Urea-hydrocarbon complex

Figure 1. (a) End-view cross section of the urea-hydrocarbon complex, (b) View of the hexagonal PHTP inclusion compound in the ab plane. PHTP inclusion compounds are composed of infinite stacks of host molecules, repeating at about 4.78 A, parallel to the molecular threefold axes. The regular packing of the stacks gives rise to parallel channels. Channel cross section in both cases is 5 A. 

Smith, A.E. The crystal structure of the urea-hydrocarbon complexes. Acta Crystallogr. 1952, 5, 224. 

In Sects. 5 and 6, a few investigations of urea-polyethylene complexes (UPEC) were discussed. The UPEC is an interesting material because a single polyethylene chain is located in an hexagonal canal of urea molecules and it must be expected that the polyethylene chain can behave differently from the bulk systems like solution-grown crystals or materials recrystallized from the melt. The inclusion complex system composed of short hydrocarbon molecules and urea molecules was studied more than 30 years ago. The crystalline structures of urea-hydrocarbon complexes are known The urea-polyethylene complex system was prepared rather recently by Monobe et al. , replacing the hydrocarbon molecules in the urea-hydrocarbon complex by... 

Nurex A process for extracting C8 - C30 linear hydrocarbons from petroleum fractions, using their ability to form urea inclusion complexes. Branched-chain hydrocarbons do not form such complexes. Developed by the Nippon Mining Company, Japan, and operated until 1979. 

Urea inclusion complexes. Normal alkanes having seven or more carbon atoms form inclusion complexes in which hydrogen-bonded urea molecules are oriented in a helical lattice in which a straight-chain hydrocarbon fits. The guest molecule is not bonded to the host but merely trapped in the channel. Discovered in 1940 by Bengen, the hydrocarbon-urea complexes have been studied particularly by Schlenk, Schiessler and Flitter, and Smith (X-ray analysis). The principal findings are as follows. 

A. E. Smith, The Crystal Structure of Urea-Hydrocarbon and Thiourea-Hydrocarbon Complexes, Journal of Chemical Physics 18 150-152 (1950). 

The strangest type of co-crystal is formed from urea and fatty acids or hydrocarbons in aqueous solutions. Nonbranched fatty acids and hydrocarbons form clathrates in water. The water molecules are ordered around the long alkyl chains mqre than in pure water, and at saturation and low temperatures water-hydrocarbon complexes may form ice structures with entrapped hydrocarbons. The hydrocarbon chains just fill the hollow space in tetrahedral ice and thereby stabilize the ice crystals and raise their melting point. however, urea is added, the urea replaces the water molecules on the hydrocarbon surface. The hydrophobic chain is now entrapped in tubular urea channels. Pure urea crystals obtained from water solutions do not contain such channels. Somehow urea, the most polar of all naturally occurring compounds (dipole moment 3.8 daltons ) develops an affinity to irregular apolar chains in the aqeous medium and entropy is overcome (Fig. 2.5.17). 

Experiments on two types of materials will be presented, namely (1) a powder of solution-grown polyethylene, Sholex 6050, and (2) a urea-polyethylene inclusion complex, in which polyethylene is of the same origin. The inclusion complexes of urea and low-molecular-weight hydrocarbon molecules have been studied extensively for more than thirty years, but the urea-polyethylene complex (UPEC) was successfully prepared rather recently. We prepared the complex according to the method described by Monobe et al. 

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. 

More than 25 different substituted urea herbicides are currently commercially available [30, 173]. The most important are phenylureas and Cycluron, which has the aromatic nucleus replaced by a saturated hydrocarbon moiety. Benzthiazuron and Methabenzthiazuron are more recent selective herbiddes of the class, with the aromatic moiety replaced by a heterocyclic ring system. With the exception of Fenuron, substituted ureas (i.e., Diuron, Fluometuron, Fig. 10, Table 3) exhibit low water solubilities, which decrease with increasing molecular volume of the compound. The majority of the phenylureas have relatively low vapor pressures and are, therefore, not very volatile. These compounds show electron-donor properties and thus they are able to form charge transfer complexes by interaction with suitable electron acceptor molecules. Hydrolysis, acylation, and alkylation reactions are also possible with these compounds. 

One type of chemical approach to the analysis of liquid and solid hydrocarbons that will probably see considerable development is that involving reaction or complex formation to yield precipitates that can be separated from the unreacted mass and subsequently be treated to regenerate the hydrocarbons or class of hydrocarbons so precipitated. This field is certainly not extensively developed. In fact very few examples come to mind but among these are Gair s (21) determination of naphthalene by precipitation with picric acid determination of benzene by Pritzker and Jungkunz (52) by an aqueous solution of specially prepared nickel ammonium cyanide Bond s (8) nitrous acid method for styrene and more recently the determination of normal alkanes in hydrocarbons of more than 15 carbon atoms by adduct formation with urea as described by Zimmerschied et al. (71). 

As expected, the influence of added nonelectrolytes can be quite different depending on whether the added compound is likely to be located in the micelles or in the in-termicellar solution. The effect of normal alcohols has been studied in detail for potassium dodecanoate the CMC is lowered for all alcohols studied but the effect increases considerably in going from ethanol to decanol (cf. Fig. 2.7). Hydrocarbons, like cyclohexane, n-heptane, toluene, and benzene, have been found to lower the CMC for many surfactants. Strongly hydrophilic substances, like dioxane and urea, have small and complex effects. At higher concentrations they markedly increase the CMC or even inhibit micelle formation. Addition of another similar surface-active agent generally gives a CMC in between the CMCs of the two surfactants. 

A great many industrial products besides those mentioned in Sections 11.2 and 11.3 depend in some way on H bonds. For finished goods that require certain catalysts (hydrous metal oxides), color removal (by clay filtration), or flotation separation, adsorption influenced by H bonds is sometimes critical. Finally, some materials require H bonds to achieve or maintain their desired form or to cairry out their functions an example is the urea complexes used in separating paraffin hydrocarbons (110). 

With the elimination of saturated acids from the olive oil hydrolyzate by crystallization from acetone, the problem remaining in isolation of oleic acid is to remove the doubly unsaturated linoleic acid. Models and cylinders show that the introduction of just one cis double bond is enough to widen the molecule to the extent that it can no longer be inserted into the 14.3-cm wide channel which accommodates -alkanes (Fig. 1). However, amodel of 3-nonyne likewise fails to fit into the 14.3-cm channel, and the fact that this acetylenic hydrocarbon nevertheless forms a urea complex indicates that the channel is subject to some stretching, namely to a diameter of 16.2 cm, as in Fig. 2. 

As the hydrocarbon chain is lengthened, more urea molecules are required to extend the channel but the ratios given in Table 1 show that the host-guest relationship is not stoichiometric. For preparation of a complex, a liquid hydrocarbon is... 

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