Molecular orbital of allyl cation

FIGURE 10 13 Their molecular orbitals of allyl cation The allyl cation has two IT electrons and they are in the orbital marked it. 

The TT molecular orbitals of allyl cation, radical, and anion. Allyl cation has two tt electrons, allyl radical has three, and allyl anion has four. 

Write the molecular orbitals of allyl anion, allyl radical and allyl cation with the number of electrons in each. Please indicate the symmetry characteristics of each orbital in respect of mirror plane and also C2 axis of symmetry. 

Refer to the molecular orbital diagrams of allyl cation (Figure 10 13) and those presented earlier in this chapter for ethylene and 1 3 butadiene (Figures 10 9 and 10 10) to decide which of the following cycloaddition reactions are allowed and which are forbidden according to the Woodward-Floffmann rules... 

Figure 13.3 The n molecular orbitals of the allyl cation. The allyl cation, like the allyl radical, is a conjugated unsaturated system. The shapes of molecular orbitals for the allyl cation calculated using quantum mechanical principles are shown alongside the schematic orbitals. 

The bonding n molecular orbital of the allyl cation contains two spin-paired electrons. 

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. 

We can summarize all this information in a molecular orbital energy level diagram, the n molecular orbitals of the allyl system the allyl cation... 

Even if silicon chemistry is new to you, you should by now have a picture of stable compounds with C-Si bonds and selective reaction with fluoride. You are already familiar with silyl enol ethers as nucleophilic enolate equivalents and allyl silanes resemble these in many ways. The missing link is the i-silyl effect. A Si atom stabilises a cation in the p-posit ion by overlap of the populated and relatively high energy C-Si c-orbital with the empty p orbital of the cation. This overlap is already present in the preferred conformation 95a of the allyl silane 95 as an anti-bonding interaction 95b between the C-Si c-orbital and the n orbital of the double bond. The resultant molecular orbital (the new HOMO) 95c increases the nucleophilic reactivity of the carbon atom in the y-position. 

The n molecular orbital energy level diagram sketched, for the allyl system under Huckel approximation and computed the delocalization energy of allyl cation ... 

Let s now look at the molecular orbitals of the allyl cation, the allyl radical, and the allyl anion. 

The carbocation of Figure 5.19 can be written either as one of the two canonical forms (5.26) and (5.27) or using an electron smear (5.28) as shown in Figure 5.20. The smear is closer to reality since the molecular orbitals of the molecule will be distributed across the three carbons of the allylic cation. However, it will be able to react as if the positive charge were localised at either end. For each individual reaction, the nature of the other reactive species, the reaction conditions and the nature of the product will determine which way round the system will react. 

Refer to the molecular orbital diagrams of allyl cation (Figure 10.14) and those... 

The bonding tt molecular orbital of the allyl cation, like that of the allyl radical (Fig. 13.1), contains two spin-paired electrons. The nonbonding tt molecular orbital of the allyl cation, however, is empty. 

Figure 1.8 -Molecular orbitals of the allyl system. From left to right, the electron occupancies are for the cation, radical and anion, which have two, three, and four x-electrons, respectively.

Figure 14-3 The Aufbau principle is used to fill up the tt molecular orbitals of 2-propenyl (allyl) cation, radical, and anion. In each case, the total energy of the TT electrons is lower than that of three noninteracting p orbitals. Partial cation, radical, or anion character is present at the end carbons in these systems, a result of the location of the lobes in the 772 molecular orbital.

Molecular orbitals are useful tools for identifying reactive sites m a molecule For exam pie the positive charge m allyl cation is delocalized over the two terminal carbon atoms and both atoms can act as electron acceptors This is normally shown using two reso nance structures but a more compact way to see this is to look at the shape of the ion s LUMO (the LUMO is a molecule s electron acceptor orbital) Allyl cation s LUMO appears as four surfaces Two surfaces are positioned near each of the terminal carbon atoms and they identify allyl cation s electron acceptor sites... 

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