Pyrrole resonance contributors

Positional selectivity in these five-membered systems, and their high reactivity to electrophitic attack, are well explained by a consideration of the Wheland intermediates (and by implication, the transition states that lead to them) for electrophilic substitution. Intermediate cations from both a- and P-attack are stabilised (shown for attack on pyrrole). The delocalisation, involving donation of electron density from the heteroatom, is greater in the intermediate from a-attack, as illustrated by the number of low-energy resonance contributors. Note that the C-C double bond in the intermediate for p-attack is not and cannot become involved in delocalisation of the charge. 

It is not immediately apparent that the electrons represented as lone-pair electrons on the nitrogen atom of pyrrole are tt electrons. The resonance contributors, however, show that the nitrogen atom is sp hybridized and uses its three sp orbitals to bond to two carbons and one hydrogen. The lone-pair electrons are in a p orbital that overlaps thep orbitals on adjacent carbons, forming a tt bond—thus, they are tt electrons. Pyrrole, therefore, has three pairs of tt electrons and is aromatic. 

The resonance contributors of pyrrole show that nitrogen donates the electrons depicted as a lone pair into the five-membered ring. 

Use resonance contributors to explain why pyrrole is protonated on C-2 rather than on... 

The resonance contributors show that lone-pair electrons of pyrrole form a tt bond in the ring of a resonance contributor thus, they are tt electrons. 

Pyrrole is an extremely weak base because the electrons shown as a lone pair in the stmcture are part of the tt cloud. That is, the nitrogen donates the lone-pair into the five-membered ring (as shown by the resonance contributors). Therefore, protonating pyrrole destroys its aromaticity. As a result, the conjugate acid of pyrrole is a very strong acid (piT, = -3.8). 

Draw arrows to show the movement of electrons in going from one resonance contributor to the next in pyrrole. 

In Section 8.6, we saw a compound s delocalization energy increases as the resonance contributors become more stable and more nearly equivalent. The delocalizahon energies of pyrrole, furan, and thiophene are not as great as the delocalizahon energies of benzene or the cyclopentadienyl anion, each a compound for which the resonance contributors are all equivalent. 

Substitution occurs preferentially at C-2 because the intermediate obtained by adding a substituent to this position is more stable than the intermediate obtained by adding a substituent to C-3 (Figure 20.1). Both intermediates have a relatively stable resonance contributor in which all the atoms (except H) have complete octets. The intermediate resulting from C-2 substitution of pyrrole has two additional resonance contributors, whereas the intermediate resulting from C-3 substitution has only one additional resonance contributor. 

When the imidazole ring is considered to be something resembling a pyrrole-pyridine combination (1) it would appear that any electrophilic attack should take place preferably at C-5 (pyrrole-or, pyridine-j8). Such a model, though, fails to take account of the tautomeric equivalence of C-4 and C-5 (Section 4.06.5.1). The overall reactivities of imidazole and benzimidazole can be inferred from sets of resonance structures in which the dipolar contributors have finite importance (Section 4.06.2) or by mesomeric structures such as (2). These predict electrophilic attack in imidazole at N-3 or any ring carbon atom, nucleophilic attack at C-2 or C-1, and also the amphoteric nature of the molecule. In benzimidazole the acidic and basic properties, the preference for nucleophilic attack at C-2 and the tendency for electrophiles to react at the fused benzene ring can be readily rationalized. 

Furan is less resonance stabilized than pyrrole because its O atom is less basic, so it donates electron density less "willingly." Thus, charge-separated resonance forms are more minor contributors to the hybrid than the charge-separated resonance forms of pyrrole. 

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