PROPERTIES

Due to cubanes exceptional structure, strain and symmetry, it is a benchmark in organic chemistry. It has been studied extensively since its synthesis in 1964, and much concerning its properties has been published.

Some of the physical parameters are given in the table below:

C - C distance

1.5727 ± 0.0019 Å

C - H distance

1.118 ± 0.008 Å

Colour

Transparent

Toxicity

Nontoxic

Stability

Inert to light, water and air

Decomposition

> 220 Deg. C

Density

1.29 g cm-3

Vapour pressure

1.1 mm (25 deg. C)

Melting point

130 - 131 Deg. C

Boiling point

133 Deg. C

Solubility

18 wt % (hexane)

Heat of formation

+ 144 kcal mol-1

Strain Energy

166 kcal mol-1

It is interesting to note that the C - C bond length is not much different that the C - C bond length found in a simple cyclobutane.

Molecular mechanics calculations, semi-empirical and ab initio quantum mechanical treatments of cubane abound. The agreements between methods is fair. The bottom line experimentally is that cubane is remarkably stable. It has been shown that the energy of activation at 230 - 260 deg. C is 43.1 ± 0.1 kcal mol-1 , an extraordinarily high value. Cubane is entirely stable to light, air, water and most common reagents.

 

The Nature of the C - H Bond

The geometry of the cubane system dictates substantial rehybridisation of the component tetravalent carbon atoms away from sp3 hybridisation. The carbon - carbon bonds become p-rich, and the exocyclic carbon orbital used for the C - H bond becomes s-rich. There is about 31% s character in this orbital as calculated from the observed 13C - 1H coupling constant of 155Hz. Thus, the hydrogen atoms of cubane are expected to be more acidic than those of strain-free, saturated hydrocarbons. The kinetic acidity of cubane is about 63 000 times that of cyclohexane. Nonetheless, this is far too low to be useful for direct synthetic applications. (See Reactivity).

 

Spectroscopic Properties

The identity of cubane follows unmistakably from the parent peak in the mass spectrum at m / e 104 and from the diagnostic simplicity or the infrared spectrum in which the only noticeable adsorptions in the region from 4000 to 660 wavenumbers appear at 3000, 1231 and 851 cm-1.

The proton NMR shows a single-line absorption at d 4.0 p.p.m., ( t 6.0 p.p.m., very low field for a cycloalkane) with a width at half-height of 0.3 c.p.s. and a J13CH of 160Hz.

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