Allyl anion electronic configuration

Electronic Configurations of the Allyl Radical, Cation, and Anion 681... 

Figure 15-12 also shows the electronic configuration of the allyl anion, which differs from the allyl radical in having an additional electron in tt2, the nonbonding orbital with its electron density divided between Cl and C3. 

Q Show how to construct the molecular orbitals of ethylene, butadiene, and the allylic Problems 15-35 and 36 system. Show the electronic configurations of ethylene, butadiene, and the allyl cation, radical, and anion. 

The molecular orbitals of the allyl system are formed by the overlap of three atomic p orbitals. Because there is an odd number of atomic orbitals, one of the molecular orbitals is a nonbonding orbital, whose energy is comparable to that of the isolated p orbitals from which it was derived. Note that if there were degenerate molecular orbitals in the allyl system, the electronic configurations of various allyl species would be different. For example, if P2 3 for the allyl system had identical energy levels, the allyl anion would have two unpaired electrons. 

It was suggestedthat bicyclobutane formation from conjugated dienes occurs in a concerted fashion from vibrationally relaxed singlet having an allyl anion-methyl cation electronic configuration. 

Fig. 7a and b. Interconversion of cyclopropyl and allyl cation (1), radical (2), and anion (3) correlation diagrams for electron configurations and states a) conro-tatorymotion b) disrotatory motion... 

There is a third kind of orbital called a non-bonding orbital. Electrons in these orbitals neither strengthen nor weaken the bonding between atoms. The 7r-electron configurations for the ground states, and first excited states of the allyl cation, free radical, and anion are ... 

As we shall see. Frontier Configuration (FC) theory tells us that stereoselectivity in 4N + 3 electron systems should be qualitatively the same as stereoselectivity in 4N electon systems. Thus, for example, we expect that 1,3-butadiene, allyl anion, and allyl radical will all undergo electrocyclization in a conrotatory fashion. While there is every indication that this is the case in the first two systems, the ring closure of allyl radical-type systems is predicted by computations to be near disrotatory with indicative (but not definitive) experimental results pointing in the same direction. What is Wrong ... 

The ground-state tt-electron configuration of the allyl system is built up by putting electrons in pairs into the MOs, starting with those of lowest energy. Thus far, we have been describing our system as the allyl radical. However, since we have as yet made no use of the number of tt electrons in the system, our results so far apply equally well for the allyl cation, radical, or anion. 

We can determine the electron configuration of the allyl carbanion using the allyl radical as a reference. Adding another electron to the radical gives the anion. The second electron must be paired with the electron already located in the MO. C-1 and C-3 share the electron pair equally, and each one has a formal charge of -Vi. 

Figure 29.7. Allyl system. Configuration of tt electrons in cation, free radical, and anion.

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