductively vary in more complex ways, for example 2-methoxypyridine (3.3) is a weaker, but 4-methoxypyridine (6.6) a stronger base than pyridine the effect of inductive with- 

Heterocyclic Chemistry 5th Edition John Joule and Keith Mills 2010 Blackwell F iblishing Ltd 

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List two photoacceptors in addition to nitrogen dioxide that provide an initial source of free radicals. 

Thiadiazole and selenodiazole rings are isoelectronic with benzene and form a fully conjugated, essentially planar structure when fused to porphyrazines at the p-pyrrolic position (168). Compared to pc, the incorporation of sulfur and selenium in addition to nitrogen atoms to the peripheral heterocyclic rings affects the electronic charge density and influences the interactions between adjacent molecular stacks. 

Attack on Nitrogen. A variety of cyclic derivatives of phosphorous acid have been converted into spirophosphoranes (51), using diethyl azodicarboxylate as the condensing agent,42 probably by initial addition to nitrogen to give (50). Several... 

D. Addition to Nitrogen-, Oxygen- and Sulfur-Attached Alkynes.. 641... 

Hexasulphamide, S6NH2.—In the preparation of nitrogen sulphide, N4S4, from ammonia and sulphur monochloride, as already described (p. 283), the mother-liquor may, on concentration, yield hexasulphamide in addition to nitrogen pentasulphide. Hexasulphamide crystallises in colourless, square plates, melting at 105° C. It is insoluble in water... 

Reaction between 5-alkoxythiatriazole and triphenylphosphine, which is a weaker nucleophile, proceeds more sluggishly.18 At 25° triphenylphosphine reacted exothermically with 5-(4-chlorophenyl)thiatriazole in ether the products were those of total fragmentation, i.e., 4-chloro-phenyl cyanate and triphenylphosphine sulfide, in addition to nitrogen. Since thiatriazaphosphorines normally are stable at room temperature... 

Schlessinger has shown that the addition of ester enolates to sulfur stabilized acceptors, e.g. ketene di-thioacetal monoxide (151) and methyl a-(methylthio)acrylate (187), is highly efficient for the synthesis of Y-ketoesters.148 Similarly, Ahlbrecht and Seebach have reported that amide and ester enolate additions to nitrogen stabilized acceptors, e.g. nitroalkenes (40) and 2-(/V-methylanilino)acrylonitrile (59 Scheme 72), are highly efficient.149... 

Addition to Oxygen-containing Multiple Bonds. Addition to Nitrogen-containing Multiple Bonds.. . . ... 

In addition to nitrogen bases, the potential of chiral phosphanes as catalysts has also been assessed. Early work on the use of P-chiral phosphines in intramolecular... 

Reduction products of nitric acid, such as nitrogen and nitrogen oxides can also be formed as the result of oxidation processes. Niederer [148] found that N2 and NO were not formed in the nitration of nitro- to dinitrotoluene, while they were present in the nitration products of di- to tri-nitrotoluene. In addition to nitrogen and nitrogen oxide, carbon mono- and dioxide were formed in the last stage of nitration. Their amount and the CO to C02 ratio increased with temperature. 

In each of the five sections of Chapter 3, the chemistry is reviewed in the following order (1) Reactivity of aromatic rings (thermal reactions not involving reagents, substitutions at carbon, additions to nitrogen, metallations) (2) Reactions of nonaromatic compounds (this enormous area, which overlaps extensively with nonheterocyclic chemistry, is reviewed with emphasis on the heterocyclic aspects) (3) Reactions of substituents (with emphasis on situations in which substituents behave somewhat differently when attached to a heterocycle note that for benzene-fused heterocycles, the benzene ring is treated as a substituent). 

In the Example 2.5 for producing ammonia, the amount of air fed is set by the stoichiometric ratio of hydrogen to nitrogen for the ammonia feed stream. In addition to nitrogen and oxygen, the air... 

The presence of fluxes such as NaCl or Na2S04 during the decomposition of ammonium dichromate ensures that Cr203-pigments are formed in addition to nitrogen and water, instead of extremely fine powders. The isolation of ammonium dichromate can therefore be dispensed with by calcining mixtures of sodium dichromate and ammonium chloride or ammonium sulfate at 800 to 1000°C. 

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