CHEMISTRY: Costs of Cyclobutadiene

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Science  21 Jul 2006:
Vol. 313, Issue 5785, pp. 275b
DOI: 10.1126/science.313.5785.275b

Chemists attribute much of the unusual stability of benzene and related aromatic hydrocarbons to the delocalized network of π-orbital electrons numbering 2 more than a multiple of 4. By the same logic, carbon rings with a π-electron count divisible by 4 ought to be especially unstable. In the case of the simplest such anti-aromatic compound, cyclobutadiene (C4H4), this electronic instability is compounded by the strain inherent in the square bonding geometry of the carbons. Nonetheless, this small square ring and a number of its derivatives have been synthesized and characterized. The thermodynamic cost of its formation from the elements, however, has eluded quantification without the help of theoretical modeling.

Fattahi et al. present a fully experimental derivation of the heat of formation of cyclobutadiene from carbon and hydrogen. They begin by gas-phase ionization of 3-chlorocyclobutene, which leads to loss of Cl and formation of the cyclobutenyl cation. Deprotonation of this compound yields cyclobutadiene. By adding various bases to bracket the enthalpy of this deprotonation step, as well as electron donors to bracket the electron affinity of the cation, they determine the reaction enthalpy for cyclobutene dehydrogenation to cyclobutadiene. They then add the cyclobutene heat of formation (known from calorimetry) to derive a cyclobutadiene heat of formation of 429 ± 16 kJ/mol, in good agreement with accompanying theoretical computations. — JSY

Angew. Chem. Int. Ed. 45, 10.1002/anie.200600839 (2006).

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