Applications of Urea Inclusion Compounds

1-bromoadamantane. In this material all of the C-Br bonds are aligned with the channel axis and do not reorientate under ambient conditions. The compound achieves almost the theoretical maximum X-ray attenuation ratio.  

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The empty urea tunnel stmeture is unstable, and it has been shown by experiment and computer simulation that the tunnel collapses if the guest molecules are removed from the inclusion compound, leading to the pure crystalline phase of urea, which does not contain empty tunnels. The instability of the "empty" urea tuimel stmeture clearly limits the potential for developing certain applications of urea inclusion compounds. 

Well-developed applications of urea inclusion compounds have been in the areas of detergents, dewaxing and petroleum chemistry. Some recent novel advances include the use of the peroxydodecanoic acid-urea inclusion compound in a laundry bleach product and in the purification of insect pheromones . ... 

Many studies of urea inclusion compounds have focused on practical applications, such as inclusion polymerization. stabilization of liquids or unstable solids in an isolated form, and molecular separation and chromatography. Chemists studying fatty acids, for example, routinely use urea inclusion compounds to carry out molecular separations. Among industrial examples, urea inclusion compound formation has been used by the petroleum industry in the dewaxing of certain oil fractions, although zeolites are now routinely used in such applications. Other applications include stabilization of diacyl peroxides and peroxy acids in laundry products, and the use of urea inclusion compounds as solid supports in gas-liquid chromatography. 

One of the main interests in urea inclusion compounds is their potential application in the separation of linear and branched hydrocarbons in the petroleum industry. Because the channel diameter is only slightly larger than the van der Waals diameter of a linear hydrocarbon, only a small amount of branching can be tolerated. Whether a particular urea inclusion compound will form or not may be assessed simply by comparison of the channel size with the diameter of the guest. For example, benzene is... 

The fortuitous discovery that urea forms inclusion compounds with many unbranched organic molecules was first reported by Bengen in 1940 15). Structural studies by Schlenk 16) and by Smith 11 18> determined the now familiar canal structure of these materials. The chemistry of these compounds has been reviewed on a number of occasions 19-21), as have specific applications in chromatography22, their use in inclusion polymerisation 23,24), and studies of their thermodynamic25 and spectroscopic 26 properties. 

There are of course more organic compounds which can be used as host molecules in the formation of inclusion complexes. For most of them, e.g. urea and thiourea, crown ethers, cholic acid, hydroquinone, tri-o-thymotides and Di-anin s compound, the literature describing SS NMR applications in structural... 

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