Conjugate pi system

Molecular Orbitals and Pericyclic Reactions of Conjugated Pi Systems... 

It is apparent that the molecular orbital theory is a very useful method of classifying the ground and excited states of small molecules. The transition metal complexes occupy a special place here, and the last chapter is devoted entirely to this subject. We believe that modem inorganic chemists should be acquainted with the methods of the theory, and that they will find approximate one-electron calculations as helpful as the organic chemists have found simple Hiickel calculations. For this reason, we have included a calculation of the permanganate ion in Chapter 8. On the other hand, we have not considered conjugated pi systems because they are excellently discussed in a number of books. 

Let s carefully compare the movements of the atoms that must occur in these reactions to cause such different stereochemical results. As shown in Figure 22.1, the process is quite simple. The two end carbons of the conjugated pi system need only rotate so that their p orbitals begin to overlap to form the new sigma bond of the cyclobutene. In the case of the thermal reaction, both carbons rotate in the same direction, a process that is... 

Sigmatropic rearrangement (Section 22.8) The intramolecular migration of a group along a conjugated pi system. 

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. 

I. Molecular orbitals and pericyclic reactions of conjugated pi systems (Section 30.1). 

The LUMOs of conjugated pi systems are typically much lower in energy than that of ethene, so that the reduction of these systems to the corresponding anion radicals is more facile. The electrochemical reduction of 1,3-butadiene in liquid ammonia solution at —78 °C, in fact, yields an anion radical which is stable enough to permit observation by ESR spectroscopy (Scheme 66) [109]. 

MOLECULAR ORBITAL THEORY FOR CYCLIC CONJUGATED PI SYSTEMS... 

Figure 12.7 A summary of the molecular orbitals and relative energies for linear conjugated pi systems of 1 to 6 p orbitals. The phase of the ends of a linear pi system molecular orbital alternates as the energy of the MO increases. The fundamentai (lowest MO) is always symmetric.

Since the degenerate pair must be completely filled or completely empty in order for the molecule to be aromatic, stable arrangements arise when 2, 6, 10,. .. or 4n + 2 pi electrons are in the cyclic conjugated pi system. Unstable half-filled degenerate shells will occur with 4, 8, 12,. .. or 4n pi electrons. The 4n systems tend to distort from planarity to diminish pi overlap and this destabilizing antiaromatic conjugation. 

Nitrogen 1 is sp hybridized and its unshared pair of electrons is in an sp nonbonding AO. Nitrogen 2 is also sp hybridized, but its unshared pair of electrons is in a p orbital and is part of the conjugated pi system. Nitrogen 3 is sp hybridized and its unshared pair of electrons is in an sp nonbonding AO. 

The electrons on the N with the H are part of the conjugated pi system and are not very basic. The electrons on the N without the H are in an AO that is perpendicular to the pi system. These electrons are not involved in resonance and are much more basic. 

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