Dienes resonance energy

Using cyclohexene as the reference standard, calculate the resonance energies of cyclohexa-1,3-diene = —224 kJ moU ), cyclohexa-1,4-diene =... 

Thus the rotation of the allylic system out of the plane (required before ring closure can occur) results in the reduction of the available allylic resonance energy by about 5 kcal mole". This particular steric requirement does not apply to the isomerizations to the dienes, and these have much the same values as for vinylcyclopropane itself, viz. ... 

Polycyclic aromatic hydrocarbons are moderately reactive as the diene component of Diels-Alder reactions. Anthracene forms adducts with a number of reactive dienophiles. The addition occurs at the center ring. There is no net loss of resonance stabilization, because the anthracene ring (resonance energy = 1.60 eV) is replaced by two benzenoid rings (total resonance energy = 2 x 0.87 = 1.74 eV).48 49... 

HMO calculations, based on localized polyenes instead of isolated alkenes, can account for the heats of atomization of furan (41.64 observed, 41.69 eV) and of dibenzofuran (109.09 observed, 108.92 eV). For resonance energies (quoted as resonance energy per electron, REPE) they give furan, 0.007 oxepin, -0.006 benzo[6]furan, 0.036 benzo[c]furan, 0.002 and dibenzofuran, 0.047 /3 (72T3657). That furan emerges as hardly more aromatic than a diene while pyrrole (REPE 0.039/3) is clearly aromatic is in line with other results, including those from MINDO/3 and topological methods (see Section... 

In the fused compounds (241) and (242) the furan ring fails to react as a diene and Diels-Alder reaction with dienophiles occurs on the terminal carbocyclic rings. However, (243) and (244) afford monoadducts with dimethyl fumarate by addition to the furan rings (70JA972). The rates of reaction (Table 2) of a number of dehydroannuleno[c]furans with maleic anhydride, which yield fully conjugated dehydroannulenes of the exo type (247), have been correlated with the aromaticity or antiaromaticity of the products (76JA6052). It was assumed that the transition state for the reactions resembled products to some extent, and the relative rates therefore are a measure of the resonance energy of the products. The reaction of the open-chain compound (250), which yields the adduct (251), was taken as a model. Hence the dehydro[4 + 2]annulenes from (246) and (249) are stabilized compared to (251), and the dehydro[4 ]annulenes from (245) and (248) are destabilized (Scheme 84). 

The Diels-Alder reaction is certainly one of the most important reactions in organic chemistry. A few other interesting examples are provided in the following equations. Benzene is not very reactive as a diene because the product would not be aromatic. However, reactive dienophiles do add to the central ring of anthracene. In this case the product, with two benzene rings, has not lost much aromatic resonance energy... 

The heat of hydrogenation of buta-1,3-diene is about 17 kJ/mol (4.0 keal/mol) less than twice that of but-l-ene, showing that buta-1,3-diene has a resonance energy of 17 kJ/mol. Figure 15-2 shows the most stable conformation of buta-1,3-diene. Note that this conformation is planar, with the p orbitals on the two pi bonds aligned. 

The extra stabilization provided by delocalization, compared with a localized stmcture. For dienes and polyenes, the resonance energy is the extra stability of the conjugated system compared with the energy of a compound with an equivalent number of isolated double bonds, (p. 669)... 

Hydrogenation of cyclohexa-1,4-diene is exothermic by 240 kJ/mol (57.4 kcal/mol), about twice the heat of hydrogenation of cyclohexene. The resonance energy of the isolated double bonds in cyclohexa-1,4-diene is about zero. 

Hydrogenation of cyclohexa-1,3-diene is exothermic by 232 kJ/mol (55.4 kcal/mol), about 8 kJ (1.8 kcal) less than twice the value for cyclohexene. A resonance energy of 8 kJ (1.8 kcal) is typical for a conjugated diene. 

Instead, these heterocycles and their derivatives most commonly undergo electrophilic substitution nitration, sulfonation, halogenation. Friedel-Crafts acylation, even the Reimer-Tiemann reaction and coupling with diazonium salts. Heats of combustion indicate resonance stabilization to the extent of 22-28 kcal/ mole somewhat less than the resonance energy of benzene (36 kcal/mde), but much greater than that of most conjugateci dienes (about Tlccal/mole). On the basis of these properties, pyrrole, furan, and thiophene must be considered aromatic. Clearly, formulas I, II, and III do not adequately represent the structures of these compounds. 

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