Conjugated diene molecular orbitals

Stability of Conjugated Dienes Molecular Orbital Theory 4133... 

Conjugated enones are more stable than nonconjugated enones for the same reason that conjugated dienes are more stable than nonconjugated dienes (Section 14.1). Interaction between the tt electrons of the C=C bond and the tt electrons of the C=0 group leads to a molecular orbital description for a conjugated enone that shows an interaction of the tt electrons over all four atomic centers (Figure 23.3). 

Figure 23.3 The - bonding molecular orbitals of a conjugated erone (propenal) and a conjugated diene (1,3-butadiene) are similar in shape and are spread over the entire %< system.

As applied to cycloaddition reactions the rule is that reactions are allowed only when all overlaps between the HOMO of one reactant and the LUMO of the other are such that a positive lobe overlaps only with another positive lobe and a negative lobe only with another negative lobe. We may recall that monoalkenes have two n molecular orbitals (p. 9) and that conjugated dienes have four (p. 36), as shown in Figure 15.1. A concerted cyclization of two monoalkenes (a 2 -f- 2 reaction) is not allowed because it would require that a positive lobe overlap with a negative lobe (Fig. 15.2). On the other hand, the Diels-Alder reaction (a 2 -f 4 reaction) is allowed, whether considered from either direction (Fig. 15.3). 

As in benzene, the substituents in ortho and para will be better conjugated than those in meta. These qualitative aspects of regioselectivity have been studied quantitatively by quantum mechanical studies. Further the molecular orbital considerations have shown that the favoured transition state will be that in which there is strongest interaction between the HOMO of the diene and LUMO of the dienophile. In the most common cases the dienophile bears an electron withdrawing substituent and the diene an electron releasing one. Here the interation is between the LUMO of the dienophile and the HOMO of diene. 

Overlapping could, of course, take place, 1,2 3,4 5,6 or 1,6 5,4 3,2, leading to formulations corresponding to the Kekule structures 4a and 46) but, as an alternative, all six adjacent p orbitals could overlap, as with conjugated dienes (p. 12), resulting in the formation of six molecular orbitals, three bonding 3) and three anti-bonding... 

Lewis structures are inadequate to represent delocalized molecules such as buta-1,3-diene. To represent the bonding in conjugated systems accurately, we must consider molecular orbitals that represent the entire conjugated pi system, and not just one bond at a time. 

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