Rapid Preliminary Determination of Melting Points GLENN W. STAHL School of Chemistry, Rutgers University, New Brunswick, N. J.
T
HE continuous gro\vth in the application of qualitative organic analysis in industry and the jvidespread inti-0in organic analysis in colleges and universiduction of ties have brought many developments in laboratory technique, especially in the determination of such frequently employed constants as the melting point. Considerable time map be consumed in ivaiting for heat reservoirs t'o warm to t'he fusion point of the solid. K i t h an unknown compound, the melting point may be overrun considerably because of too rapid heating of the bath and still
inch nonluminous Bunsen burner flame, the solid resting on the top surface of the bulb. Attention is focused on the thermometer bulb as it is slowly lowered 1 inch every 10 seconds. When the solid fuses at any point on the upper surface of the bulb, the temperature is quickly read without withdrawing the bulb from above the flame. The accuracy of the determination may he increaied by spreading the molten solid in a thin film along the bulb by means of a clean spatula or glass rod, if the solid has not decomposed on melting. When allowed to cool and become crystalline, the film becomes clearly visible on the shiny bulb. The heating procedure is repeated and the melting point taken when the film disappears.
TABLE I. RAPIDPRELIMINARY DETERMISATION OF MELTING POINTS Compound Phenol Benzophenone 1 3-Diphenoxy-2-propanol Ethyl-3,5-dinitrobensoate Dibeneaiaretone Benzoic acid Cinnamide 2,4-Dinitrophenylhydrazone of propionaldehyde 3-Bromoethyl amine hydrobromidc
I
I1
42 48 78 92 111 122 146
41 49 80 95 113 122 148
157 177
157 172
Determination I11
1'
Average
Oil B a t h
Literature
41 48 80 96 112 118 147
41 49 80 94 113 120 147
40-41 49-50 79.5-80 92-92.5 111-112 121-122 145-146
41 49-51 80-81 93 112 122 147
158 177
154 173
156 174
154-1 55 170-171
154 173
172 193 200 232 260
170 191 202 233 258
170 191 202 230 259
170-171 187-188 204-205 231-232 259-260
173 189 204-5 234 261
... ...
IV
Procedure .1 41 42 50 48 83 81 93 94 113 114 118 119 146 147 154 171 Procedure B
3-Bromoethyl amine hydrobromide Yernicarbazone a-methyl-6-ethylacroleili 3,5-Dinitrobenzoic acid Phthalimide Phenolphthalein
168 190 200 228 258
171 189 205 230 264
170 191 202 228 257
Procedure, A 174 194 209 236 26.5
Mixed Melting Points, Procedure A Mixture
{ %:amic {~ ~
acid
~
~ El-methyl-P-ethy,acrolein ~ ~ ~ z f n
103-8
106-10
106-9
103-10
104-8
Range 103-110
98-110
e142-50
1-15-63
145-52
148-55
143-49
142-155
146-155
more time is required to alloiv the bath to cool and to prepare a second capillary tube. Beginning students tend to heat the ordinary oil or sulfuric acid baths to too high temperatures, resulting in excessive fuming and destruction of the oil through decomposition. In rarer instances determination of the melting point by capillary tube methods may result in the loss of an appreciable fraction of the derivative, if only a small amount is available. K h e r e a solid is being purified by successive recrystallizations, determination of the melting point by ordinary methods after each crystallization results in considerable loss of time. The chance observation that the melting point may be determined with minute amounts of solids by the simple method outlined below was considered to merit further investigation because of its surprising accuracy and economy in time and materials. The method is in no way meant to supplant the final and more accurate determinations by the bath-capillary tube techniques \There sufficient amounts of solid are available (1).
...
...
Results of the determination of the melting points of a number of solids compared with the known melting points as determined in an oil bath are listed in Table I, which includes examples of application to the determination of mixed melting points. The average time required for any single determination is less than a minute. However, in the neighborhood of 175", the heat exchange is SO variable that the individual melting point determinations occasionally vary by over 5'. PROCEDURE B, MELTINGPoIsm ABOVE 175" C. The melting point is determined by Procedure A, 10" are subtracted from this temperature, and the solid is again heated, the eye being focused on the scale of the thermometer until melting point A - 10 or temperature B is attained. If the solid has not melted, the mean between A and B may be taken as the melting point. This mean melting point is not equal experimentally to the melting point by Procedure A less 5", because of the uneven heat exchange at higher temperatures which occasionally leads to overrunning the melting point as determined by Procedure A. If the melting point has been overrun considerably the solid will melt at temperature B , 10" are then subtracted from B, and the solid is again heated until B - 10 or C is attained. If the solid has not fused at C, the melting point is taken as the mean between B and C. However, the accuracy is such that only in rare instances is this third heating necessary.
PROCEDURE A, MELTISGPOIXTS BELOW 175" C. Two milligrams or less of the finely ground solid are placed upon the bulb of an ordinary 150" or 360" C. laboratory thermometer having a thin glass mercury reservoir. The position of the solid is such that the temperature may be read by a direct glance at the graduations of the thermometer, no solid being within 0.25 inch of the tip of the mercury bulb. The thermometer is held in a horizontal position with the bulb approximately 8 inches above a 2- or 3-
Literature Cited (1) Morton, "Laboratory Technique in Organic Chemistry", 1st ed., pp. 21-49, New York, McGraw-Hill Book Co., 1938.
545
