Urea-hydrocarbon adducts

Pemberton. R.C. Parsonage, N.G. Thermodynamic properties of urea + hydrocarbon adducts. Part I. Heat capacities 0/adducts o/n-CioH22, n-Ci2H26, n-Ci6H34, and n-C2oH42- Trans. Faraday Soc. 1965. 61. 2112. 

The composition of the nonstoichiometric urea-n-hydrocarbon adduct can be calculated from a formula given by Smith [57h] Urea/hydrocarbon mole ratio = 0.684 (n-1) + 2.175, where n is the number of carbon atoms in the extended zigzag chain (by taking C-C= 1.54 A, C-C-C = 109.5 , and radius of CH3 group = 2.0 A). 

Picolinic acid also accelerates the H2O2 oxidations but less efficiently than pyrazine-2-carboxylic acid. It has been demonstrated recendy that the vanadium complex with picolinic acid, VO(PA)2 , encapsulated into the NaY zeolite retains solution-like activity in the liquid-phase oxidation of hydrocarbons [16a], It is noteworthy that pyrazine-2-carboxylic acid accelerates the hydrocarbon oxidation catalyzed by CH3Re03 [25 b]. Employing a (+)-camphor derived pyrazine-2-carboxylic acid as a potential co-catalyst in the CHsReOj-catalyzed oxidation of methyl phenyl sulfide with urea-H202 adduct, the corresponding sulfoxide was obtained with an e.e. of 15% [16b]. 

An example of this type of process of selective solute partitioning is urea based adductive crystallization. A saturated solution of urea in water at 70 °C may be mixed with a mixture of aromatic and paraffinic hydrocarbons present in a solvent at 40 °C (the Edeleanu process). Under conditions of appropriate refrigeration, lumps of urea-n-paraffin adducts appear as crystals (Findlay, 1962 Fuller, 1972). As shown in Figure 4.1.23(a), the host compound, urea, has crystallized into a form having a central tunnel open at both ends the tunnel accommodates the "guest paraffinic hydrocarbon molecule and holds it by van der Waals forces with m urea molecules forming the tunnel ... 

Urea has found acceptance as a protein substitute for ruminant animals (cattle, goats, and sheep). In this application, it can efficiently replace about one-third of the natural protein in ruminant feeds, but care in formulating must be exercised to optimize the concentration. Urea is toxic in large doses. It is reported in a recent patent that urea-hydrocarbon solids adducts are more palatable... 

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. 

Norma/andBranc/jedAlip/jatic Hydrocarbons. The urea-adduction method for separating normal and branched aHphatic hydrocarbons can be carried out in sulfolane (38,39). The process obviates the necessity of handling and washing the soHd urea—normal paraffin adduct formed when a solution of urea in sulfolane is contacted with the hydrocarbon mixture. OveraH recovery by this process is typicaHy 85% normal paraffin purity is 98%. 

The microcrystalline or amorphous waxes separated from the crude fractions boiling above paraffin distillate are predominantly of the naphthene-containing paraffin structure (62, 70). Thus urea adducting of 149° to 165° F. melting point wax (Superla of Indiana) isolated but 15% of adductive material assumed to be normal or terminally branched paraffins. The microcrystalline waxes consist mostly of hydrocarbon with 34 to 60 carbon atoms (70) and have a melting range from 140° to 200° F. (16). 

A part of the wax portion, which contained a considerable amount of oil in addition to the true wax, was further processed by fractionation by adsorption and by treatment with urea to form adducts of the latter with the n-paraffins. The wax portion was found to contain about 8% of aromatic hydrocarbons, which had been imperfectly separated from the main bulk of the aromatic hydrocarbons occurring in the extract oil portion. Of the remaining 92% of the wax portion, about 39% was determined to be n-paraffins and 53% cycloparaffins, with possibly a relatively small amount of branched paraffins. 

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). 

Use Fertilizer, animal feed, plastics, chemical intermediate, stabilizer in explosives, medicine (diuretic), adhesives, separation of hydrocarbons (as urea adducts), pharmaceuticals, cosmetics, dentifrices, sulfamic acid production, flameproofing agents, viscosity modifier for starch or casein-based paper coatings, preparation of biuret. 

FIGURE 9.7 Relative sizes of urea adduct and specific hydrocarbons. 

Selenourea, like urea and thiourea, can form inclusion compounds with a variety of hydrocarbons. Its inclusion properties were studied by van Bekkum and coworkers in 1969 [79], well over 20 years after the discovery of the analogous urea [54] and thiourea [55] inclusion compounds. The 1 3 adduct of adamantane with... 

Other separating techniques may be used to separate total hydrocarbons into different classes. Thus, the normal paraffins are selectively removed by 5 A molecular sieve (Mortimer and Luke, 1967) or by urea adduction, although it is less specific than the former method. Unsaturated hydrocarbons are separated from the saturated fraction by thin layer or column chromatography on silicic acid/AgNOj. 

Separation and Sub-fractionation of Alkanes Saturated hydrocarbons were separated from the neutral oil by silica-gel (60-120 mesh, dehydrated at 150°C for 5 h) chromatography in a 1 m X 30 mm i.d. column eluted with distilled n-hexane. n-Alkanes were separated from iso-octane solutions of total alkanes by adsorption for one week on 5 X molecular sieve (freshly dehydrated for 2h h at U00°C). Washing with iso-octane, followed by Soxhlet extraction, freed the molecular sieve from inwanted non-adsorbed compo mds n-alkanes were recovered by desorption after refluxing the molecular sieve for several hours with n-hexane. For the Kuwait crude and fluidized-bed tar, the molecular-sieve treatment was preceded by urea-adduction of n-alkanes and thiourea-adduction of branched-chain alkanes. 

Qathrate compounds are of this type molecules of one substance are trapped in the open structure of molecules of another. Hydroquinone forms clathrate compounds with SO2 and methanol, for example. Urea and thiourea also have the property of forming complexes, known as adducts, with certain types of hydrocarbons. In these cases molecules of the hydrocarbons fit into holes or channels in the crystals of urea or thiourea the shape and size of the molecules determine whether they will be adducted or not. 

In 1940 the simple organic molecule urea, (NH2)2C=0, was discovered to form adducts. These differ from the previous two examples in this section in that the host-guest interaction occurs only in the solid state. Urea crystallizes in the presence of long-chain hydrocarbons, as illustrated in Fig. 20. Thiourea behaves in the same fashion with only minor modifications. 

When urea crystallizes, channels with a diameter of 0.8-1.2nm are formed within its crystals and can accomodate long-chain hydrocarbons. The stability of such urea adducts of fatty acids parallels the geometry of the acid molecule. Any deviation from a straight-chain arrangement brings about weakening of the adduct. A tendency to form inclusion compounds decreases in the series 18 0 >18 1 (9) >18 2 (9, 12). 

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