Nitrogen in pyrroles

A further chracterization of these pyrrole type nitrogen compounds in the very weak base concentrate can be made by using the colorometric pyrrolic nitrogen value of 6.8% (Table III) as the value for a, -unsub-stituted pyrrole type compounds. This leaves 22.2% of the nitrogen in pyrroles and indoles which have both a- and -substitution. The affi-unsubstituted pyrroles and indoles also have no N-substitution because these N-substituted compounds would titrate as weak bases and not as very weak bases. The lack of N-substitution on the pyrroles and indoles is consistent with the research of Jacobson (18, 19) who reported that N-alkylpyrroles and N-alkylindoles thermally and irreversibly isomerize to give the a and alkyl isomers and therefore would not likely be present in crude shale oil. 

Inventing heterocycles by further replacement of CH groups by nitrogen in pyrrole leads to two compounds, pyrazole and imidazole, after one replacement and to two triazoles after two replacements. 

Furan and thiophene have structures that are analogous to the structure of pyrrole. Where nitrogen in pyrrole carries a hydrogen atom, the oxygen or sulfur carries an unshared pair of electrons in an sp orbital. Like nitrogen, the oxygen or... 

If the heteroatom contributes two electrons to the tr-electronic structure of the molecule (e.g., nitrogen in pyrrole or indole, oxygen in furan, sulfur in thiophene), it essentially represents a charge-transfer donor-type substituent. Its effect on the electronic structure and spectra will be dominated by its ability to donate electrons and to a lesser extent by considerations of electronegativity. Thus, the interactions of nitrogen, oxygen, and sulfur in, say, indole, benzofuran, and benzothiophene, with their aromatic systems, are similar to the interactions of amino, hydroxy, and mercapto exocyclic groups with their aromatic systems. 

Pyrrole is more acidic = —17) than the analogous saturated amine (p Ta = 36), because the nitrogen in pyrrole is sp hybridized and is, therefore, more electronegative than the sp nitrogen of a saturated amine (Table 21.1). Pyrrole s acidity also is increased as a result of its conjugate base being stabilized by electron... 

Both pyrroles and indoles can be synthetically modified by reactions at the oc-position of alkyl substituents. The special reactivity of benzylic positions in carbocyclic aromatics is well-appreciated and in pyrroles and indoles this reactivity is further enhanced by the electron-releasing capacity of the heterocyclic nitrogen. In pyrroles and indoles, these substitution reactions frequently take place under base-catalyzed conditions by an elimination-addition mechanism. Even groups such as alkoxy. 

In both pyridine and pyrrole the unshared electron pair occupies that orbital which provides the most stable structure. It is a different orbital in each case. In pyridine it is an p -hybridized orbital localized on nitrogen. In pyrrole it is a orbital of nitrogen that overlaps with the p orbitals of the ring carbons to give a delocalized 7T system. 

Electrostatic potential maps of pyridine and pyrrole. The color range is the same for both. In pyridine the unshared electron pair is responsible for the concentration of electron density red) near nitrogen. In pyrrole the corresponding electron pair is delocalized into the tt system of the ring. 

The sp nitrogen in pyrrole is more electronegative than the sp nitrogen in a saturated amine (Section 2.6). As a result, pyrrole (p/Cg 17) is more acidic than the analogous saturated amine (pK 36). The partial positive charge on the nitrogen atom (apparent in the structure of the resonance hybrid) also contributes significantly to pyrrole s increased acidity. 

Notice that there are four dipolar forms in which a positive charge is placed on the heteroatom and a negative charge successively on each of the carbons. This picture suggests that the heteroatom should be relatively electron poor and the carbons relatively electron rich. The electrostatic potential maps below confirm this expectation. Thus (on the same scale), the nitrogen in pyrrole is less electron rich (orange) than that in its saturated counterpart azacyclopentane (red), whereas the diene portion in pyrrole is more electron rich (red) than that in 1,3-cyclopentadiene (yellow). 

Pyrrole is a much weaker base than azacyclopentane (pyrrolidine) for which of the following reasons (a) The nitrogen in pyrrole is more electropositive than that in pyrrolidine (b) pyrrole is a Lewis acid (c) pyrrole has four electrons (d) pyrrolidine can give up the proton on the nitrogen atom more readily than can pyrrole (e) pyrrole is aromatic. 

The lone pair of electrons on nitrogen in pyrrole is a part of the aromatic sextet, and pyrrole Is a very poor base. It Is, however, a moderately good acid with a pK of 17. Pyrrole, furan, and thiophene are electron-rich heterocycles and react readily with electrophiles at the 2-position. Mild conditions are normally used as pyrrole and furan are sensitive to acid. 

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