Allyl cation molecular orbitals

Molecular orbitals allyl cation, 397 [10]-annulene, 425 benzene, 407, 424 bonding and antibonding, 34—35 1,3-butadiene, 397—398 cyclobutadiene, 424 cycloheptatrienyl cation, 427—428 cis, tram -1,3-cyclooctadiene, 524 cyclooctatetraene, 424 cyclopentadienide anion, 428 ethylene, 386—397 frontier, 386... 

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... 

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

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. 

Removal of an electron from an allyl radical gives the allyl cation => the electron is removed from the nonbonding tz molecular orbital. 

The ring opening of cyclopropyl cations (pp. 345, 1076) is an electrocyclic reaction and is governed by the orbital symmetry rules.389 For this case we invoke the rule that the o bond opens in such a way that the resulting/ orbitals have the symmetry of the highest occupied orbital of the product, in this case, an allylic cation. We may recall that an allylic system has three molecular orbitals (p. 32). For the cation, with only two electrons, the highest occupied orbital is the one of the lowest energy (A). Thus, the cyclopropyl cation must... 

Are the carbon-carbon bond distances in allyl cation, allyl radical and allyl anion all similar, or are they significantly different The three molecules differ mainly in the number of electrons they assign to one particular molecular orbital. (This is the lowest-unoccupied molecular orbital (LUMO) in allyl cation, and the highest-occupied molecular orbital (HOMO) in allyl radical and allyl anion.) Examine the shape of this orbital. Are the changes in electron occupancy consistent with the changes in CC bond length Explain. 

In the allyl cation, with two tt electrons, and in the anion, with four -n electrons, there are two in M(V Note that the nonbonding >Pmo2 is concentrated at the ends of the chain the molecular orbital pictures for these species thus correspond closely to the resonance pictures (see 8, 9, 10, p. 6), which show the charge or unpaired electron to be concentrated at the ends. 

Conjugated compounds undergo a variety of reactions, many of which involve intermediates that retain some of the resonance stabilization of the conjugated system. Common intermediates include allylic systems, particularly allylic cations and radicals. Allylic cations and radicals are stabilized by delocalization. First, we consider some reactions involving allylic cations and radicals, then (Section 15-8) we derive the molecular orbital picture of their bonding. 

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. 

Bromine is more electronegative than carbon and so the C-Br bond is polarized towards the bromine. If this bond were to break completely, the bromine would keep both electrons from the C-Br bond to become bromide ion, Br, leaving behind an organic cation. The end carbon would now only have three groups attached and so it becomes trigonal (sp2 hybridized). This leaves a vacant p orbital that we can combine with the n bond to give a new molecular orbital for the allyl system. 

So once again we have three p orbitals to combine. This is the same situation as before. We have the same atoms, the same orbitals, and so the same energy levels. In fact, the molecular orbital energy level diagram for this compound is almost the same as the one for the allyl cation the only difference is the number of electrons in the it system. Whereas in the allyl cation Jt system we only had two electrons, here we have three (two from the 7t bond plus the single one). Where does this extra election go Answer in tire next lowest molecular orbital—the nonbonding molecular orbital,... 

Where is the electron density in the allyl anion it system The answer is slightly more complicated than that for the allyl cation because now we have two full molecular orbitals and the electron density comes from a sum of both orbitals. This means there is electron density on all three carbon atoms. However, the HOMO for the anion is now the nonbonding molecular orbital. It is this orbital that contains the electrons highest in energy and so most reactive. In this orbital there is no electron density on the middle carbon it is all on the end carbons. Hence it will be the end carbons that will react with electrophiles. This is conveniently represented by curly arrows. 

Molecular orbitals demonstrate the smooth transition from the allyl silane, which has a k bond and a C-Si O bond, to the allylic product with a new K bond and a new o bond to the electrophile. The intermediate cation is mainly stabilized by O donation from the C-Si bond into the vacant p orbital but it has other a-donating groups (C—H, C-C, and C-E) that also help. The overall process is electrophilic substitution with allylic rearrangement. Both the site of attachment of the electrophile and the position of the new double bond are dictated by the silicon. 

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