Benzene energies

Thermochemical stabilization (in kcal/mol) based on difference between A/ and summation of standard bond energies (benzene RE = 45.8 kcal/mol)."... 

Difference in SCF-MO total energy (in kcal mol) for heterocycle and sum of localized polyene energies (benzene = 20 kcal/mol). ... 

The aromatic hydrocarbons contain at least one unsaturated ring system with the general structure C6R6, where R is any functional group (see Chap. 1). The parent hydrocarbon of this class of compounds is benzene (C6H6), which exhibits the resonance, or delocalization of electrons, typical of unsaturated cyclic structures. Owing to its resonance energy, benzene is remarkably inert. 

It is not always easy to ascertain if the addition reaction proceeds via the triplet excited state of the arene. Benzene and its simple derivatives such as anisole and benzonitrile have high triplet energies (benzene, 84 kcal/mol 1 anisole, 81 kcal/mol" benzonitrile, 77 kcal/mol-1) which makes sensitization impracticable. Results of quenching experiments are sometimes difficult to interpret, as has become evident from the work of Cantrell. He found [109] that the formation of adducts from benzonitrile and 2,3-dimethylbut-2-ene, vinyl acetate, and 2-methoxypropene in solutions 0.5 M in cri-l,3-pentadiene occurs at a rate only approximately one-fourth that in the absence of added quencher. Five years later, the author reported [110] that m-l,3-pentadicnc itself adds slowly to benzonitrile to give ortho adducts. When a correction was made for the reaction of benzonitrile with the quencher, it became apparent that little, if any, triplet quenching had occurred. 

Ag+ has a low-lying LUMO, so it interacts strongly with one of the relatively high-energy benzene HOMOs. o (I2) lies at much higher energy, so it overlaps better with the VP1 benzene orbital. 

Hydrocarbons and Energy benzene butadiene butylene cumene ethylene propylyene styrene and steam and power. 

TABLE la UHF/MCSCF Energy Benzene -Dewar Benzene 6 orbital CAS Transition State Geometry... 

The larger the tt-system, the smaller the distance between the orbital energy levels and the smaller excitation energies. Benzene, naphthalene, anthracene, and letra-cene, with one, two, three, and four 6-cycles merged along the bonds, respectively, absorb in the UV region, Tetracene though also absorbs in the visible (blue) and is pale yellow. 

Fig. 6.7 Representation of ligand /r-molecular orbitals and their possible interactions with metal orbitals. The highest energy benzene m.o Q is omitted as it makes little or no contribution to bonding. 

Because of its large delocalization energy, benzene is an extremely stable compound. Therefore, it does not undergo the electrophilic addition reactions that are characteristic of alkenes except under extreme conditions. (Notice the conditions that Sabatier had to use in order to reduce benzene s double bonds on page 332.) Now we can understand why benzene s unusual stability puzzled nineteenth-century chemists, who did not know about delocalized electrons (Section 8.1). 

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