Ladenburg benzene prismane

PROBLEM 13.30 Write arrow formalisms for the conversions of Dewar benzene, Ladenburg benzene (prismane), benzvalene, and 3,3 -bicyclopropenyl (Fig. 13.4) into benzene. Some of these tasks may be quite challenging. 

The X-ray crystal structure of benzene, proving the equivalence of the six C-C bonds, appeared in 1929 and 1932, and Pauling reported its electron-diffraction data in 1931. Note that several of the structures proposed in the 19th century, such as Dewar benzene (non-planar) and Ladenburg s prismane, which are valence isomers of benzene, have now actually been prepared from benzene derivatives photochemically. They are kinetically stabilized, since they do not spontaneously revert to benzene or its derivatives [17-20]. 

Prismane = Ladenburg benzene=tetracyclo[2.0.0.02/6.03/S], featuring two pairs of equivalent protons (single 1H, single 13C line). Time-average symmetry D3h.92... 

In the course of the study of the structure of benzene, several structural formulas have been proposed for it. Well known structures are Kekule s cyclohexatriene, Ladenburg s prismane, Thiele s partial bond structure, and Dewar s bicyclohexadiene. Later, Huckel proposed benzvalene as another structure of benzene. Since Kekule s structure was gradually accepted as the structure of benzene, other structures are only of historical importance. 

The three Dewar structures 5, 11 and 12 (Dewar benzene) are also considered to contribute to the resonance hybrid (according to valence bond theory, approximately 20% in total) and to the extra stability. Dewar benzene has now been prepared. It is a bent, non-planar molecule and is not aromatic. It gradually reverts to benzene at room temperature. The Ladenburg structure, prismane i6), is an explosive liquid. Dewar benzene and prismane are valence isomers of benzene. 

Derivatives of Ladenburg "benzene," called prismanes, have actually been made.)... 

Ladenburg benzene An incorrect structure for BENZENE in which the six carbon atoms are at the corners of a triangular prism. It is named for Albert Ladenburg (1842-1911). The actual compound, known as prismane, was synthesized in 1973. 

Bicyclo[2.2.0]hexadienes and prismanes are valence isomers of benzenes. These compiounds actually have the structures that were proposed for benzenes in the nineteenth century. Prismanes have the Ladenburg formula, and bicyclo[2.2.0]-hexadienes have the Dewar formula. Because of this bicyclo[2.2.0]hexadiene is often called Dewar benzene. On page 32 it was mentioned that Dewar formulas are canonical forms (though not very important) of benzenes. Yet, they also exist as separate compounds in which the positions of the nuclei are different from those of benzenes. 

Another extraordinarily strained polycyclic hydrocarbon that has been prepared in recent years is prismane (the Ladenburg structure for benzene, see Exercise 1-6). 

Valence isomer A type of structural isomer that has the same basic skeleton, but differs in the exact bonding arrangement, e.g. the prismanes (or Ladenburg structure for benzene), and the Dewar and Kekule structures for benzene (i.e. the bicyclo[2.2.0]hexadienes), are all valence isomers of each other. 

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