Ethylene dimerization state correlation

Fig. 19 State correlation diagram for the dimerization reaction of two ethylene molecules...

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 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.,... 

Figure 7-13. State correlation diagram for the ethylene dimerization,...

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 < ...

The simplest cycloaddition, if the dimerization of two methylenes to form ethylene (Section 4.3) can be excluded, is the (l-f-2)-addition to ethylene of a singlet carbene CX2, where X is a halogen or another electronegative substituent. The reactions of methylene itself, and other carbenes that have triplet ground-states, will be deferred until the symmetry properties of electron spin are taken up. An incidental practical advantage of setting up the correlation digrams in Fig. 6.1... 

---