Electrophilic Addition at Nitrogen

The corresponding equation for a base involves the dissociation of the conjugate acid of the base, so we 

Heterocycles which contain an imine unit (C=N) as part of their ring structure, pyridines, quinolines, isoquinolines, 1,2- and 1,3-azoles, etc., do not utilise the nitrogen lone pair in their aromatic tr-system (cf. 

2) and therefore it is available for donation to electrophiles, just as in any simpler amine. In other words, such heterocycles are basic and will react with protons, or other electrophilic species, by addition at nitrogen. In many instances the products from such additions - salts - are isolable. 

Heterocyclic Chemistry 5th Edition John Joule and Keith Mills 

For the reversible addition of a proton, the position of equilibrium depends on the p/faH of the heterocycle, and this in turn is influenced by the substituents present on the ring electron-releasing groups enhance the basicity and electron-withdrawing substituents reduce the basic strength. The p/faH of simple pyridines is of the order of 5, while those for 1,2- and 1,3-azoles depend on the character of the other heteroatom pyrazole and imidazole, with two nitrogen atoms, have values of 2.5 and 7.1, respectively. 

Other factors can influence the rate of quatemisation all the diazines react with iodomethane more slowly than does pyridine. Pyridazine, much more weakly basic (pA a 2.3) than pyridine, reacts with iodomethane faster than the other diazines, a result which is ascribed to the a effect , i.e. the increased nucleophilicity is deemed to be due to electron repulsion between the two immediately adjacent lone pairs. Reaction rates for iodomethane with pyridazine, pyrimidine and pyrazine are respectively 0.25, 0.044, and 0.036 relative to the rate with pyridine. 

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Electrophilic addition reactions. See also Addition reactions with butylenes, 4 405-408 of maleic anhydride, 75 490 with methacrylic acid/derivatives, 76 236-237 of propylene, 20 774 Electrophilic aromatic substitution, benzene, 3 599-601 Electrophilic attack, at nitrogen and carbon, 27 98... 

The pKa of a nitrogen is a convenient measure of its nucleophilicity in proton addition steric effects are unimportant. All other types of electrophilic attack at nitrogen are sensitive in varying degrees to steric effects from a-substituents. (Exception certain ring formation reactions as in metal chelation.)... 

As proposed S-14 is a very reactive electrophile, as demonstrated, firstly, by the thermal addition of nitrogen at 25 K to give 13 back, secondly, by the addition of CO to yield 2i/-imidazol-2-ylideneketene (15) (T-10 reacts in a similar fashion31), and thirdly, by formation of a complex 14-Xe of the carbene with xenon. In a xenon matrix complexes of this type were also found by Sander et al.37 for difluorovinylidene and by Maier and Lautz38 for C2. 

Both processes can be minimized by lowering the C,C-coupling temperature. No evidence for the third possible side reaction, e g., the addition of the nitro carbanion at the nitroso group of conjugated nitrosoalkene (process c), has been observed in C,C-coupling reactions due apparently to insufficiently high electrophilicity of the nitrogen atom. 

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