The Thermodynamic Stability of Benzene

In 1911, Richard Willstatter succeeded in synthesizing cyclooctatetraene. Willstatter found, however, that it is not at all like benzene. Cyclooctatetraene reacts with bromine by addition, it adds hydrogen readily, it is oxidized by solutions of potassium permanganate, and thus it is clearly not aromatic. While these findings must have been a keen disappointment to Willstatter, they were very significant for what they did not prove. Chemists, as a result, had to look deeper to discover the origin of benzene s aromaticity. 

We have seen that benzene shows unusual behavior by undergoing substitution reactions when, on the basis of its Kekule structure, we should expect it to undergo addition. Benzene is unusual in another sense It is more stable thermodynamically than the Kekule structure suggests. To see how, consider the following thermochemical results. 

Cyclohexene, a six-membered ring containing one double bond, can be hydrogenated easily to cyclohexane. When the AH° for this reaction is measured, it is found to be —120 kJ mol very much like that of any similarly substituted alkene  

We would expect that hydrogenation of 1,3-cyclohexadiene would Ubaate roughly twice as much heat and thus have a AH° equal to about -240 kJ mol . When this experiment is done, the result is AH° = 232 kJ mol This result is quite close to what we calcu- 

If we extend this kind of thinking, and if benzene is simply 1,3,5-cyclohexatriene, we would predict benzene to liberate approximately 360 kJ moF [3 X (—120)] when it is hydrogenated. When the experiment is actually done, the resnlt is surprisingly different. The reaction is exothermic, but only by 208 kJ moF  

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The Discovery of Benzene 627 Nomenclature of Benzene Derivatives 628 Reactions of Benzene 630 The Kekule Structure for Benzene 631 The Thermodynamic Stability of Benzene 632 Modern Theories of the Structure of Benzene 634 Huckel s Rule The 4n +2 tt Electron Rule 637 Other Aromatic Compounds 645... 

The orbital phase theory can be applied to the thermodynamic stability of the disubstituted benzene isomers. The cyclic orbital interaction in the benzene substituted with two EDGs is shown in Scheme 21. The orbital phase is continuous in the meta isomer and discontinuous in the ortho and para isomers (Scheme 22, cf. Scheme 4). 

The stabilization of iminoboranes can yield five different tj ies of products cyclodimers (1,3,2,4-diazadiboretidines, Di), cyclotrimers (borazines, Tr), bicyclotrimers (Dewar borazines, Tr ), cyclotetramers (octahydro-l,3,5,7-tetraza-2,4,6,8-tetraborocines, Te), and polymers (polyiminoboranes, Po) these substances are isoelectronic with cyclobutadienes, benzenes, Dewar benzenes, cyclooctatetraenes, and polyalkynes, respectively, which are all known to be products of the thermodynamic stabilization of alkynes. 

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... 

The authors suggested244 that proton loss [Eq. (185)] is controlled by stereo-electronic considerations and not by the thermodynamic stability of the product. In the preferred conformation, the tertiary hydrogen on the isopropyl group of the p-cymene radical cation is located in the nodal plane of the benzene ring where its interaction with the n system in the transition state is minimized. The methyl group, on the other hand, rotates rather freely, and the loss of any one of the hydrogens is, therefore, not conformationally restricted. 

TABLE 30. Calculated substituent effects on the thermodynamic stabilities of substituted benzenes and of ipso-substituted silabenzenes (3-21G AE in kcalmol - lf... 

These various approaches for comparing the thermodynamic stability of aromatic compounds with reference compounds all indicate that there is a large stabilization of benzene and an even greater destabilization of cyclobutadiene. These compounds are the best examples of aromaticity and antiaromaticity, and in subsequent discussions of other systems we compare their stabilization or destabilization to that of benzene and cyclobutadiene. 

Consequently, the benzene oxidation mechanism was further developed by considering additional decomposition and oxidation steps. Sethuraman et al. proposed that phenyl radical decomposition can occur by either of two key pathways (3-scission of phenyl radical or by breakdown of the phenylperoxy radical formed by the oxidation of phenyl radical (Fig. 9). Using PM3 calculations,which were ultimately verified by DFT studies,Carpenter predicted that another species, 2-oxepinoxy radical (3 in Fig. 9b), is an important intermediate due to its relative stability, formed via a spirodioxiranyl intermediate (2 in Fig. 9b) from phenylperoxy radical. Pathway A in Fig. 9b is the thermodynamically preferred pathway at temperatures increasing up to 432 K, while pathway B has an entropic benefit at higher temperatures. While pathway B essentially matched the traditional view of benzene combustion, pathway A introduced a new route for phenylperoxy radical, which could resolve discrepancies observed using previous models. 

Even if the addition of DMSO to protic solvents has a dramatic effect on the rate and equilibrium constants for adduct formation, to a first approximation it does not affect the relative stability of adducts formed from the same substrate. As already observed in the chemistry of Meisenheimer adducts in the benzene series, the ratios Kk/Kt, kf/kj, and k /k] are practically independent of the composition of the medium.46 Under such circumstances it is possible to obtain approximate kinetic or thermodynamic data for a given adduct in a particular medium whenever kinetic and... 

Aromatic compounds have a special place in ground-state chemistry because of their enhanced thermodynamic stability, which is associated with the presence of a closed she of (4n + 2) pi-electrons. The thermal chemistry of benzene and related compounds is dominated by substitution reactions, especially electrophilic substitutions, in which the aromatic system is preserved in the overall process. In the photochemistry of aromatic compounds such thermodynamic factors are of secondary importance the electronically excited state is sufficiently energetic, and sufficiently different in electron distribution and electron donor-acceptor properties, ior pathways to be accessible that lead to products which are not characteristic of ground-state processes. Often these products are thermodynamically unstable (though kinetically stable) with respect to the substrates from which they are formed, or they represent an orientational preference different from the one that predominates thermally. 

TABLE 2. Calculated substituent effects on the thermodynamic stability, AE (in kcalmol-1), of (pso-substituted silabenzenes and of substituted benzenes (HF/3-21G)"... 

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