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Acidity of pyrroles

Chemical evidence led to the conclusion that the conjugate acids of pyrrole probably exist predominantly as 16 or 17 rather than as X52,4,5 Although infrared spectra were initially interpreted on the... [Pg.3]

ICR has been used to determine the gas phase acidities of pyrrole (79JA6046), pyrazole (86JA), and imidazole (86JA), as well as the basicity of the compounds given in Table V (Section IV,A). [Pg.196]

For a review of the basicity and acidity of pyrroles, see Catalan Abboud Elguero Adv. Heterocvcl. Chem. 1987, 41, 187-274. [Pg.270]

The higher acidity of pyrroles and indoles bearing electron-withdrawing substituents at the a- or /3-positions permits their alkylation under mildly basic conditions, but although the thallium salt of 2-formylpyrrole is Af-alkylated, the corresponding alkylation of the thallium salts of ethyl pyrrole-2-carboxylate yields a complex mixture of products resulting from iV-alkylation and transesterification (B-77MI30502). N-Alkylation of pyrrolyl and indolyl esters is most conveniently effected under phase-transfer conditions. [Pg.236]

The spectra of conjugate acids of pyrrole and of pyrrolyl anions have also been investigated (see Section III, A, 1). Protonation of the... [Pg.463]

We have presented evidence that pyrrole-2-carboxylic acid decarboxylates in acid via the addition of water to the carboxyl group, rather than by direct formation of C02.73 This leads to the formation of the conjugate acid of carbonic acid, C(OH)3+, which rapidly dissociates into protonated water and carbon dioxide (Scheme 9). The pKA for protonation of the a-carbon acid of pyrrole is —3.8.74 Although this mechanism of decarboxylation is more complex than the typical dissociative mechanism generating carbon dioxide, the weak carbanion formed will be a poor nucleophile and will not be subject to internal return. However, this leads to a point of interest, in that an enzyme catalyzes the decarboxylation and carboxylation of pyrrole-2-carboxylic acid and pyrrole respectively.75 In the decarboxylation reaction, unlike the case of 2-ketoacids, the enzyme cannot access the potential catalysis available from preventing the internal return from a highly basic carbanion, which could be the reason that the rates of decarboxylation are more comparable to those in solution. Therefore, the enzyme cannot achieve further acceleration of decarboxylation. In the carboxylation of pyrrole, the absence of a reactive carbanion will also make the reaction more difficult however, in this case it occurs more readily than with other aromatic acid decarboxylases. [Pg.372]

The acidities of the five parent compounds are compared with that of pyrrole in Table 3. The acidity of the ring system increases as the number of nitrogens increases, the acidity of pyrrole increasing by approximately 2,4.5, and 5 pK3 units for each successive inclusion of a nitrogen atom. 1,2,3-Triazole is slightly more acidic than 1,2,4-triazole, but the effect on NH acidity of nitrogen orientation is much less than the effect of the total number of nitrogens. [Pg.489]

As a result, the pKa of the conjugate acid of pyrrole is much less than that for the conjugate acid of pyridine. [Pg.972]

Experimental gas-phase acidities of pyrrole and imidazole reveal that there is a free-energy exchange of Arxnf 298 = 9.3 0.6 kcal mol for deprotonation of imidazole by anionic pyrrole (Equation 1) <2005JPCA11504>. [Pg.184]

Pyrrole is an extremely weak base because the electrons shown as a lone pair are part of the rr cloud. Therefore, when pyrrole is protonated, its aromaticity is destroyed. Consequently, the conjugate acid of pyrrole is a very strong acid (p/fa = 3.8) that is, it has a strong tendency to lose a proton. [Pg.898]

Estimate the pKg of the conjugate acid of pyrrole given that pyrrole is about 10 -10 times less basic than pyridine and that the p/conjugate acid of pyridine is 5.2. Is the conjugate acid of pyridine strong or weak What about the conjugate acid of pyrrole ... [Pg.466]

How can the MO theory account for the unusually high acidity of pyrrole ... [Pg.80]

Pyrrole was discovered in 1834 by Friedlieb Ferdinand Runge in coal tar, where it is present in a concentration of less than 0.01%. Production from coal tar is, therefore, not economical. Pyrrole is synthesized by the reaction of furan with ammonia. Pyrrole is used in the production of polypyrrole and pharmaceuticals, such as the anti-inflammatory drug tolmetin. In tolmetin synthesis, the acidity of pyrrole is made use of. It is transformed into the potassium salt, which reacts with methyl chloride to form 1-methylpyrrole. The reaction of methylpyrrole with formaldehyde and dimethylamine produces the Mannich base, which is then quatemized... [Pg.390]

In this chapter, we explained why pyrrole is such a weak base, but we did not discuss the acidity of pyrrole. In fact, pyrrole is 20 orders of magnitude more acidic than most simple amines. Draw the conjugate base of pyrrole and explain its relatively high acidity. [Pg.1136]


See other pages where Acidity of pyrroles is mentioned: [Pg.50]    [Pg.361]    [Pg.221]    [Pg.379]    [Pg.393]    [Pg.50]    [Pg.392]    [Pg.50]    [Pg.901]    [Pg.268]   
See also in sourсe #XX -- [ Pg.11 ]

See also in sourсe #XX -- [ Pg.1134 ]




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