Butadiene, bond orbitals

Mixed labeling involving both x and a orbitals occurs in certain molecules the 5BU molecular orbitals of ra is-2-butene (III.78) and transoid 1,3-butadiene (III.65) are labeled ch3> ( cc) and 7rcn2> ( cc) because one lobe of the x orbital overlaps well with the adjacent CC bond-orbital to form a delocalized combination. In cisoid acrolein, orbitals 9A and 10A are labeled TCH2y nodal surfaces of the two localized orbitals coincide and allow for a delocalized combination (III.G8). 

Butadiene has two n bonds. The interaction between the two n bonds is one of the simplest models to derive molecular orbitals from bond orbitals. A n bond in butadiene is similar to that in ethylene. The n bond is represented by the bonding and antibonding orbitals. The interactions occur between the n bonds in butadiene. The bond interactions are represented by the bond orbital interactions. 

The bonding orbitals and of ethylenes are combined in phase to be the lowest TZ molecular orbitals (tTj) of butadiene (Scheme 18). The out-of-phase combined orbital (jtj) is the highest occupied molecular orbital (HOMO). The in-phase combination of... 

Scheme 18 The Jt molecular orbitals of butadiene from the bond orbitals...

While the sigma nonbonded interaction of lone pairs is ambiguous, the situation improves when one considers nonbonded interactions of sigma bonds. This is illustrated by reference to the molecule 1,3-butadiene. The orbitals of the C-C coupling unit are assumed to be the same as the ones shown in Fig. 11 for the C-C coupling unit of 1,2-difluoroethylene. The sigma MO s of the CH2 group and the... 

In the rightmost column (reactant-like orbitals), we can see that the two butadiene n-bonds are formed by the pairs of orbitals ( /i, /2) and ( /3, /4), respectively. Orbital /2 has a small additional bulge towards /3, which is an indication of the much weaker n-orbital interaction across the central carbon-carbon bond. Orbital /e and its symmetry-related counterpart, j/5, are responsible for the % bond on the ethene fragment. 

This equivalence of the valence bond and molecular orbital descriptions of the bonding in these complexes arises from the alternant1 properties of the metal-butadiene bonding network. A similar equivalence between the two theories occurs for benzene and other polyenes that have alternant 7r-systems (73, 140). 

In the reverse transformation, the ring opening of cyclobutene to gi butadiene, the conversion involves net transformation of a c-bonding orbital to --bonding orbital. The 4 orbitals of cyclobutene are a, a and tc, tS (Figure 8.6). The 4 orbitals of the product are the familiar MO s of... 

Figure 21-8 Energies and schematic representations of the n molecular orbitals of localized 1,3-butadiene. The orbitals are the n orbitals of two isolated ethene bonds and the total u-electron energy is 4(a + /3) = 4 a + 4/3.

Butadiene has two Jt bonds and so four electrons in the n system. Which molecular orbitals are these electrons in Since each molecular orbital can hold two electrons, only the two molecular orbitals lowest in energy are filled. Let s have a closer look at these orbitals. In, the lowest-energy bonding orbital, the electrons are spread out over all four carbon atoms (above and below the plane) in one continuous orbital. There is bonding between all the atoms. The other two electrons are in 2. This orbital has bonding interactions between carbon atoms 1 and 2, and also between 3 and 4 but an antibonding interaction between carbons 2 and 3. Overall, in both the occupied Jt orbitals there are... 

The overall energy of the two bonding butadiene molecular orbitals is lower than that of the two molecular orbitals for ethene. This means that butadiene is more thermodynamically stable than we might expect if its structure were just two isolated double bonds... 

A simple theoretical model of the butadiene system predicts the energy of the bonding 1 orbital to be [a + 1.62PJ and that of bonding orbital to be [ex + 0.62f)]. With both of these orbitals fully occupied, the total energy of the electrons is[4a + 4.48 j. Remember that the energy of the bonding n molecular orbital for ethene was [a +13] (p. 152) so, if we were to have two localized 71... 

The correlation diagrams for the conrotatory process show that there is a good correlation between the bonding orbitals v 1 and v 2 of butadiene and ct- and TT-orbitals of cyclobutene (Fig. 8.40). Thus, the ring opening of cyclobutene to butadiene or the reverse reaction is thermally allowed and occurs by the conrotatory process. The reaction proceeds with conservation of orbital symmetry. The photochemical conrotatory process in this case will be symmetry forbidden. 

If we examine the classification of orbitals of starting material and product with respect to each of the two symmetry elements in turn, we see that the bonding orbitals, and for starting material and product share symmetry only with respect to the C2 axis. (With respect to the a plane, both bonding orbitals of the cyclobutene system are symmetric, but in the butadiene system, only is symmetric with respect to the a plane.)... 

In conrotatory ring opening, the reoriented a orbitals derived from cyclobutene look like part of the butadiene molecular orbitals 1 2 and %. The orbitals derived from the double bond of cyclobutene look like part of the butadiene molecular orbitals and 3. Because the signs of the cyclobutene orbitals can be correlated with bonding orbitals of butadiene by... 

The 7T bonding orbital of cyclobutene arises from reorganization of the butadiene tt system. This process is considered in Woodward, R. B. Hoffmann, R. The Conservation of Orbital Symmetry, Verlag Chemie Weinheim, 1970 p. 38 ff. and references cited therein. 

For 1,3-butadiene, with four component p orbitals, there are four molecular orbitals for tt electrons (Fig. 29.6). The ground state has the configuration that is, there are two electrons in each of the bonding orbitals, and 2 The higher... 

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