Pyrroles resonance forms

The amino form is usually much more favored in the equilibrium between amino and imino forms than is the hydroxy form in the corresponding keto-enol equilibrium. Grab and XJtzinger suggest that in the case of a-amino- and a-hydroxy-pyrroles, structure 89 increases the mesomeric stabilization and thus offsets the loss of pyrrole resonance energy, but the increase due to structure 90 is not sufficient to offset this loss. Similar reasoning may apply to furans and... 

Thiophene is present in the benzene fraction from the distillation of coal tar. As with pyrrole and furan, the same type of resonance forms contribute to its overall molecular constitution, and the compound is aromatic in character. There is a difference between thiophene and furan, however, because sulfur is less electronegative than oxygen. Thus, the chemistry of thiophene tends to be closer to that of pyrrole than to that of furan. For example, thiophene does not enter easily into [4 + 2] cycloaddition reactions and quite severe conditions, high pressure (15 bar) and a temperature of 100 C, are necessary in order to force a cycloaddition between it and maleic anhydride. 

The synthesis and metal complexes of the pyrrole dithiocarbamate ligand were reported by Kellner et al. (386). At a later date two independent studies dealt with the coordination chemistry of this ligand (26, 211). An outstanding feature of this molecule is the pyrrole ring, which, by preserving aromaticity makes the contribution of resonance form B (Fig. 4) insignificant (26) and creates a dominant -accepting character at the sulfur atoms (26). 

The very interesting Cu(R2Dtc)2 complex (R2 = pyrrole) has been synthesized. The tendency of the pyrrole ring to preserve aromaticity (Fig. 4), makes the resonance form R2 N=CS2 quite unimportant in the structure of the ligand. Complexes with this ligand, including the Cu(II) complex, show the C N vibration between 1250 and 1350 cm"1 (26). 

Molybdenum bisalkyne complexes form more readily in the pyrrole-/V-carbodithioate ligand system ( pyrroledithiocarbamate ) than in the corresponding dialkyldithiocarbamate systems (88). The pyrrole nitrogen is reluctant to share electron density with the attached CS2 moiety since the aromatic stabilization of the five-membered NC4 ring is lost in resonance form ii. As a result of decreased electron donation from the... 

The contribution of the resonance forms XXI, XXII, XXIII, and XXIV to the structure of the anions is frequently overlooked, yet many base-catalyzed condensation reactions of phenol and pyrrole undoubtedly proceed through these resonance structures at the moment reaction occurs. The condensation of phenol with aqueous formaldehyde, the Kolbc synthesis (p. 197), and the Reimer-Tiemann reaction (p. 202) are striking examples of reactions which occur through the seemingly less important carbanion structure of the resonance hybrid. (See p. 133.)... 

The for loss of the A-hydrogen of pyrazole is 14.2, thus it is an appreciably stronger acid than pyrrole (pK 17.5), and the same as that of imidazole the anion has, like the imidazolyl anion, two, equally-contributing resonance forms. 

Write two different resonance fornns for pyrrole in which nitrogen has a formal charge of + 1. Are comparable resonance forms possible for pyridine ... 

Compared with chemical oxidation polymerization, electrochemical polymerization is performed at an electrode (conductive substrate) using the positive potential [97-104]. Whereas the powder forms are obtained by chemical polymerization, the electrochemical method leads to films deposited on the anode. When a positive potential is apphed at the electrode, pyrrole monomer such as a heterocychc compound is oxidized to form a delocalized radical cation, which includes the possible resonance forms. Radical-radical coupling reaction produces the dimerization of the monomer radicals at the a-position. Removal of 2H+ ions consequently forms the neutral dimer. Next step is chain propagation which includes the oxidation of the neutral dimer to form the dimer radical. The resultant radical can react with other monomer or dimer and this radical coupling and the electrochemical oxidation processes repeat in order to extend the polymer chain. The final step involves the termination of chain growth and the resultant PPy film is formed on the anodic electrode. 

The delocalization of the lone pair in the l-hetero-2,4-cyclopentadienes can be described by charge-separated resonance forms, as shown for pyrrole. 

Indole is related to pyrrole in the same way that naphthalene is related to benzene. Its electronic makeup is indicated by the various possible resonance forms that can be formulated for the molecule. Although those resonance forms that disturb the cyclic six-rr-electron... 

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. 

Some of the intermediate steps in the synthesis of a dipyrrylmethene have been studied chemically (36) and the spatial changes are similar to those postulated by Michaelis (93) for the triphenylmethane dyes. The condensation of the pyrrole-a-aidehyde (5) goes through an intermediate carbinol stage (4) in which the central carbon atom is bound tetrahedrally. The carbinol readily loses hydroxyl ion in the presence of very weak acid to form the planar carbonium ion 5), which represents one of the resonance forms of dipyrrylmethene usually written as (6),... 

Unsymmetrically substituted dipyrromethanes are obtained from n-unsubstitued pyrroles and fl(-(bromomethyl)pyiToIes in hot acetic acid within a few minutes. These reaction conditions are relatively mild and the o-unsubstituted pyrrole may even bear an electron withdrawing carboxylic ester function. It is still sufficiently nucleophilic to substitute bromine or acetoxy groups on an a-pyrrolic methyl group. Hetero atoms in this position are extremely reactive leaving groups since the a-pyrrolylmethenium( = azafulvenium ) cation formed as an intermediate is highly resonance-stabilized. 

Pyrrole is soluble in alcohol, benzene, and diethyl ether, but is only sparingly soluble in water and in aqueous alkaUes. It dissolves with decomposition in dilute acids. Pyrroles with substituents in the -position are usually less soluble in polar solvents than the corresponding a-substituted pyrroles. Pyrroles that have no substituent on nitrogen readily lose a proton to form the resonance-stabilized pyrrolyl anion, and alkaU metals react with it in hquid ammonia to form salts. However, pyrrole pK = ca 17.5) is a weaker acid than methanol (11). The acidity of the pyrrole hydrogen is gready increased by electron-withdrawing groups, eg, the pK of 2,5-dinitropyrrole [32602-96-3] is 3.6 (12,13). 

Hydroxypyrroles. Pyrroles with nitrogen-substituted side chains containing hydroxyl groups are best prepared by the Paal-Knorr cyclization. Pyrroles with hydroxyl groups on carbon side chains can be made by reduction of the appropriate carbonyl compound with hydrides, by Grignard synthesis, or by iasertion of ethylene oxide or formaldehyde. For example, pyrrole plus formaldehyde gives 2-hydroxymethylpyrrole [27472-36-2] (24). The hydroxymethylpyrroles do not act as normal primary alcohols because of resonance stabilization of carbonium ions formed by loss of water. 

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