Anthracene resonance energy

This trend is revealed, for example, by the rates of Diels-Alder addition reactions of anthracene, naphthacene, and pentacene, in which three, four, and five rings, respectively are linearly fused. The rate data are shown in Table 9.3. The same trend can be seen in the activation energy and the resonance energy gained when cycloreversion of the adducts 9-12 yields the aromatic compoimd, as shown in Scheme 9.3. 

This treatment could be applied to anthracene and phenanthrene, with 429 linearly independent structures, and to still larger condensed systems, though not without considerable labor. It is probable that the empirical rule6 of approximate proportionality between the resonance energy and the number of benzene rings in the molecule would be substantiated. 

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... 

The resonance energies of fused systems increase as the number of principal canonical forms increases, as predicted by rule 6 (p. 35).75 Thus, for benzene, naphthalene, anthracene, and phenanthrene, for which we can draw, respectively, two, three, four, and five principal canonical forms, the resonance energies are, respectively, 36, 61, 84, and 92 kcal/mol (152, 255, 351, and 385 kJ/mol), calculated from heat-of-combustion data.76 Note that when phenanthrene, which has a total resonance energy of 92 kcal/mol (385 kJ/mol), loses the 9,10 bond by attack of a reagent such as ozone or bromine, two complete benzene rings remain, each with 36 kcal/mol (152 kJ/mol) that would be lost if benzene was similarly attacked. The fact that anthracene undergoes many reactions across the 9,10 positions can... 

In this reaction the excited anthracene molecule is supposed to abstract a chlorine atom from CC14, a process facilitated by the resonance energy of. CCIS radical. As the energy of 1A is 322 kJ (77 kcal) mol-1 and the bond energy of C—Cl in CC14 is 293 kJ (70 kcal) mol-1 there is enough energy for the process. 

A similar treatment of naphthalene17 leads to the value 2.04a, which on equation to the empirical resonance energy 75 kcal/mole fixes a at 37 kcal/mole, in approximate agreement with the result for benzene. Calculations for anthracene and phenanthrene1 lead to 2.95a and 3.02a, respectively, for the resonance energy, giving a = 36 and 35 kcal/mole on comparison with the empirical values. 

This is due to the uniform initial condition k(t), which starts from its maximal value ki) Wrrd3r and drops with time, approaching the stationary limit from above. Contrary to this famous result, the experimental study of delayed fluorescence of anthracene in viscous solution [262] showed quite the opposite time behavior of k t). As it is initially much less than ko, the rate constant increases with time, approaching the long-time asymptote (3.56) from below. The authors of Ref. 262 called this anomaly the anti-Smoluchowski time behavior of the delayed fluorescence. They properly attributed it to a nonuniform distribution of triplets generated by the intersystem conversion from singlets that are preliminary quenched by the resonant energy transfer. 

Anthracene and phenanthrene are isomeric compounds with three fused benzene rings. Their resonance energies are calculated to be 84 kcal/niol (352 kJ/mol) and 92 kcal/mol (385 kJ/mol), respectively. Many other polycyclic aromatic hydrocarbons are known. Chrysene and benzo[a]pyrene are typical examples. 

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... 

Fig. 18. The adduct isolated from the dehydration of dideoxykohnkene 45 is only one of many possible constitutionally-isomeric products. Symmetry considerations played an important part in the identification of 47 [122] as the major isolated product because it is the only constitutionally-asymmetric hydrocarbon. In calculating the resonance energies (RE s) displayed in the boxes, A, B, and N represent Anthracene, Benzene, and Naphthalene units, respectively, and are assumed, to a first approximation, to have RE s of 84, 36, and 61 kcal mol-1, respectively...

Enantiopure epoxides (3/ ,4Y)-dibenz[ 7, ]anthracene 3,4-oxide and (3iJ,4Y)-phenanthrene 3,4-oxide were synthesized via involved routes and were observed to spontaneously racemize. This racemization of arene oxides is in accordance with perturbation molecular orbital predictions based on resonance energy considerations, and presumably occurs via an electrocyclic rearrangement to the corresponding (undetected) oxepine tautomer (Scheme 17) <2001J(P1)1091>. 

Since both the anthraquinone and the dihydroanthracene have two non-conjugated benzenoid rings, the resonance stabilization energy of each molecule is 2 x 150 kJ mol The resonance energy of anthracene is 351 kj mol and so the loss ot resonance stabilization energy associated with ihe Oxidation and reduction reactions is only of the order of 50 kJ nio , ... 

Phenanthrene is best represented as a hybrid of the five canonical forms 20-24. It has a resonance energy of 380 kJ mol and is more stable than anthracene. In four of the five resonance structures, the 9,10-bond is double and its length is about the same as an alkenic C=C bond. The numbering system for phenanthrene is shown in 20. Five different mono-substituted products are possible. 

ProMem 30.20 How much resonance energy would be sacrificed by oxidation or reduction of one of the outer rings of anthracene Of phenanthrene ... 

If anthracene stoppers are attached, this pH driven locomotion of the CD rings becomes completely reversible and can be monitored in situ using fluorescence resonance energy transfer (FRET) of suitable fluorescent probes attached to the CD rings and to both ends of the polymer [269],... 

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