1.2.5- Hexatrienes

Linear polyenes (butadiene, hexatriene, etc.) absorb ultraviolet radiation. They have absorption maxima at the approximate wavelengths given in Table 6-1. 

Using Program SCF for ethylene and 1,3,5-hexatriene, list the electron repulsion integrals in the foiiii Yjj, Yj2, and so on. Take the coordinates from Figure 8-6. Try small variations in the atomic coordinates to see what their influence is on Yy. 

The electronic spectra of benzenoid systems differ in a characteristic manner from their acyclic analogues. Thus benzene, unhke hexatriene. 

The reaction was very vigorous and external heating was not necessary. A mixture of hexatriene and hexanol condensed in the receiver (note 1). The conversion was terminated by external heating, so that 5-10 ml of hexanol distilled at 55-50 C/ /15 mmHg. The contents of the receiver were "redistilled", using the apparatus... 

We reach a similar conclusion when comparing benzene with the open chain con jugated tnene (Z) 13 5 hexatriene Here we compare two real molecules both conju gated tnenes but one is cyclic and the other is not The heat of hydrogenation of... 

The precise value of the resonance energy of benzene depends as comparisons with 13 5 cyclohexatriene and (Z) 13 5 hexatriene illustrate on the compound chosen as the reference What is important is that the resonance energy of benzene is quite large SIX to ten times that of a conjugated triene It is this very large increment of resonance energy that places benzene and related compounds m a separate category that we call aromatic... 

The most easily obtained information from such calculations is the relative orderings of the eneigy levels and the atomic coefficients. Solutions are readily available for a number of frequently encountered delocalized systems, which we will illustrate by referring to some typical examples. Consider, first, linear polyenes of formula C H 2 such as 1,3-butadiene, 1,3,5-hexatriene, and so forth. The energy levels for such compounds are given by the expression... 

Carrying out the numerical operations for 1,3,5-hexatriene gives the results shown in Table 1.15. Because the molecule is a six-rr-electron system, j, doubly... 

Table 1.15. Energy Levels and Coefficients for HMOs of 1,3,5-Hexatriene...

Fig. 1.10. Graphic representation of K-molecular orbitals of 1,3,5-hexatriene as combinations of 2p AOs. The sizes of the orbitals are roughly proportional to the coefficients of the Hiickel wave functions.

Correlation diagrams can be constructed in an analogous fashion for the disrotatory and conrotatory modes for interconversion of hexatriene and cyclohexadiene. They lead to the prediction that the disrotatory mode is an allowed process whereas the conrotatory reaction is forbidden. This is in agreement with the experimental results on this reaction. Other electrocyclizations can be analyzed by the same method. Substituted derivatives of polyenes obey the orbital symmetry rules, even in cases in which the substitution pattern does not correspond in symmetiy to the orbital system. It is the symmetry of the participating orbitals, not of the molecule as a whole, that is crucial to the analysis. 

Analysis of the hexatriene-cyclohexadiene system leads to the conclusion that the disrotatory process will be favored. The basis set orbitals for the conrotatory and disrotatory transition states are shown below. 

An especially interesting case of the hexatriene-cyclohexadiene type interconversion is the rapid equilibrium between cycloheptatrienes and bicyclo[4.1.0]hepta-2,4-dienes ... 

The prediction on the basis of orbital symmetry analysis that cyclization of eight-n-electron systems will be connotatoiy has been confirmed by study of isomeric 2,4,6,8-decatetraenes. Electrocyclic reaction occurs near room temperature and establishes an equilibrium that favors the cyclooctatriene product. At slightly more elevated temperatures, the hexatriene system undergoes a subsequent disrotatory cyclization, establishing equilibrium with the corresponding bicyclo[4.2.0]octa-2,4-diene ... 

The cyclohexadiene-hexatriene system seems to be less complicated than the cyclobutene-butadiene system. Cyclohexadiene undergoes photochemical electrocyclic ring opening ... 

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