Bullvalene

Tricyclo [ 3.3.2.02,8 ] deca -3 ,6,9 -triene

Crystalline solid

Fixed

96 ° C

From 400 ° C: decomposition to naphthalene and hydrogen

Template: Infobox chemical / molecular formula search available

Bullvalene is an unsaturated polycyclic hydrocarbon, which is extremely stable despite a cyclopropane skeleton at low temperatures.

History and name

The name of the substance bullvalene was composed of the name of the scientist, William " Bull" Doering, from the team that predicted the properties of the molecule basis of theoretical calculations in 1963, and the property of the molecule to exist in very many valence tautomeric forms. The first synthesis succeeded in the same year G. Schröder by photolysis of a dimeric cyclooctatetraene with removal of benzene.

Properties, Preparation and Reactions

Bullvalene forms colorless crystals having a melting point of 96 ° C. Only at 400 ° C decomposes the substance in naphthalene and hydrogen. Bullvalene is synthetically very accessible from cyclooctatetraene and discusses the expected addition reactions by each 4 moles of bromine or hydrogen for three double bonds and cyclopropane ring are consumed.

Structure

Bullvalene in the three carbon atoms of the cyclopropane ring with three methine vinyl bridges are star -connected. This complicated polycyclic structure is surprisingly stable and is formed by reaction with quite high yield.

The reason lies in the stabilization of the system by valence tautomerism, in this case by a [3,3 ] sigmatropic rearrangement, the so-called Cope rearrangement. In a discrete Bullvalenmolekül are, as Figure 1 shows three possible Cope systems before. In the Cope rearrangement in bullvalenes forms over a six-membered transition state, a new cyclopropane ring, while the old breaks and shift the double bond, as shown in Figure 2 Due to the specific structure of bullvalene seen from this rearrangement, however, again a Bullvalenmolekül out, just take the carbon atoms individually in other places a new molecule.

From 100 ° C, the Cope rearrangements between the more than 1.2 million possible arrangements take place (just 10! / 3 = 1,209,600 options ), the Automeren (see tautomerism ), so quickly that the 1H -NMR spectrum of the hydrogen atoms has only a single, sharp signal at 4.2 ppm. This means that at this temperature all of the hydrogen atoms are equivalent, and led to the expression of " structural fluctuating ". This assumption was proven by X-ray crystallography at room temperature and neutron diffraction experiments at 100 K.