Benzene relative aromaticity

Brown noticed that the reactivities of toluene relative to benzene in aromatic substitutions were proportional to the ratios in which toluene underwent p- and -substitutions. This point is illustrated in table 7.3. 

Paradoxically, although they are electron-rich, S-N compounds are good electron acceptors because the lowest unoccupied molecular orbitals (LUMOs) are low-lying relative to those in the analogous carbon systems. For example, the ten r-electron [SsNs] anion undergoes a two-electron electrochemical reduction to form the trianion [SsNs] whereas benzene, the aromatic hydrocarbon analogue of [SsNs], forms the monoanion radical [CeHg] upon reduction. ... 

Rate coefficients have also been measured at a range of temperatures for some aromatics in aqueous perchloric acid-trifluoroacetic acid (Table 168)468, and, surprisingly, the lower reactivity of benzene relative to toluene and /-butylbenzene appears to arise from a more negative activation entropy. This effect if real is... 

Unless the immunoassay kit is benzene sensitive, the kit may display strong biases, such as the low affinity for benzene relative to toluene, ethylbenzene, xylenes, and other aromatic compounds. This will cause an underestimation of the actual benzene levels in a sample, and since benzene is often the dominant compound in leachates due to its high solubility, a low sensitivity for benzene is undesirable. 

From the relative roles of the primary and tertiary radicals which are statistically formed on p-cymene, the termination constant of p-cymene should lie between that of cumene and that of toluene or xylene. Experimentally, this interpretation can be easily checked (12) by comparing the oxidation of p-cymene diluted by benzene with aromatic hydrocarbon mixtures having equal concentrations of methyl and isopropyl groups in comparable mixtures (Table V). 

Magnetic susceptibility anisotropy has been used for the estimation of relative aromaticity of some azines in comparison with benzene (77JCS(P2)897). If the extent of ir-electron delocalization for benzene is taken as 1.0, the corresponding values for azines are pyridine 0.7, pyridazine 0.7, pyrimidine 0.5, and 1,3,5-triazine 0.3. 

The quantum mechanical methods described in this book are all molecular orbital (MO) methods, or oriented toward the molecular orbital approach ab initio and semiempirical methods use the MO method, and density functional methods are oriented toward the MO approach. There is another approach to applying the Schrodinger equation to chemistry, namely the valence bond method. Basically the MO method allows atomic orbitals to interact to create the molecular orbitals of a molecule, and does not focus on individual bonds as shown in conventional structural formulas. The VB method, on the other hand, takes the molecule, mathematically, as a sum (linear combination) of structures each of which corresponds to a structural formula with a certain pairing of electrons [16]. The MO method explains in a relatively simple way phenomena that can be understood only with difficulty using the VB method, like the triplet nature of dioxygen or the fact that benzene is aromatic but cyclobutadiene is not [17]. With the application of computers to quantum chemistry the MO method almost eclipsed the VB approach, but the latter has in recent years made a limited comeback [18],... 

How does this concept of aromaticity apply to typical heterocycles such as pyridine 5.1 and pyrrole 2.1 Pyridine can formally be derived from benzene by replacement of a CH unit by an sp2 hybridised nitrogen atom. Consequently, pyridine has a lone pair of electrons instead of a hydrogen atom. However the six 7t electrons are essentially unchanged, and the pyridine is a relatively aromatic heterocycle. 

Magnetic susceptibility anisotropy has been used to estimate relative aromaticities of some azines <1977JOC897>. If the extent of -electron delocalization for benzene is taken as 1.0, the corresponding values for azines are pyridine 0.7, pyridazine 0.7, pyrimidine 0.5, and 1,3,5-triazine 0.3. Another quantitative magnetic index is the exaltation of the total magnetic susceptibility (A). All aromatic systems reveal large A values, whereas for nonaromatic compounds A is close to zero and it is assumed that aromaticity increases with A. For six-membered monocycles the following values of A have been reported (in units of cm3 mol-1 x —106) benzene (17.9), pyridine (18.3), pyridazine (8.7), pyrimidine (18.2), pyrazine (12.7), l-ethyl-2-pyridone (13.0), and 1,3,5-triazine (19.0). 

Perusal of CC bond distances and the accompanying 7r-bond orders presented in Table 7 reveals that an appreciable MN effect occurs in systems 20-22. This is evidenced by alternation of their IBDs values and differences (benzene) relative to free benzene gauge value. Bond fixation is reflected in the corresponding bond localization indices L(d). They read 0.064(0.068), 0.099(0.167), 0.155(0.242) and 0.153(0.238) for 12, 20, 21, and 22, respectively, where the first number refers to the MP2 results whereas the Hartree-Fock model calculations are given within parentheses. It follows that oxa-heteroanalog 21 exhibits the largest MN effect. It is also apparent that the HF model exaggerates the bond fixation in the aromatic moiety, but its qualitative predictions are correct in most cases. 

Isodesmic and homodesmotic reactions are frequently used for the study of aromaticity from the energetic point of view. However, the energy of the reactions used experimentally or in calculations may reflects only the relative aromaticity of benzene and not its absolute aromaticity. A new homodesmotic reactions based on radical systems predict an absolute aromaticity of 29.13 kcal mol (121.9 kJ mol ) for benzene and an absolute antiaromaticity of 40.28 kcal mol (168.5 kJ moP ) for cyclobutadiene at the MP4(SDQ)/ 6-31G-(d,p) level.2 ... 

The phosphorus analogues of the cyclopentadienyl anion P (CH)5 have been examined by SINDOl for their aromaticity. For n = 0 and 5 the aromaticity index is comparable to that of benzene. Of the two diphospholides (n = 2), the 1,3-isomer (88%) has a slightly higher relative aromaticity than the 1,2-isomer (86%) <92JOC3694>. 

Dr. B. Chawla (68a) in this laboratory has recently carried out a study of solvent effects on the shifts of several aromatic compounds. A series of typical results is given in Table 16. It shows the influence of the environment on the chemical shifts of the para and meta carbon atoms of a,a,a-trifluoromethyl-benzene relative to the signal of benzene. 

Figure 8 Sammon map obtained for the training data set, showing the relative distances between the input variabies [ASE, A, NICS(1)zz, and HOMA] in the original space. The color scale indicates the Euclidean distances between the weight vector of each neuron and the neuron activated by benzene. ASE, aromatic stabilization energy HOMA, harmonic oscillator model of aromaticity NICS, nucleus-independent chemical shift A, magnetic susceptibility exaltation. Reprinted with permission from Aionso and Herradon (2010JCC917). Copyright 2009 Wiiey Periodicais, inc.

The chemistry of benzene and its derivatives is quite different from that of alkenes and alkynes, but even though we do not study the chemistry of arenes until Chapters 21 and 22, we will show structural formulas of compounds containing aryl groups before then. The three double bonds in a six-membered ring create a special stabilization called aromaticity, which lowers the reactivity of benzene relative to other alkenes. What you need to remember at this point is that an aryl group is not chemically reactive under any of the conditions we describe in Chapters 6 through 20. 

So, once again, ortho or para substitution of an electrophile results in a less stable intermediate. This time it is not two full positive charges that must be adjacent, but a full and partial positive charge. No matter—meta substitution, which keeps the charges further apart, will still be favored (Fig. 14.78). The rate of the reaction will be slow relative to that for benzene. An aromatic ring with a carbonyl group attached is deactivated. 

Figure 7. (a) Aromatic stabilization of benzene relative to the hypothetical localized triolefin cyclohexatriene. (b) Aromatic stabilization of considering both complete and deltahedral (octahedral) delocalization for the core bonding. 

Figure 16 (a) Benzene and the corresponding spectrum of Gc-(b) The aromatic stabilization of benzene relative to a localized cyclohexatriene... 

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