Prismane or 'Ladenburg benzene' is a polycyclic

hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ...

with the formula C₆H₆. It is an isomer of

benzene Benzene is an organic chemical compound with the molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar ring with one hydrogen atom attached to each. Because it contains only carbon and hydrogen atoms ...

, specifically a

valence isomer In organic chemistry, two molecules are valence isomers when they are constitutional isomers that can interconvert through pericyclic reactions. Benzene There are many valence isomers one can draw for the C6H6 formula benzene. Some were originally ...

. Prismane is far less stable than benzene. The carbon (and hydrogen) atoms of the prismane

molecule A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioche ...

are arranged in the shape of a six-atom

triangular A triangle is a polygon with three edges and three vertices. It is one of the basic shapes in geometry. A triangle with vertices ''A'', ''B'', and ''C'' is denoted \triangle ABC. In Euclidean geometry, any three points, when non- collinea ...

prism Prism usually refers to: * Prism (optics), a transparent optical component with flat surfaces that refract light * Prism (geometry), a kind of polyhedron Prism may also refer to: Science and mathematics * Prism (geology), a type of sedimentary ...

—this compound is the parent and simplest member of the

prismanes The prismanes are a class of hydrocarbon compounds consisting of prism-like polyhedra of various numbers of sides on the polygonal base. Chemically, it is a series of fused cyclobutane rings (a ladderane, with all- cis/all- syn geometry) that wra ...

class of molecules. Albert Ladenburg proposed this structure for the compound now known as

. The compound was not synthesized until 1973.

History

In the mid 19th century, investigators proposed several possible structures for benzene which were consistent with its empirical formula, C₆H₆, which had been determined by combustion analysis. The first, which was proposed by Kekulé in 1865, later proved to be closest to the true structure of benzene. This structure inspired several others to draw structures that were consistent with benzene's empirical formula; for example,

Ladenburg Ladenburg is a town in northwestern Baden-Württemberg, Germany. It lies on the right bank of the river Neckar, northwest of Heidelberg and east of Mannheim. The town's history goes back to the Celtic and Roman Ages, when it was called Lopo ...

proposed prismane, Dewar proposed Dewar benzene, and Koerner and Claus proposed Claus' benzene. Some of these structures would be synthesized in the following years. Prismane, like the other proposed structures for benzene, is still often cited in the literature, because it is part of the historical struggle toward understanding the mesomeric structures and

resonance Resonance describes the phenomenon of increased amplitude that occurs when the frequency of an applied Periodic function, periodic force (or a Fourier analysis, Fourier component of it) is equal or close to a natural frequency of the system ...

of benzene. Some computational chemists still research the differences between the possible isomers of C₆H₆.

Properties

Prismane is a colourless liquid at room temperature. The deviation of the carbon-carbon bond angle from 109° to 60° in a triangle leads to a high ring strain, reminiscent of that of cyclopropane but greater. The compound is explosive, which is unusual for a hydrocarbon. Due to this ring strain, the bonds have a low bond energy and break at a low activation energy, which makes synthesis of the molecule difficult; Woodward and Hoffmann noted that prismane's thermal rearrangement to benzene is symmetry-forbidden, comparing it to "an angry tiger unable to break out of a paper cage." On account of its strain energy and the aromatic stabilization of benzene, the molecule is estimated to be 90 kcal/mole less stable than benzene, but the activation of this highly exothermic transformation is a surprisingly high 33 kcal/mol, making it persistent at room temperature. The substituted derivative hexamethylprismane (in which all six hydrogens are substituted by methyl groups) has a higher stability, and was synthesized by rearrangement reactions in 1966.

Synthesis

The synthesis starts from benzvalene (1) and 4-phenyltriazolidone (2), which is a strong dienophile. The reaction is a stepwise Diels-Alder like reaction, forming a carbocation as intermediate. The adduct (3) is then hydrolyzed under basic conditions and afterwards transformed into a copper(II) chloride derivative with acidic copper(II) chloride. Neutralized with a strong base, the

azo compound Azo compounds are organic compounds bearing the functional group diazenyl (, in which R and R′ can be either aryl or alkyl groups). IUPAC defines azo compounds as: "Derivatives of diazene (diimide), , wherein both hydrogens are substituted ...

(5) could be crystallized with 65% yield. The last step is a photolysis of the azo compound. This photolysis leads to a biradical which forms prismane (6) and

nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...

with a yield of less than 10%. The compound was isolated by preparative gas chromatography.

References

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External links

Molecules with silly or unusual Names
Hydrocarbons Molecular geometry Tetracyclic compounds Cyclobutanes Cyclopropanes Explosive chemicals Substances discovered in the 1970s

History

Properties

Synthesis

See also

References

External links