Urea clathration

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. 

The history of iaclusion compounds (1,2) dates back to 1823 when Michael Faraday reported the preparation of the clathrate hydrate of chlorine. Other early observations iaclude the preparation of graphite iatercalates ia 1841, the P-hydroquiaone H2S clathrate ia 1849, the choleic acids ia 1885, the cyclodexthn iaclusion compounds ia 1891, and the Hofmann s clathrate ia 1897. Later milestones of the development of iaclusion compounds refer to the tri-(9-thymotide benzene iaclusion compound ia 1914, pheaol clathrates ia 1935, and urea adducts ia 1940. 

A 90% yield of isoquinoline (>95% pure) was reported by treating a cmde fraction with hydrochloric acid followed by addition of an alcohoHc solution of cupric chloride in a mole ratio of 1 2 CUCI2/isoquinoline (40). A slighdy lower yield of 2-methylquinoline [91-63-4] (97.5% pure) was obtained from bituminous coal using 30% aqueous urea to form a clathrate (41). 

The highest fixed nitrogen-containing fertilizer 46.7 wt %, urea is a white solid that is soluble in water and alcohol. It is usually sold in the form of crystals, prills, flakes, or granules. Urea is an active compound that reacts with many reagents. It forms adducts and clathrates with many... 

Another example of such a behavior is provided by the interesting polymerization of butadiene molecules imprisoned in tubes of clathrates of urea.9 Of course, the configuration of the resulting polymer is strongly influenced by the order introduced in the assembly of monomers and thus all trans polybutadiene is formed. 

Urea solution + /i-alkane — urea-n-alkane clathrat + H2O... 

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

An interesting PLC variation of the nrea clathration techniqne has been proposed by Chaffee and Johns [82]. Component mixtnres are applied onto TLC plates (20 cm X 20 cm X 0.5 mm) coated with Kieselgnhr G/nrea (2 1), prepared from a slnrry in urea-saturated methanol (Iml/g powder). Spotted plates are left in methanolic atmosphere overnight to allow clathrate formation. To remove methanol, plates are air dried for 2 to 3 h at room temperatnre and then developed in -heptane. Two bands of acychc (Rf 0.9 to 1.0) and cyclic (Rf 0.0 to 0.1) hydrocarbons are distinguished, and components are recovered qnantitatively by extraction with chloroform. 

Systems that react in this manner fall into two classes. In the first of these the framework that dominates the crystal structure scarcely participates in the reaction. This is the case, for example, in the reaction of an organic molecule intercalated in graphite or a clay, or of a guest molecule held in a clathrate of urea or thiourea. Some cases of this sort will be treated in the next section. 

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. 

An important advantage of the inclusion complexes of the cyclodextrins over those of other host compounds, particularly in regard to their use as models of enzyme-substrate complexes, is their ability to be formed in aqueous solution. In the case of clathrates, gas hydrates, and the inclusion complexes of such hosts as urea and deoxycholic acid, the cavity in which the guest molecule is situated is formed by the crystal lattice of the host. Thus, these inclusion complexes disintegrate when the crystal is dissolved. The cavity of the cyclodextrins, however, is a property of the size and shape of the molecule and hence it persists in solution. In fact, there is evidence that suggests that the ability of the cyclodextrins to form inclusion complexes is dependent on the presence of water. Once an inclusion complex has formed in solution, it can be crystallized however, in the solid state, additional cavities appear in the lattice, as in the case of the hosts previously mentioned, which enable the inclusion of further guest molecules. ... 

Clathration is the third sorption situation. Guests are incorporated during the growth of the host lattice. Their liberation occurs on heating or lowering the pressure with lattice breakdown. This situation is found for water, phenol, quinol, cresol, urea, and Dianin s compound. 

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... 

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