Urea clathrates

For physical processes, two examples are the elimination of normal paraffins from a mixture by their adsorption on 5 A molecular sieves or by their selective formation of solids with urea (clathrates)... 

Urea has the remarkable property of forming crystalline complexes or adducts with straight-chain organic compounds. These crystalline complexes consist of a hoUow channel, formed by the crystallized urea molecules, in which the hydrocarbon is completely occluded. Such compounds are known as clathrates. The type of hydrocarbon occluded, on the basis of its chain length, is determined by the temperature at which the clathrate is formed. This property of urea clathrates is widely used in the petroleum-refining industry for the production of jet aviation fuels (see Aviation and other gas-TURBINE fuels) and for dewaxing of lubricant oils (see also Petroleum, refinery processes). The clathrates are broken down by simply dissolving urea in water or in alcohol. 

Aliphatic hydrocarbon fractions are sometimes fractionated further into the fractions of normal and branched/cyclic alkanes by urea clathration or molecular sieves... 

That the situation is different for photochemical reactions is indicated by a particularly interesting recent study of some dialkylketones (239). In solution, 5-nonanone, 152, reacts photochemically to yield the cyclobutanol 153 and its isomer 154 in comparable amounts. Within the urea clathrate, however, 153 is the dominant product, with only traces of 154 being formed. The cyclobutanols analogous to 153, that is, having methyl and hydroxyl cis, also predominate in the urea-clathrate-mediated photocyclization of 2-hexanone and 2-undecanone. It might be expected that the bulky cyclobutane derivatives, which almost certainly cannot be crystallized in a urea clathrate, would also not be formed in such a clathrate. There are decomposition pathways (cleavage reaction 0 of the diradical intermediate that occur both in the clathrate and in solution. Nevertheless, the ring closure is a major pathway of reaction even in the clathrate. 

With larger guest molecules that do not fit the cavity, all three principal cyclodextrins are capable of forming channel structures in which the cyclodextrin cavities line up in order to produce an extended hydrophobic channel into which guests can be threaded in a similar way to urea clathrates... 

While the resonance of nitrogen in a urea clathrate containing heptane appeared to be unobservable80), due to a fluctuation in the vicinity of urea molecules, the nitrogen resonance was recently found to be visible in thiourea-cyclohexanc clathrate81) in which a well-defined crystalline structure leads to a unique environment for each thiourea molecule. This observation needs further confirmation. 

There is continued interest in micellar and surface effects, and unusual reaction media such as polymers, de Mayo and his co-workers have reported further examples of the way in which photoreactions can be modified if one reactant is adsorbed on to silica gel. Irradiation of nonan-5-one as a urea clathrate leads to reduced fragmentation and relatively increased and more selective cyclization in comparison with behaviour in methanol (Casal and... 

Clathrates provide cavities of a specific size and shape and therefore they can be used very effectively for separating gases with different sizes of molecules. For example, urea clathrates have been used to separate linear from branched hydrocarbons. The hydroquinone clathrate can be used to store and deliver radioactive krypton. In addition, if the host is chiral, there can be chiral discrimination so that one enantiomer of a guest is enclosed in the clathrate structure in preference to the other enantiomer. The trapped guest molecules can be liberated, for example, by solution in an apolar solvent, at the convenience of the user,... 

Bitumens, were separated by chromatography, urea clathration and 5A molecular sieve occlusion before and after analyses of many of the aliphatic sub-fractions by GC and gas chromatography-mass spectrometry (GC-MS). Experimental details are noted in a previous publication (16) in which the distribution of cyclic alkanes in two lacustrine deposits of Devonian (N.E. Scotland) and Permian (Autun, France) age, (the D and C series samples) were discussed, Chromatographic separation into aliphatic, aromatic and polar compounds of the bitumens extracted from the shales gave the results shown in Table VI. Carbon Preference Indices and pristane/phytane ratios were measured in this work space limitations precluded... 

Host guest ratios are ill-defined and depend on guest length, and the structures are incommensurate, rather like urea clathrates (Section 7.3). The host channels arise from the arrangement of 12 TOT molecules that form the channel repeat unit. Two sequences of six molecules form a doublehelix structure (Figure 7.26). 

From the examples in Figure 9.8, it can be seen that these urea clathrates (also known as inclusion compounds, adducts, or channel or cage compounds the process itself has been dubbed extractive crystallization17) are not confined to n-alkanes, but are also formed with straight-chain olefins, alcohols, esters, ketones, halides, etc. An X-ray structure determination18 first demonstrated the spiral hexagonal structure formed by the urea and the approximately 0.7 urea molecules per carbon of chain length.15... 

The slight difference between the lattice energies of the diastereomeric urea clathrates leads to low ee values and renders the steering of the recrystallization a difficult process which depends strongly on experimental conditions. Therefore, the development of new more universally applicable host substances has largely replaced urea for the resolution of racemates. 

This article provides a brief overview of the theory of IR spectroscopy and Raman spectroscopy (for more in-depth descriptions of these methods see Refs. [1-3]). This is followed by a review of vibrational spectroscopic studies performed on clathrate hydrates (a class of inclusion compound) and macrocyclic suprainolecular compounds. Clathrate hydrates were highlighted in this article because of all the clathrate compounds, they are particularly amenable to vibrational spectroscopy and are of great industrial significance. Similar IR/Raman methods can be applied to other well-known clathrate compounds. including quinol and urea " clathrates. Finally, future directions on the use of vibrational spectroscopy in supramolecular compounds will be given. 

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