Protonation of pyrrole

This is reflected in the basicity of pyrrole. Pyrrole is a particularly weak base, with pA a of the conjugate acid —3.8. First, we should realize that protonation of pyrrole will not occin on nitrogen nitrogen has... 

Although 2 protonation of pyrrole is favoured, there is sufficient of the 3-protonated form to initiate the addition of hydroxyl-amine. Once this occurs, ring opening may follow, as shown below. Subsequent addition of a second equivalent of hydroxylamine and elimination of ammonia (as ammonium chloride) then gives the dioxime. 

JA307 C-Protonation of pyrroles and indoles and relatively slow preocesses that... 

Protonation of pyrrole, furan and thiophene derivatives generates reactive electrophilic intermediates which participate in polymerization, rearrangement and ring-opening reactions. Pyrrole itself gives a mixture of polymers (pyrrole red) on treatment with mineral acid and a trimer (146) under carefully controlled conditions. Trimer formation involves attack on the neutral pyrrole molecule by the less thermodynamically favored, but more reactive, (3-protonated pyrrole (145). The trimer (147) formed on treatment of thiophene with phosphoric acid also involves the generation of an a-protonated species. 

The orientation of association of the benzene molecule about the pyrrole NH group is open to some conjecture. It was assumed from IR studies118 that the orientation was orthogonal (18). Such a model is also compatible with solvent-dependence studies of the molar Kerr constant.155 The model does not, however, adequately account for the solvent effect of benzene upon NMR chemical shifts of the a- and /3-protons of pyrrole (see Section III, B, 3, a). The NMR observations are best explained in terms of a nonplanar configuration (19) resulting not from NH-7T bonding but, instead, from a 7r-dipole interaction.151 In an attempt to confirm the orthogonal model, LeFevre et a/.155... 

The cation derived from protonation of pyrrole at C-2 is a hybrid of the following resonance structures. 

Protonation of pyrrole occurs at a ring carbon, not the N atom, as noted in Problem 17.43. 

The favorable effect of water is due to its proton scavenging action, which prevents protonation of pyrrole and its consequent subtraction from oxidation [97]. 

Electrochemistry has elucidated the role of water in the deposition of polypyrrole [216]. The favourable effect of water [217] was found to be due to its protonscavenging action which eliminates protonation of pyrrole (protons are released in coupling) and its consequent subtraction from oxidation. The effect of the counteranion on structure and conductivity properties of polypyrrole is well documented [98,215,217-25]. It was found that deposition from carboxylate anions of different basicity [223] produces polymers with a conductivity which increases as the anion basicity is decreased. Sulfonate anions produce the... 

The faster polymer generation at rising proton concentrations in aqueous solutions can be explained from the initial protonation of pyrrole in the a position, as a previous step in the general mechanism of electropolymerization [142]. As is well known, pyrrole can be protonated in all the possible positions (a, (i and N) resulting in three kinds of cationic species (Figure 10.14) [21,136], although the more stable position for protonation is the a carbon. A general feature of protonated pyrrole molecules is that they are not aromatic and hence they are more easily oxidable. In this way we can propose an electrochemical mechan-... 

Another example of electron delocalization can be seen in a comparison of the electrostatic potential maps of pyrrole and pyridine in Figure 18.16. The lone pair of pyrrole is delocaUzed due to its participation in estabhshing aromatic stabilization. As a result, pyrrole is an extremely weak base because protonation of pyrrole would result in the loss of aromaticity. In contrast, the lone pair in pyridine is locahzed and does not participate in estabhshing aromaticity. As a result, pyridine can function as a base without destroying its aromatic stabilization. For this reason, pyridine is a stronger base than pyrrole. In fact, inspection of the pA values in Table 23.1 reveals that pyridine is five orders of magnitude ( 100,000 times) more basic than pyrrole. 

In fact, pyrrole is protonated only in strong acid, and protonation occurs at C(2), not on nitrogen. First, explain why pyridine and imidazole are basic and pyrrole is not. Then, explain why evenm-al protonation of pyrrole is at carbon, not nitrogen. Caution You win have to write full Lewis structures to answer this question. 

Nalewajski, R. R, Korchewiec, J. (1989b). Protonation of Pyrrole A Model Hardness/ Softness Sensitivity Study. Croatica Chem. Acta 62, 603-616. 

Nalewajski, R. R, Korchowiec, J. (1989a). Chemical reactivity and charge sensitivities of reactants interaction energy and apphcations to protonation of pyrrole and cyclopen-tadiene. Acta Phys. Polon. A76, 747-788. 

Proton magnetic resonance studies have played a central part in the elucidation of the sites of protonation of pyrroles by acids. Thus, in hydrochloric acid 2 j the following situations arise ... 

Stereospeciflc protonation of pyrrole-2-carbaldehyde oximes may be rationalized by through-space stabilization of positive charge in the pyrrole ring with hydroxyl group ( H, NMR, quantum chemical calculations using B3LYP/6-311G (d, p)... 

SCHEME 2.147 Stereoselective protonation of pyrrole-2-carbaldehyde oximes. 

SCHEME 2.148 Rationale of the stereoselectivity of the protonation of pyrrole-2-carhal-dehyde oximes. 

Trofimov, B.A., A.V. Ivanov, LA. Ushakov et al. 2011. Stereospecific protonation of pyrrole-2-carboxaldehyde Z-oximes as a result of through-space cation stabilization with oxime hydroxyl. Mendeleev Commun 21 103-105. 

The formation of the complex between the free base (PH2) and the metal ion has been extensively studied. It was found that the complicated metal incorporation mechanism can be best interpreted for the reaction of the dipositive metal ion and the neutral chelating ligand. However, this does not mean that the anionic porphyrin species (PH and P ) are not reactive. In fact, they are present in very small concentrations, so small that they can hardly be detected. On the other hand, the cationic porphyrin species are definitively unreactive. Earlier works, of some twenty years ago, preferred the mechanism of metal incorporation which assumes a pre-equilibrium between the free base (PH2) and the dissociated protons of pyrrole nitrogens, so that the metal cation would actually react with the P " dianion. Later works, however, indicate the existence of the so-called SAT (sitting-atop) complex intermediate, which was already foreseen in 1960 by Fleischer and Wang. It was later shown that the SAT metal-ion intermediate can deform porphyrin, thus... 

Despite the lone pair on nitrogen, pyrrole is a very weak base, because the lone pair is a part of the aromatic system. The p/Cj, of the cation formed by protonation of pyrrole, 12.20, is -3.8 the cation is strongly disfavored, as the aromaticity of pyrrole has been lost. However, pyrrole is relatively easy to deprotonate its p/Cj, is 17, comparable to that of cyclopentadiene at 16. The anion still has a full octet of electrons at nitrogen, and the aromatic system remains intact. 

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