Molecular orbitals allyl anion

Like the allyl anion, the orbitals in the allyl cation are a combination of three atomic p orbitals, one from each carbon. So we can use the same molecular orbital energy level diagram as we did for the anion, simply by adjusting the number of electrons we put into the orbitals. This time, there are only two electrons, from the alkene, as those which were in the C—Br bond have left with anionic bromide. 

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. 

Molecular orbital calculations by Hofmann (162) indicated that an if-allyl anion complex with a noncomplexed butadiene group is more stable than an -butadiene complex, in which the allyl anion portion of the seven-carbon ring is not bonded to the metal. We have now been able to confirm this result by an X-ray structure analysis (see Fig. 8) on [Ph4As][C7H7Fe(CO)3] (163). 

This molecular orbital representation of the allyl anion is consistent with the resonance forms shown earlier, with a negative charge and a lone pair of nonbonding electrons evenly 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. 

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. 

Such predictions from a consideration of the molecular orbitals are confirmed both by the reactions of the allyl anion and by its NMR spectrum. It is possible to record a carbon NMR spectrum of the allyl anion directly (for example, as its lithium derivative). The spectrum shows only two signals the middle carbon at 147 p.p.m. and the two end carbons both at 51 p.p.m. 

The molecular orbital energy diagram for the carboxylate anion is the very similar to that of the allyl system. There are just two main differences. 

Nonetheless, some useful information regarding methylation has been obtained from molecular orbital calculations. One CNDO/2 study on the allyl and pentadienyl anions indicated that in all cases the methyl group would serve to withdraw net electron density from the anions196. For the allyl anion, a 1-methyl substituent would withdraw a substantial 0.102 electrons, while at the formally uncharged 2 position the effect was consider-... 

The cation, radical and anion have the same a framework 1.4, with fourteen bonding molecular orbitals filled with 28 electrons made by mixing the Is orbitals of the five hydrogen atoms either with the 2s, 2px and 2py orbitals of the three carbon atoms or with the sp2 hybrids. The allyl systems are bent not linear, but we shall treat the % system as linear to simplify the discussion. 

The n system is made up from the three pz orbitals on the carbon atoms. The linear combination of these orbitals takes the form of Equation 1.9, with three terms, creating a pattern of three molecular orbitals, tpi, ip2 and 3. In the allyl cation there are two electrons left to go into the n system after filling the a framework (and in the radical, three, and in the anion, four). 

As we shall see later, the most important orbitals with respect to reactivity are the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). These are the frontier orbitals. For the allyl cation, the LUMO is ip2, and the drawings of the allyl cation 1.1a and 1.1b emphasise the electron distribution in the LUMO. Similarly, the corresponding drawings of the allyl anion emphasise ip2, the HOMO for that species. It is significant that it is the LUMO of the cation and the HOMO of the anion that will prove to be the more important frontier orbital in each case. Similarly in the allyl radical, the localised... 

The molecular orbitals in the HF2 system in Fig. 2.17 resemble those of the allyl anion—a low-energy orbital with no nodes, and an orbital with a node at the central atom. The node at the hydrogen atom leaves it with no interactions with the two fluorine atoms, which are far enough apart to be essentially nonbonding. For this arrangement to be stabilised, i/ , and ip2 must together be lower in energy... 

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. 

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