E. J. Cowlas and M. T. Pike' University of Minnesota D U I U ~ ~Minnesota ,
II
Semimicro Modification of the fhst Method for Determining Molecular Weights
In the course of research involving small-scale organic syntheses, we have experienced the need for a method of determining molecular weights which might be suitable for the undergraduate organic or physical chemistry laboratory but would eliminate some of the drawbacks of the usual methods. For example, the Beckmann procedureZ involves the inconveniences of the thermometer itself and requires relatively large samples, the micro Rast method3 requires the accurate weighing of very small samples and the forcing of waxy camphor through small-bore tubmg, and the usual macro Rast method4 requires large samples and permits loss of camphor through sublimation during the mixing process. We have obtained satisfactory results with the relatively simple semimicro h a t procedure described below.
Camphor, chosen as a solvent because of its high freezing point depression constant and its ability to form homogeneous solutions with the type of aromatic compounds being studied, was sublimed and stored in glass-stoppered weighing bottles. A 10- X 75-mm Pyrex test tuhe, to be used for the melting point tuhe, was weighed to the nearest tenth of a milligram on a chainomatic balance. Five to 15 mg of powdered sample was picked up in a melting-point capillary tube, the outside of the capillary was brushed off, and the sample was pushed out of the capillary into the bottom of the test tube by means of a fine glass rod in such a way as to leave no solid adhering to the walls of the test tube. The test tuhe was reweighed, and enough small lumps of camphor to form a 5 1 5 % solution of "unknown" was carefully placed in the bottom of the test tuhe by means of a nickel spatula. The tuhe was reweighed. I n a second, uuweighed test tuhe about the same amount of camphor was placed, to be used for determining the melting point of the pure solvent. Each test tuhe was sealed with an oxygen-natural gas torch as follows: The closed end of the tube was wrapped with a strip of wet paper toweling to prevent heating and sublimation of the sample. Approximately 3 em above the sample, the tube was heated with a Table 1. Results of Molecular Weight Determinations
Wt. Form- Wt. Solula Salute vent Compound Wt. (mg) (mg) A T ( W K,
Figure 1. Molecvlor weight apparotu=.
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National Science Foundation Undergraduate Research Participation Program participant. 'DANIELS, F., MATHEWS,J. H., WILLIAMS,J . W., BENDER, P., MURPAY,G. W., AND ALBERTY,R. H.,"Experimental Physical Chemistry," 4th ed., McGraw-Hill Book Co., Inc., New York, 1949, pp. 8 4 4 . J . B., %TniTni~ro QuantitaCHERONIS, N. D., AND ENTRIKIN, tive Organic Analysis," 2nd ed., Interscienoe Publishers, Inc., New York, 1958, pp. 147-8. R. L., and F u s o ~R. , C., "The Systematic Identiii'SSKRINER, cation of Organic Compounds," 3rd ed., John Wiley and Sons, Inc., New York, 1948, pp. 50-1.
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Determination of the melting point depression Biphenyl 154.2 8 . 7 93.1 25.25 Acetanilide 135.2 4 . 7 76.9 18.5 Bensoioacid 122.1 12.3 124.6 32.5 Av. Results with semimicro a p p a m t s s Naphthalene1282 18.1 167.1 33.5 Biphenyla 154.2 15.2 147.2 27.0 Mdeic anhydrideb 98.1 11.3 99.5 44.0 Synthktic product 332( 9.7 1A5.3 7.0 Trial 1 Trial 2 33ZC 8 . 5 142.4 6 . 5
Mol. Wt. BeckMol. mann Wt. MethExp. od2
eastant 41.6 . . . 40.8 . . . 40.2 . . . 40.9"
. .. . .. . ..
. . . 132
. ..
125
156
151
. ..
106
.. .
. . . 382 . . . 375
. ..
.. .
This value compares favor%bly with that reported by MELW. B., SAXER,L. P., AND JONES, T. O., J.A m . Chem. Soe., 65.2023 (1943) who sueeest a value of about 39.7 far solutions of cohcentritionigreater %an 0.2 molal. ' This was handed out as an unknown in phyaical chemistry or organic qualitative analysis and was not identified until after the molecular weight had been determined. This is thought to he a polymer of a. compound of molecular weight 166 or 168. DRUM,
small, hot flame and drawn out to a diameter of about 4 mm. After cooling somewhat, the constriction was heated sufficientlyto complete the seal. Figure 1shows the sealed tubes (a) attached to a 360' thermometer (b) by means of a coil spring (c). Various types of Thiele tubes were tried as melting-point baths, but were discarded in favor of a 150-ml beaker (e) filled with glycerol. The bath was heated with a microburner bearing a chimney, and was stirred with a carefully regulated stream of compressed air (d). A large magnifying glass ( j ) aided the observation of the samples.
The mixture was alternately melted and solidified until homogeneity was achieved. Melting points of both the camphor and the mixture were read within 0.25°C. The standard equation for this type of determination was used: Molecular weight = 1000 KIg/(GAT)
where K , = melting point depression constant (40.9), g = solute weight, G = solvent weight, and AT = depression of melting point. Table 1 shows some results obtained by undergraduate students.
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