Spiropentane


Spiropentane is a hydrocarbon with formula C5H8. It is the simplest spiro-connected cycloalkane, a triangulane.
It took several years after the discovery in 1887 until the structure of the molecule was determined. According to the nomenclature rules for spiro compounds, the systematic name is spiropentane. However, there can be no constitutive isomeric spiropentanes, hence the name is unique without brackets and numbers.

Synthesis

After Gustavson produced cyclopropane by reacting 1,3-dibromopropane with dust fine zinc, he tried the same reaction with 2,2-Bis-1,3-dibromopropane. The starting material is easily obtained by reacting pentaerythritol with hydrobromic acid. A molecule with the formula C5H8 was obtained. It was referred to as vinyltrimethylene in the initial publication. In 1907, Fecht expressed the assumption that it must be spiropentane, a constitutional isomer of vinylcyclopropane.Further Evidence for the structure of the hydrocarbon comes from the fact that it could also be obtained from 1,1-bis-cyclopropane.
Spiropentane is difficult to separate from the other reaction products and the early procedures resulted in impure mixtures. Decades later, the production method was improved. The spiro hydrocarbon can be separated from the byproducts by distillation.

Properties

Physical Properties

Structural determination by electron diffraction showed two different C-C lengths; the bonds to the quarternary carbon atom are shorter than those between the methylene groups. The C–C–C angles on the spiro C atom are 62.2°, larger than in cyclopropane.

Chemical Properties

When heating molecules of spiropentane labelled with deuterium atoms, a topomerization or "stereomutation" reaction is observed, similar to that of cyclopropane: cis-1,2-dideuteriospiropentane equilibrates with trans-1,2-dideuteriospiropentane.
Gustavson reported that heating spiropentane to 200 °C caused it to change into other hydrocarbons. A thermolysis in the gas phase from 360 to 410 °C resulted in ring expansion to the constitutional isomer methylenecyclobutane, along with the fragmentation products ethene und propadiene. Presumably, the longer – and weaker – bond is broken first, forming a diradical intermediate.