Antiaromaticity, An Unusual Destabilizing Effect

In contrast to the stability of aromatic systems, a planar tt system with 4n electrons is generally observed to be unstable, and is called antiaromatic. The instability of such systems is quite severe, as shown with one example in the Connections highlight on the next page. In Section 14.5.6 we will show that such systems can have biradical character, and 

Cyclopropenyl cation is stable enough to be isolated and stored in a bottle, albeit at -20 °C. It can be prepared by chloride abstraction from 3-chlorocyclopropene, as shown below. The H NMR spectrum in nitromethane solvent shows a singlet at a very downfield shift 11.2 ppm. 

One can compare the reactivity of cyclopropenyl cation to allyl cation as a means of estimating the stability imparted by aromaticity. In the following equilibrium, it 

Estimating the destabilization imparted by the antiaromaticity of cyclobutadiene is more of a challenge than estimating the stability imparted to benzene. It was not until late 1999 that AHf° for this prototype structure was determined by Snyder and Peters. Using photoacoustic calorimetry, a AH° value of 114 11 Real/mol was determined for cyclobutadiene. Using this and other relevant data, we find that Eq. 2.33, the direct analogue of Eq. 2.32, is endothermic by 46 Real / mol 11 Real / mol. It is not obvious exactly how to incorporate strain effects, but to first order it is a good approximation to say that Eq. 2.33 is strain neutral. This analysis leads to the conclusion that the antiaromaticity of cyclobutadiene is more substantial than the aromaticity of benzene. This difference is especially true when we consider these values on a per carbon basis. 

Cyclobutadiene does not adopt a perfectly square structure. It rapidly interconverts between two rectangular forms. The distortion from a square arises from a pseudo-Jahn-Teller effect, which leads to a lowering of the energy of the system. This dynamic process (Eq. 2.34) has a very low activation barrier, estimated to be 5-10 Rcal/mol. 

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