Symmetry imposed barrier

If the reaetants eould be prepared, for example by photolysis, in an exeited state having orbital oeeupaney 7112713 713, then reaetion along the path eonsidered would not have any symmetry-imposed barrier beeause this singly exeited eonfiguration eorrelates to a singly-exeited eonfiguration of the produets. The faet that the reaetant and produet... 

It should be stressed that although these symmetry considerations may allow one to anticipate barriers on reaction potential energy surfaces, they have nothing to do with the thermodynamic energy differences of such reactions. Symmetry says whether there will be symmetry-imposed barriers above and beyond any thermodynamic energy differences. The enthalpies of formation of reactants and products contain the information about the reaction s overall energy balance. 

The situation is reversed for the tt2s + n4s addition. Figure 11.16 illustrates this case now the bonding orbitals all transform directly to bonding orbitals of the product and there is no symmetry-imposed barrier. As with the electrocyclic processes, the correlation diagrams illustrate clearly the reason for the striking difference observed experimentally when the number of electrons is increased from four to six. The reader may verify that the 4s + 4s reaction will be forbidden. Each change of the total number of electrons by two reverses the selection rule. 

The state correlation diagram for a [2+2] cycloaddition showing the symmetry-imposed barrier E... 

On the other hand, stereospecificity is not always complete, and many ketene cycloadditions take place only when there is a strong donor substituent on the alkene. An ionic stepwise pathway by way of an intermediate zwitterion 3.34 is therefore entirely reasonable in accounting for many ketene cycloadditions. It seems likely that some of these reactions are pericyclic and some not, with the possibility of there,being a rather blurred borderline between the two mechanisms, with one bond forming so far ahead of the other that any symmetry in the orbitals is essentially lost. But when it is pericyclic, how does it overcome the symmetry-imposed barrier ... 

Correlation diagrams have given us a convincing sense of where the barriers come from for those reactions that we have been calling forbidden. In principle, of course, no reaction is forbidden—what these reactions have is a formidable symmetry-imposed barrier, and something very unusual is needed if barriers of this magnitude are to be crossed. 

The original explanation of Woodward and Hoffmann involved construction of an orbital correlation diagram for the reaction under consideration, and then carrying out the reaction in such a manner so that the symmetries of the reactant and product orbitals matched exactly. If the correlation diagram indicates that the reaction may occur without encountering a symmetry-imposed barrier, it is termed symmetry-allowed. If a symmetry is present, the reaction is designated symmetry-forbidden. 

But if an electron of one ethene molecule is promoted to the antibonding orbital, then there is no symmetry-imposed barrier to the reaction. Thus, photochemically it is symmetric allowed. 

Xas->sa °f the same symmetry aa (dashed lines). Configuration interaction between xo of symmetry, v,v and the doubly excited configuration of the same symmetry splits the two crossing states of symmetry ss apart (full lines), but a symmetry-imposed barrier remains, which classifies the reaction as forbidden in the ground state. 

As there is no symmetry-imposed barrier in the ground state MOs of both the reactants and the product, the reaction is thermal in nature. Although there is one-to-one symmetry correspondence among all the anti-bonding MOs of the reactants and the product, there is a huge energy-imposed barrier to the reaction because the first excited state of the diene-alkene complex, i.e., 2> does not correlate with the first excited state of cyclohexene, i.e., it. ... 

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