Aromaticity of benzene

The pattern of orbital energies is different for benzene than it would be if the six tt electrons were confined to three noninteracting double bonds The delocalization provided by cyclic conjugation in benzene causes its tt electrons to be held more strongly than they would be in the absence of cyclic conjugation Stronger binding of its tt electrons is the factor most responsible for the special stability—the aromaticity—of benzene... 

In summary, most of the presently available criteria point to an order of decreasing aromaticity of benzene > thiophene > selenophene pyrrole > tellurophene > furan. 

Nuclear magnetic resonance measurements have led to the conclu-sion that 2-pyridones have about 35% of the aromaticity of benzene and that the formally related l,2-dihydro-2-methylenepy-ridine is not aromatic. A substantial contribution by such resonance is indicated by the electronic spectrum of 2-quinolone, which is... 

This mesomerism (or resonance )59 between equivalent Kekule structures was recognized as the quintessential feature underlying the aromaticity of benzene, conferring highly distinctive symmetry, stability, and reactivity patterns. 

Structures and nomenclature for the most important five-membered monocycles with one or more heteroatoms are depicted in Scheme 1. The aromaticity scale in five-membered heterocycles has been long debated.97-101 The decreasing order of aromaticity based on various criteria is (DRE values in kcal/ mol) benzene (22.6) > thiophene (6.5) > selenophene > pyrrole (5.3) > tellurophene > fur an (4.3). Pyrrole and furan have comparable ring strains (Scheme 38). The aromaticity of furan is still controversial 100 the NMR shielding by ring current estimated it at about 60% of the aromaticity of benzene, and other methods reviewed earlier102 estimated it at less than 20%. 

Nitrogen. Pyridine is one of the most important heterocycles. The aromaticity of pyridine was intensively connected to structural considerations and chemical behavior. The relative difference between the aromaticity of benzene and pyridine is controversial generally calculations give similar orders of magnitude and differences depend on the criterion of aromaticity considered and the mode of calculation used. A comprehensive review on the theoretical aspects in connection with the aromaticity of pyridine was published.191 Pyridine is about as aromatic as benzene according to theoretical calculations and to experimental data, while quinoline is about as aromatic as naphthalene and more aromatic than isoquinoline.192193 The degrees of aromaticity of pyridine derivatives strongly depend on their substituents. 

At this point, it is appropriate to draw a parallel with the straightforward MO explanations for the aromaticity of benzene using approaches based on a single closed-shell Slater determinant, such as HMO and restricted Hartree-Fock (RWF), which also have no equivalent within more advanced multi-configuration MO constructions. The relevance of this comparison follows from the fact that aromaticity is a primary factor in at least one of the popular treatments of pericyclic reactions Within the Dewar-Zimmerman approach [4-6], allowed reactions are shown to pass through aromatic transition structures, and forbidden reactions have to overcome high-energy antiaromatic transition structures. 

The conclusion appears to be that furan, if regarded as an aromatic compound, probably has less than about 20% of the aromaticity of benzene, and perhaps has none. Only a definition of aromaticity that identifies it with NMR shielding by ring currents, and little else, clearly suggests an aromaticity of 60% or more. 

Does formation of bromobenzenium ion lead to disruption of the aromaticity of benzene Is the ion highly delocalized Examine the geometry of bromobenzenium ion, and measure CC bond distances. Are they all the same (as in benzene) or do you see alternation between short and long distances How do they compare to bond distances in benzene, and to typical single and double bond distances (1.54A and 1.32A, respectively). Draw a Lewis structure (or series of Lewis structures) to convey what you observe. Examine atomic charges as well as the electrostatic potential map for bromobenzenium ion. Where is the positive charge Is it localized on a single center or delocalized over several centers ... 

Using the appropriate bond-separation reactions, the UF/3-21G aromatic stabilization energies are calculated to be 47.2, 36.4 and 22.5 kcal mol-1 for 22,11 and 21, respectively, compared to 59.0 kcal mol-1 for benzene503 thus the meta-, para- and ortho-isomers have 80, 62 and 38% of the aromaticity of benzene. The different orders of the thermodynamic stability of the three isomeric disilabenzene and of their aromatic stabilization energies... 

Watts and Bunton examined the effect of Cr(CO)3 coordination on pAR for the tropylium ion.194 Because of a competing reaction at high pH in water, the study was conducted in methanolic solutions in which pAR is decreased by up to 5 log units.195 This allowed determination of pAR = 6.6, in methanol which translates to perhaps 9.5 in water. For the uncoordinated tropylium ion pAR is reported as 2.15 in methanol compared with 4.7 in water.195 The moderate stabilization of the tropylium ion by coordination of Cr(CO)3 is pertinent to Schleyer s conclusion that coordination of Cr(CO)3 does not impair the aromaticity of benzene.181... 

An example of a typical addition reaction with an organic solute is that of benzene, shown in Eq. (5). The aromaticity of benzene is destroyed, opening the way for ring-opening reactions. 

A natural way to study aromaticity would be to make use of the concept of two structures, as introduced by Kekule. The first to use this view were Pauling and Wheland [47]. They used an approximate form of the valence bond (VB) method developed by Heitler and London [48], for describing the aromaticity of benzene. 

The dipole moments of the analogous dibenzo- and tetraphenyl-sesquiful-valenes182 are essentially smaller than those of the tropones. There is effectively no ring current in pyridone-2-methide, while 2-pyridone is said to have about 25% of the aromaticity of benzene.183... 

The aromaticity of benzene is linked, in spin-coupled theory, to the particular mode of coupling of the electron spins, and so it seems reasonable to suppose that the orbital descriptions of Dih cyclobutadiene and of benzene could be fairly similar, but for these to be associated with very different modes of spin coupling. To a first approximation, this indeed turns out to be the case. With benzene-like orbitals ordered a,b,c,d around the ring, the symmetry requirements of an overall Bu state are such that the electron spins associated with each diagonal (ale and bid) must be strictly triplet coupled. These two triplet subsystems combine to a net singlet. A characteristic feature of antiaromatic situations in spin-coupled theory is the presence of such triplet-coupled pairs of electrons. 

Sakai, S. Theoretical study on the aromaticity of benzenes annelated to small rings, 7. Phys. Chem. A 2002,106, 11526-11532. 

This arrangement accounts for the extra stability or aromaticity of benzene. The six overlapping p orbitals can be pictured as forming a delocalized 7i-electron cloud comprising of two rings (think of them as doughnuts ), one above and one below the molecular plane as shown in Figure 1.4. There are no localized C=C bonds as there are in alkenes. 

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