Urea, clathrates with

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 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... 

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... 

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,... 

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... 

Chekhova, G.N. Dyadin, Yu.A. Rodionova, T.U. Clath-rates of urea and thiourea. Part IV. Stoichiometry of thiourea clathrates with small molecules and the nature of their packing in the matrix channels. Izv. Sibirskogo Otd. Akad. Nauk SSSR, Ser. Khim. Nauk 1979, N5 (78). 

Living radical polymerization (atom transfer radical pol5mierization) has been developed which allows for the controlled polymerization of acrylonitrile and comonomers to produce well defined linear homopolymer, statistical copolymers, block copolymers, and gradient copolymers (214-217). Well-defined diblock copolymers with a polystyrene and an acrylonitrile-styrene (or isoprene) copolymer sequence have been prepared (218,219). The stereospecific acrylonitrile polymers are made by solid-state urea clathrate polymerization (220) and organometallic compounds of alkali and alkaline-earth metals initiated polymerization (221). 

We need now to translate the above theoretical results into practice. First, let us start from an orthorhombic lattice with two molecules per unit cell and lower the symmetry to a supermolecular organization in which the cell can be considered as isolated and decoupled from its neighbors. This situation can be achieved in the following case (i) isotopic solid solutions (ii) molecule as a guest in a chemically different host lattice (e.g., urea clathrates) [143] (iii) liquid crystalline-like systems and (iv) systems with conformationally distorted chain ends, but with an ordered bulk [103]. The following changes of the Raman spectrum are expected (and observed) ... 

At present, clathrate compounds are considered as both ideal [l] and nonideal solid solutions [2-3] of the guest component in a crystalline framework of the host component. In the absence of the guest component, clathrate 3-modification is considered to be metastable with respect to the stable a-modification under ordinary conditions, i.e. Ap.= i - i <0, where [x, is the chemical potential of the corresponding modification. A number of experimental data on water, urea and thiourea clathrates, hardly consistent with the classical theory, made us study the formation of clathrates with unstable (labile) host frameworks. A comprehensive motivation of such possibility is given in [4]. 

Solid state NMR experiments on LP/urea indicate a high de e of guest mobility under ambient conditions, a result consistent with similar studies of long-chain hydrocarbon/urea clathrates. ... 

The most famous clathrates are clathrates of hpids with urea. Urea forms a cyhndrical structure, formed by spiral molecules bound by hydrogen bonds, which entrap fatty acid or other derivatives with a long unbranched saturated chain (e.g. hydrocarbons). Branched or cis unsaturated fatty acids bind much less tightly, triacylglycerols do not form clathrates with urea, because their volume is bigger and the molecules do not fit into the housing formed by urea molecules. 

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