Orbital correlation diagram ethylene dimerization

Figure 7.10 An orbital correlation diagram for ethylene dimerization. Left two widely separated ethylene molecules. Center two ethylene molecules close enough for significant interactions to occur. Right cyclobutane electron configurations correspond to the ground state for each stage.

The dimerization of acyclic polyenes in which all n bonds are lost would lead to the open structures of (54) and (55). A schematic orbital correlation diagram (Fig. 15) for process (54) shows that allyl dimerization is improbable. The cyclization of higher acyclic polyenes, e.g. to cis-or trans-7 in (55), is subject toa similar prohibition, but the formation of 8 is allowed. In general, processes in which the products retain elements of symmetry inherent in the reactants are symmetry-forbidden the argument used to demonstrate this is analogous to that used for ethylene. One dimerization of 1,3-butadiene, namely to 9, is unique this... 

The rules for the state correlation diagrams are the same as for the orbital correlation diagrams only states that possess the same symmetry can be connected. In order to determine the symmetries of the states, first the symmetries of the MOs must be determined. These are given for the face-to-face dimerization of ethylene in Table 7-1. The D2h character table (Table 7-2) shows that the two crucial symmetry elements are the symmetry planes a(xy) and v"(yz). The MOs are all symmetric with respect to the third plane, vide supra). The corresponding three symmetry operations will unambiguously determine the symmetry of the MOs. Another possibility is to take the simplest subgroup of D2t, which already contains the two crucial symmetry operations, that is, the C2v point group (cf.,... 

The orbital correlation diagram for the concerted dimerization of ethylene to form cyclobutane or for the reverse reaction, the fragmentation of cyclobutane into two ethylenes, may be obtained most easily by applying the principle of conservation of orbital symmetry. A mirror plane perpendicular to the molec-... 

FIGURE 11.1. An orbital correlation diagram lor the concerted dimerization reaction of ethylene. 

Fig. 14. Orbital and state symmetry correlations for the dimerization of ethylene. The orbital symmetry designations in the upper diagram are with respect to the planes of symmetry tjyt and < ...

MOs, while tlie two 7t c orbitals lead to the tt and tt MOs. In the initial stage of (he dimerization, the interaction between two ethylencs is weak so that 7t+ and tt. lie far below the n+ and tt levels, so that only 7t+ and rr are occupied. Of the a orbitals of cyclobutane described earlier, only those related to the tt., 7t1 and nl levels by symmetry are shown in Figure 11.1. Not all the occupied MOs of the reactant lead to occupied orbitals in the product. In particular, tt. correlates with one component of the empty set in cyclobutane. The tt+ combination ultimately becomes one component of the filled set in cyclobutane. So the reaction is symmetry forbidden. The reader should carefully compare the correlation diagram for ethylene dimerization here with the Ho + O2 reaction in ITgure 5.8. flie two correlation diagrams are very similar, as they should be, since in this instance the spatial dfstributions of tt and n " are similar to those of and respectively, in H2. These two reactions are probably the premier examples of symmetry-forbidden reactions. A related symmetry-allowed example is the concerted cycloaddition of ethylene and butadiene, the Diels-Alder reaction. We shall not cover the orbital symmetry rules for organic, pericyclic reactions. There are several excellent reviews that the reader should consult.But it should be pointed out that the orbital symmetry rules have stereochemical implications in terms of the reaction path and products formed. The development of these rules by Woodward and Hoffmann... 

It is a short step from acetylene to ethylene, but instead of constructing a correlation diagram for addition of H2 across the acetylenic triple bond, let us consider the dimerization of two coplanar methylene molecules. For consistency with the axis convention of Figs. 4.1-4.4, the methylene molecules on the the left side of Fig. 4.7 are placed in in the zx plane and allowed to approach each other along their common C2 axis (x), leaving the Py orbitals free for tt bonding. The symmetry of the field exerted on the electrons by the nuclear frame is D2/1 no hypothetical external field need be postulated. 

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