Aromatic sextet

The circle m a hexagon symbol was first suggested by the British chemist Sir Robert Robinson to represent what he called the aromatic sextet —the six delocalized TT electrons of the three double bonds Robinson s symbol is a convenient time saving shorthand device but Kekule type formulas are better for counting and keeping track of electrons especially m chemical reactions... 

The sydnones may be represented by structures (123a-d), of which the zwitterionic structure (123a) most clearly implies an aromatic sextet. The diamagnetic susceptibility exaltation for Af-phenylsydnone of ll.Ox 10 cm moP is comparable with the corresponding value for pyrrole (10.2x10 ). 3-p-Bromophenylsydnone (123 R = H, R = p-bromophenyl) is essentially planar however, the O—N bond and 0(1)—C(5) bond lengths are not very different from normal single bond distances. 

The carbon atoms of azole rings can be attacked by nucleophilic (Section 4.02.1.6 electrophilic (Section 4.02.1.4) and free radical reagents (Section 4.02.1.8.2). Some system for example the thiazole, imidazole and pyrazole nuclei, show a high degree of aromati character and usually revert to type if the aromatic sextet is involved in a reaction. Othei such as the isoxazole and oxazole nuclei are less aromatic, and hence more prone to additio reactions. 

Acylimidazoles and related amides in which the nitrogen atom is part of an aromatic ring hydrolyze much more rapidly than other amides. A major factor is the decreased resonance stabilization of the carbonyl group, which is opposed by the delocalization of the nitrogen lone pair as part of the aromatic sextet. 

The article "A History of the Structural Theory of Benzene—The Aromatic Sextet and Huckel s Rule" in the February 1997 issue of the Journal of Chemical Education (pp. 194-201) is a rich source of additional information about this topic. 

Note that nitrogen atoms have different roles depending on the structure of the molecule. The nitrogen atoms in pyridine and pyrimidine are both in double bonds and contribute only one tt electron to the aromatic sextet, just as a carbon atom in benzene does. The nitrogen atom in pyrrole, however, is not in a double bond and contributes two tt electrons (its lone pair) to the aromatic sextet. In imidazole, both kinds of nitrogen are present in the same molecule— a double-bonded "pyridine-like" nitrogen that contributes one v electron and a pyrrole-like" nitrogen that contributes two. 

The vast majority of aromatic compounds have a closed loop of six electrons in a ring (the aromatic sextet), and we consider these compounds first. It is noted that a formula periodic table for the benzenoid polyaromatic hydrocarbons has been developed. ... 

Aromatic sextets can also be present in five- and seven-membered rings. If a five-membered ring has two double bonds and the fifth atom possesses an unshared pair of electrons, the ring has five p orbitals that can overlap to create five new orbitals— three bonding and two antibonding (Fig. 2.6). There are six electrons for these orbitals the four p orbitals of the double bonds each contribute one and the filled orbital contributes the other two. The six electrons occupy the bonding orbitals and constitute an aromatic sextet. The heterocyclic compounds pyrrole, thiophene, and... 

In contrast to pyridine, the unshared pair in canonical structure A in pyrrole is needed for the aromatic sextet. This is why pyrrole is a much weaker base than pyridine. 

In sharp contrast to cyclopentadiene is cycloheptatriene (41), which has no unusual acidity. This would be hard to explain without the aromatic sextet theory, since, on the basis of resonance forms or a simple consideration of orbital overlaps. 

Another seven-membered ring that shows some aromatic character is tropone (44). This molecule would have an aromatic sextet if the two C=0 electrons stayed away from the ring and resided near the electronegative oxygen atom. In fact, tropones are stable compounds, and tropolones (45) are found in nature. However, analyses of dipole moments, NMR spectra, and X-ray diffraction measurements show that tropones and tropolones display appreciable bond alternations. ... 

The tropylium ion has an aromatic sextet spread over seven carbon atoms. An analogous ion, with the sextet spread over eight carbon atoms, is 1,3,5,7-tetra-methylcyclooctatetraene dication (47), which is stable in solution at -50°C, is diatropic and approximately planar and is not stable above about -30°C. ... 

The proton NMR ( H NMR) spectrum of 90 (called kekulene) showed that in a case where electrons can form either aromatic sextets or larger systems, the sextets... 

Homoaromatic Compounds. When cyclooctatetraene is dissolved in concentrated H2SO4, a proton adds to one of the double bonds to form the homotropylium ion (107). In this species an aromatic sextet is spread over seven carbons, as in the tropylium ion. The eighth carbon is an sp carbon and so cannot take part in the aromaticity. The NMR spectra show the presence of a diatropic ring current H/, is found at 5= - 0.3 at 5.1 6 Hj and H7 at... 

In order for the orbitals to overlap most effectively so as to close a loop, the sp atoms are forced to lie almost vertically above the plane of the aromatic atoms. In 107, Hb is directly above the aromatic sextet and so is shifted far upfield in the NMR. All homoaromatic compounds so far discovered are ions, and it is questionable as to whether homoaromatic character can exist in uncharged systems. Homoaromatic ions of 2 and 10 electrons are also known. 

Carbocations can stabilize themselves in various ways (see p. 227), but for this type of ion the most likely way is by loss of either or . The aromatic sextet is then restored, and in fact this is the second step of the mechanism ... 

The orbital symmetry rules also help us to explain, as on pages 1083 and 1433, the unexpected stability of certain compounds. Thus, 102 could, by a thermal [1,3] sigmatropic rearrangement, easily convert to toluene, which of course is far more stable because it has an aromatic sextet. Yet 102 has been prepared and is stable at dry ice temperature and in dilute solutions. ... 

Garratt, P. J. Aromaticity. New York McGraw Hill 1971 88) Clar, E. The aromatic sextet. New York Wiley 1972... 

Heterocyclic systems have played an important role in this historical development. In addition to pyridine and thiophene mentioned earlier, a third heterocyclic system with one heteroatom played a crucial part protonation and methylation of 4//-pyrone were found by J. N. Collie and T. Tickle in 1899 to occur at the exocyclic oxygen atom and not at the oxygen heteroatom, giving a first hint for the jr-electron sextet theory based on the these arguments.36 Therefore, F. Arndt, who proposed in 1924 a mesomeric structure for 4//-pyrone, should also be considered among the pioneers who contributed to the theory of the aromatic sextet.37 These ideas were later refined by Linus Pauling, whose valence bond theory (and the electronegativity, resonance and hybridization concepts) led to results similar to Hiickel s molecular orbital theory.38... 

In imidazole [82], one of the nitrogen lone pairs (N-1) forms part of the aromatic sextet, and so protonation on N-3 is expected. The fact that imidazole is a stronger base than pyridine (p/i a = 6-99 vs. 5-23) and not weaker, as might have been expected owing to the... 

Pyrazole(2-azapyrrole, [100]) is also an aromatic ring, in which the N-1 atom donates its electron pair to the aromatic sextet. 

Pyrrole [165] is the simplest aromatic ring, in which the nitrogen may be regarded as being conjugated with the ol- and 3-carbon atoms of the ring. The nitrogen lone electron pair is part of the aromatic sextet and N-protonation is not expected. Pyrrole itself is unstable in dilute acid (trimerization occurs), but in more concentrated acid solutions, in which protonation occurs, a spectrophotometric study... 

---