Crystalline adducts of urea with n-alkanes (C(8)~C(20)) and n-l-alkanols (C(12)~C(18)) were prepared. From thermal analysis and X-ray diffraction study on the adducts, we found that the composition of adducts and the heat of decomposition can be obtained easily from the thermogram of adducts

Monobe, Kazuo

Murakami, Yushi

Yokoyama, Fumiyoshi

English

Okayama University Scientific Achievement Repository

MEMOIRS OF THE SCHOOL OF ENGINEERING, OKAYAMA UNIVERSITY Vol. 6, No.1, SEPTEMBER 1971Thermal Study on Adducts of Urea with n-Alkanesand n-l-AlkanolsKazuo MONOBE, Yushi MURAKAMIand Fumiyoshi YOKOYAMADepartment of Industrial Chemistry(Recieved May 6, 1971)SynopsisCrystalline adducts of urea with n-alkanes (Ca-C2o) and n-l-alkanols (CI2-CIa) were prepared. From thermal analysis and X-ray diffraction study on theadducts, we found that the composition of adducts and the heat of decompo-sition can be obtained easily from the thermogram of adducts.§ 1. IntroductionSince urea adducts were first discovered byBengenl), many investigations2) on the chara-cteristic property and structure have beencarried out, due to the fundamental interestsand the utility for the separation of hydrocarb-ons and their derivatives. All urea adductsare characterized by the same crystal lattice;urea molecules (host molecules) form hexagonalpipes in which the organic molecules (guestmolecules) are included. Generally, it isrequired that the guest molecules in the forma-tion of the adducts are long and unbranched,since with most branched and cyclic hydro-carbons and their derivatives, the adducts arenot formed 3J •In this study, the adducts on urea with n-alkanes and n-I-alkanols were prepared fromthe saturated solutions of urea in methanoland examined by X-ray diffraction and the-rmal analysis. From these measurements, weestablished the method to determine the com-position of urea and to measure the heat ofdecomposition.§ 2. ExperitnentalMaterials and SolventsUrea as host molecule, n-alkanes (Ca-C2o)and n-I-alkanols (C I2-Cla) as guest moleculesand methanol as solvent were used in this studyand were reagent grade.Crystallization Procedurea. Adducts with n-alkanes57We prepared the saturated solutions of ureain methanol at room temperature, and a slightexcess amount of n-alkane required for the for-mation of the adducts was added. In the caseof solid n-alkanes, guest agents solved withseveral drops of hot benzene were added. Thewhole was stirred and immediately needlelikecrystals precipitated out. They were collectedon glass filters and washed with n-pentane andthen dried.b. Adducts with n-I-alkanolsA slight excess of n-I-alkanol required for theformation of the adducts, was added to thesaturated solutions of urea in methanol at40°C. In the case of solid n-I-alkanol, guestagents was added to the solutions at 60°C. Thesolution was stirred and then cooled to roomtemperature to obtain the crystalline precipi-tates of the adducts.X-Ray Diffraction StudyX-ray powder patterns were obtained by adiffractometer by use of CuK a radiation. Theprecipitates of urea-n-alkanes and urea-n-l-alkanols gave the same diffraction patterns asthose already shown by other investigators4)5),Therlllal AnalysisA Perkin-Elmer Differential Scanning Calo-rimeter (D. S. C.) was used for the thermalanalysis. The temperature scale was calibratedby the onset of the melting of Indium at 429°K, hea ted at the same rate as tha t in thesample.§ 3. Results and Discussion58 K. MONoBE and Y. MURAKAMIThermogram at the heating rate of 100C/min. for the adduct of urea with eicosane (n-C20 Hd is shown in Figure 1. The thermogramhas two dististinct endothermic peaks in whichthe higher temperature peak agrees with themelting temperature (133°C) of pure urea, andthe lower temperature peak is considerablyhigher than the melting temperature (36.4°C)of pure eicosane and is lower than it's vaporiz-ation temperature. The lower temperaturepeak may correspond to the decompositionof adduct. In Figure 2, we show the X-raydiffraction patterns for the adduct of ureawith eicosane annealed at the temperaturesabove and below the lower temperaturepeak (125°C). As shown in Figure 2, the X-raydiffraction pattern of the specimen annealedat 115°C for 30 min. is the same pattern asthat of the original specimen. However, an-nealing of the adduct at 129°C for 30 min.results in a new diffraction pattern whichagrees with that of pure urea. Thus we shallconclude that the lower temperature endothe-rmic peak in the thermogram of Figure 1means the decomposition of the adduct accom-paning with the transition of crystalline ureafrom hexagonal to tetragonal and the higherpeak means the fusion of the tetragonal ureacrystals.We can obtain the similar thermograms forthe adducts of urea with other n-alkanes and_~,IJ'------lL(d)-.J~.L..L-....I..-I1'---_ (e)______,J'------'L (hi_--...LJ'J'------'L (c,)111Diffraction Angle (2(1)Fig.2 X-ray powder patterns of the adduct of ureawith eicosane. (a) original specimen, (b) anr.ealedat 115°C, (c) annealed at 129°C a·od (d) specimen(c) kept for 24 hrs at room temperature.n-I-alkanols as in Figure 1. Figures 3 and 4show eight thermograms in which the highertemperature endothermic peaks are at constanttemperature, but the temperature of the lowerpeaks reflects the stability of the adducts. TheFiOn-Eicos~ncll-DodcC'<lllCn-1-lcxndcc;\ nen.Ocl~ncTemperature (0e)Fig. 1 D. S. C. thermo"ram of the adduct of ureawith eicosane.Temperature (OC)Fig.3 D. S. C. thermograms of the adducts of ureawith n-alkanes.Thermal Study on Adducts oj Urea 59(1)m=0.69n+ 1.50guest and the heat of adduct formation by thefollowing method. Firstly, the standard areaAo under the endothermic peak of pure ureaof Wo (g) are measured. Secondly, the area AIunder the higher temperature peak and A2under the lower temperature peak are me-asured. So the mass of urea in the adduct isrepresen ted by (At!Ao) X W 0 (g) and the massof the guest molecule is also represented bysubtracting this value from the whole weightof the adduct. Thus, the heat of adductformation can be represented by (A2/ Ao) X(60Wo) (cal/mole-guest), using the heat offusion of pure urea 60 (caljg).In Figure 6 are shown the mole ratios aga-inst the carbon number of the guest moleculesin the adducts. For n-alkanes, the relationbetween the mole ratio Tn and the carbonnumber II is given by,\lyristyl alcoholCetyl alcohol""-",....,,-----i ",,"" ,,""'"J~_140 120 110 100Temperature (OC)Fi,.4 D. S. C. thermo"rams of the adducts of ureawith n.l-alkanols.Carbon ::\umberFig. 6 Mole ratio of urealguest for the adducts ofurea vs carbon number of guest molecules.(2)m" O. 69n +0.50The sImilar result have already been prese-nted by Smith~). Similarly, for II-l-alkanols,we obtained,Vertical distance between two lines repre-sents the difference in the effect of chain ends,CH3 and OH, that is, alkanols in the channelCarbon NumberFi5.5 Decomposition temperature of the adductsof urea with n-alkanes and n-l-alkanols vs carbonnumber of guest molecules.relation between the stability (the temperaturesof lower peaks of thermograms) of the adductsand chain length (carbon numbers) of guestmolecules is shown in Figure 5. The stabilityof the adducts increases with chain length ofguest molecules. All adducts in this studydecompose by annealing at the temperatureabove the lower temperature endothermic peakand are stable for heating at the temperaturebelow the lower peak. This was examined byX-ray diffraction.Now, we can obtain the mole ratio of ureal60 K. MONOBE and Y. MURAKAMI30V 30'0S-...."@u~::r:: (J)..,10 (l)~:of urea pack more compactly than the corres-ponding alkanes in carbon number.In Figure 7 are shown the heat of decompo-sition of the adducts with n-alkanes measuredin our experiment and the heat of formationalready represented by Zimmershied and co-workers2). Thermodynamically, in the absolutevalue, the heat of formation of adduct mustagree with the heat of decomposition of adduct.However, two lines are parallel and the valuesin our data lower by about 10% than those intheir data. The reason why there are diff-erences between the heat of formation and theheat of decomposition is not clear in present.Therefore, further examinations are required.ReferencesCarbon NumberFig. 7 The heat of formation and the heat ofdecompositio::l. The heat of formation (a)(Zimmershied's data) and the heat of de-composition (b) (our data) vs carbon numberof guest molecules.I) BENGEN and SHLENK: Experientia, 5 (1949), 200.2) W.J. ZIMMERSHIED: Ind. Eng. Chern., 42 (1949),1300.3) W. SCHLENK: Ann., 565 (1949), 204.4) HENDRICKS: J. Am. Chern. Soc., 50 (1928), 2455.5) A. E. SMITH: Acta. Cryst., 5 (1952), 224.

Thermal Study on Adducts of Urea with n-Alkanes and n-l-Alkanols

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