Furans delocalized lone electron pairs

Figure 25 1 Orbital pictures of (A) pyrrole and (B) furan p< = O), and thiophene = S). The het-erocitom in each is sp hybridized and bears one delocalized lone electron pair.

Pyrrole, furan, and thiophene contain delocalized lone electron pairs... 

Pyrrole, furan, and thiophene are l-hetero-2,4-cydopentadienes. Each contains a butadiene unit bridged by an -hybridized heteroatom bearing lone electron pairs. These systems contain delocalized tt electrons in an aromatic six-electron framework. This section considers the structures and methods of preparation of these compounds. 

In Summary The donation of the lone electron pair on the heteroatom to the diene unit in pyrrole, furan, and thiophene makes the carbon atoms in these systems electron rich and therefore more susceptible to electrophilic aromatic substitution than the carbons in benzene. Electrophilic attack is frequently favored at C2, but substitution at C3 is also observed, depending on conditions, substrates, and electrophiles. Some rings can be opened by hydrolysis or by desulfurization (for thiophenes). The diene unit in furan is reactive enough to undergo Diels-Alder cycloadditions. Indole is a benzopyrrole containing a delocalized tt system. 

Pyrrole, furan, and thiophene are five-membered-ring heterocycles. Each has three pairs of delocalized rr electrons Two of the pairs are shown as rr bonds, and one pair is shown as a lone pair on the heteroatom. Furan and thiophene have a second lone pair that is not part of the rr cloud. These electrons are in an sp orbital perpendicular to the p orbitals. Pyrrole, furan, and thiophene are aromatic because they are cyclic and planar, every carbon in the ring has a p orbital, and the tt cloud contains three pairs of tt electrons (Sections 15.1 and 15.3). 

Pyrrole, with 6 7u-electrons, is an QlQcXron-excessive (also known as electron-r/c/ ) aromatic heterocycle because the electron density on each ring atoms is greater than one. Its lone pair electrons take part in the delocalization thus essential to pyrrole s aromaticity. Pyrrole s aromaticity is between furan and thiophene, which is in accordance with Pauling s electronegativity for O (3.5), N (3.0), and S (2.5) ... 

Heterocyclic aromatic compounds contain C and H atoms other than carbon and hydrogen (Fig. 6). For the monocyclic molecules Hiickel s rule is applicable. For example both pyridine and pyrrole contain six ti electrons. Unlike the former the lone pair of the latter is delocalized. Armit and Robinson have shown a connection between the electronic sextet and the heteroaromaticity. Due to the electronegativity difference between carbon and nitrogen the bonds in pyridine are not of equal length and the delocalization is not perfect. Five membered heteroaromatics with oxygen and sulfur are furan and thiophene respectively. Pyrazole/imidazole, triazoles and tetrazoles are five membered heteroaromatics with two, three and four nitrogen atoms respectively. Three important aromatic six membered heterocyclic molecules are pyrimidine, pyrazine and pyridazine. Benzofused... 

When we studied the idea of aromaticity in Chapter 3, we encountered the cyclopentadienyl anion, 12.19, which is a charged Hiickel system with six n-electrons delocalized over five atoms. The heterocyclic analogies of the cyclopentadienyl anion are furan, thiophene, and pyrrole (Figure 12.54). In each of these, there are four electrons from two t-bonds, and the other two are provided by a lone pair of electrons on the heteroatom. Thus, the lone pair is an integral part of the aromatic sextet of electrons and must occupy an sp orbital parallel to those that form the n-bonds in the ring. For furan and thiophene (though not pyrrole), there is an additional lone pair of electrons in an sp orbital in the plane of the ring O, N, and S are sp hybridized in all these molecules. 

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