Application of Resonance and of the 4n+2

Rule to Cyclic Ions

The hydrogens of the −CH2− group of 1,3-cyclopentadiene are acidic. In fact, they are considerably more acidic than the ethyne hydrogens of the 1-alkynes. This means that 1,3-cyclopentadiene is at least 1030 times more acidic than the ordinary alkanes. The reason is that loss of one of the CH2 protons of cyclopentadiene results in formation of an especially stabilized anion:

The structure of the anion may be described as a hybrid of five energetically equivalent structures, 34a through (34e). The unshared electron pair therefore is delocalized over five carbon atoms, and the resulting delocalized anion is much more stable than expected for any one of the equivalent localized structures:

This looks very reasonable, although the simple beauty is seemingly destroyed by the fact that the cyclopentadienyl cation is not very stable, despite the five structures, 35a through 35e , that may be written for it:

The experimental evidence is clear that, not only is the cation not stabilized in the same way as the anion, it also has a triplet electronic configuration. These facts agree with the molecular orbitals of Figure 21-13 for a cyclic system with five p orbitals, and also with the 4n+2 rule, because 34 has six π electrons, whereas 35 has only four.

Extension of these ideas to the other ring sizes of Figure 21-13 suggests that all of the following ions, which have (4n+2) π electrons, should be unusually stable:

In contrast, the following should be unstable with 4n π electrons and triplet electronic configurations:

These predictions indeed are borne out by many experiments, some of which we will discuss later.

It should be recognized that the term “stability” is subject to many interpretations. One criterion of stability would be whether an isolated molecule would fragment spontaneously in interstellar space, such as one would expect for a “molecule” consisting of two neon atoms 1.5Å

apart . A different criterion would be whether a molecule could be preserved in the presence of the same or other kinds of molecules at some specified temperature. The classical criterion would be whether the substance could be isolated, put into a bottle and preserved for at least a short time. All of the existing evidence indicates that cyclobutadiene molecules would not spontaneously decompose in interstellar space, but they do react with each other extremely readily, even at low temperatures, and can be preserved only by being held isolated from one another in a rigid matrix of a very inert material, such as solid argon. “Stability” in the sense of “lack of reactivity” has to be carefully defined in terms of experimental conditions. For example,  is very unstable in the presence of nucleophiles such as water or methanol, whereas it is quite stable in “super-acid solutions” where no good nucleophiles are present.