Cyclobutadiene, angle strain stability

DE, since the 7c-electron energy is 4a + 4/3, the same as that for two independent double bonds. Thus, at this level of approximation, HMO theory prediets no stabilization for eyelobutadiene from delocalization and furthermore predicts that the molecule will have unpaired electrons, which would lead to very high reactivity. In addition, cyclobutadiene would suffer angle strain, whieh is not present in benzene. The extreme instability of eyelobutadiene is then understandable. Higher-level MO ealculations modify this picture somewhat and predict that eyelobutadiene will be a rectangular molecule, as will be diseussed in Chapter 9. These ealculations, nevertheless, agree with simple HMO theory in... 

The first synthesis of an isolable cyclobutadiene (71) kinetically stabilized by bulky substituents was achieved starting from the angle strained cycloalkyne (31) via the PdCl2 complex (69)9 198,199). Here again the high reactivities of (10), (31) and (79)... 

Cyclobutadiene has two pairs of rr electrons, and cyclooctatetraene has four pairs of tt electrons. Unlike benzene, these compounds are not aromatic because they have an even number of pairs of rr electrons. There is an additional reason why cyclooctatetraene is not aromatic—it is not planar but, instead, tub-shaped. Earlier, we saw that, for an eight-membered ring to be planar, it must have bond angles of 135° (Chapter 2, Problem 28), and we know that sp carbons have 120° bond angles. Therefore, if cyclooctatetraene were planar, it would have considerable angle strain. Because cyclobutadiene and cyclooctatetraene are not aromatic, they do not have the unusual stability of aromatic compounds. 

---