Bonding orbital 1, 3-butadiene

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...

The energy associated with the 4 electrons in their atomic orbitals is 4a and so the dissociation energy of the delocalized bond in butadiene is 4.4/3. Each orbital contributes an amount of energy ej = a + m.j/3 and it is said to be bonding, anti-bonding or non-bonding for positive, negative or zero values of mr... 

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. 

Molecular-orbital bond orders are 1.930 for the C-l.C-2 bond, 1.859 for the C-3.C-6 bond and 1.363 for the C-2.C-3 bond.34 Comparing these values with those for butadiene (p. 31), we see that the C-l,C-2 bond contains more and the C-3.C-6 bond less double-bond character than the double bonds in butadiene. The resonance picture supports this conclusion, since each C-l.C-2 bond is double in three of the five canonical forms, while the... 

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... 

The simplest compound that can have two conjugated Jt bonds is butadiene. As we would now expect, this is a planar compound that can adopt two different conformations by rotating about the single bond. Rotation is somewhat restricted (around 30 kj mol-1) but nowhere near as much as in an amide (typically 60-90 kj mol-1). What do the molecular orbitals for the butadiene Jt system look like The lowest-energy molecular orbital will have all the p orbitals combining in-phase. The next lowest will have one node, and then two, and the highest-energy molecular orbital will have three nodes (that is, all the p orbitals will be out-of-phase). 

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... 

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. 

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