Nitrogen positively-charged species

As noted earlier, positively charged species such as the ammonium ion, NH4+, are known as cations and negatively charged species such as the chloride ion, Cl , are known as anions. The point here is that spermidine (and putrescine and cadaverine) is a base that is, it has the capacity to add a proton. In fact, it has three sites to which a proton might be added, the three amino nitrogen atoms. In principle, it could add three protons and end up with three positive charges. In fact, under the conditions of most living cells, it adds two protons and exists as a dication (dye-cation) ... 

Exposure of a pyridine to a medium containing electrophilic species immediately converts the heterocycle into a pyridinium cation, with the electrophile (or a proton from the medium) attached to the nitrogen. The extent of conversion depends on the nature and concentration of the electrophile (or protons) and the basicity of the particular pyridine, and is usually nearly complete. Obviously, the positively charged pyridinium cation is many orders of magnitude less easily attacked by the would-be electrophile, at carbon, than the original neutral heterocycle. The electrophile, therefore, has Hobson s choice - it must either attack an already positively charged species, or seek out a neutral pyridine from the very low concentration of uncharged pyridine molecules. 

That electrophilic attack on thienopyridines can occur on neutral or positively charged species receives support from the observations of Dressier and Joullie.48 These workers found that, under conditions where complete protonation of nitrogen was not to be expected, 7-methyl-thieno[2,3-c pyridine (56 pA, 5.81) underwent the Friedel-Crafts reaction whereas 4-methylthieno[3,2-c]pyridine (57 ptfa 6.17) did not, which the authors ascribed to the effect of the stronger basicity of 57. 

Tar bases are nitrogen-containing compounds that occur in the coal tar and react with hydrogen ions (H) to form positively charged species. 

Quatemaiy ammonium ions have four alkyl and/or aryl groups bonded to nitrogen, resulting in a positively charged species. 

Notice that the mechanisms for imine, enamine, hydrate, and acetal formation are similar. The nucleophile in each reaction has a lone pair on its attacking atom. After the nucleophile has added to the carbonyl carbon, water is eliminated from a protonated tetrahedral intermediate, forming a positively charged species. In imine and hydrate formation, a neutral product is achieved by loss of a proton from a nitrogen and an oxygen, respectively. (In hydrate formation, the neutral product is the original aldehyde or ketone.) In enamine formation, a neutral product is achieved by the loss of a proton from an a-carbon. In acetal formation, a neutral compound is achieved by the addition of a second equivalent of alcohol. 

There is another important factor in the low reactivity of pyridine derivatives toward electrophilic substitution. The —N=CH— unit is basic because the electron pair on nitrogen is not part of the aromatic n system. The nitrogen is protonated or complexed with a Lewis acid under many of the conditions typical of electrophilic substitution reactions. The formal positive charge present at nitrogen in such species further reduces the reactivity toward electrophiles. 

Addition to R—" "N=C is not a matter of a species with an electron pair adding to one atom and a species without a pair adding to the other, as is addition to the other types of double and triple bonds in this chapter and Chapter 15. In these additions, the electrophile and the nucleophile both add to the carbon. No species add to the nitrogen, which, however, loses its positive charge by obtaining as an unshared pair one of the triple-bond pairs of electrons ... 

Another approach to reduction of an amide group in the presence of other groups that are more easily reduced is to convert the amide to a more reactive species. One such method is conversion of the amide to an O-alkyl derivative with a positive charge on nitrogen.102 This method has proven successful for tertiary and secondary, but not primary, amides. 

All positively charged nitrogen species and all organic nitrogen-containing compounds are readily converted to nitrite and then to nitrate in soil. Because of this ready conversion, all are sources of nitrogen for plants. 

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