Melting points are characteristic of a particular compound, and in fact are often used to help identify an unknown.
Of course with so many thousands of known organic compounds, melting points alone cannot provide proof of an unknown's identity, but rather provide corroborative evidence of identity. For example, if you have have just completed the synthesis of Aspirin from Salicylic Acid (mp = 160°C) and you find that your product melts at 131-134°C, when the literature melting point of aspirin is 136°C, you can be fairly certain that your product is indeed aspirin.
Melting points are sensitive to the purity of the organic compound, since the crystal lattice of the compound is disrupted by the prescence of an impurity.
Figure One (right) shows a hypothetic binary phase diagram depicting the compostion of a liquid mixture as a function of temperature. This diagram is generated by studying the freezing point (equivalent to melting point here) as the composition of a binary mixture of A and B is changed from pure A to pure B.
As the temperature of a mixture of composition X is increased, it will reach a phase transition point at the eutectic point (point E on the curve), whereupon the solid will begin to melt, and the liquid formed will be comprised of A and B in the composition shown for the "eutectic mixture" (approximately 50:50 in this diagram). As the temperature rises, the eutectic mixture will continue to melt until all of the minor component is used up. At that point, the solid will begin to become enriched in the major component and the melting point will move up the curve. If the mixture is mostly A, then the melting point will follow the left side of the diagram beginning at the "Eutectic Point" and ending at the temperature corresponding to the initial composition of the mixture. If it is mostly B, it will follow the right side of the diagram, but either way, the mp of the mixture is Lower than that of either pure component. In addition, as long as the mixture does not begin with the eutectic composition, the mp of the mixture will have a Broader range than that of the pure compound.
Mixed Melting Points:
When two different compounds have the same melting point, how can they be distinguished from one another? The answer is to take a mixed melting point. Say for example that an unknown is either A or B, both of which have a mp of 110°. If one were to mix the unknown with first A and then B, and take a mp of each mixture, only one of them would maintain the mp of 110°C. Since different molecules do not fit into eacother's crystal lattices in exactly the same way, the presence of even a small amount of unknown in a sample of A will lower the mp of A unless of course the unknown is A.
The Mel-Temp apparatus
Taking a melting point is very simple using a mel-temp apparatus. The solid is packed into the end of a small capillary tube, then inserted into the apparatus, where the melting can easily be viewed along with the temperature through a lighted, magnification lens.
The melting point is recorded as a range. The first temperature recorded is the temperature at which the first liquid droplet can be seen (not the temperature at which the first change is noticed). The second temperature recorded is the temperature at which all of the solid has melted. When comparing an experimental melting point to a literature melting point, always compare the top number, since literature values are typically obtained from highly purified crystalline compounds, and so their melting points will typically be somewhat higher than those obtained in an undergraduate laboratory.
It is very important that the rate of heating be kept slow when measuring a melting point. Once the approximate mp has been ascertained, aim for a rate of 1° C per minute. Any faster will lead to erroneous results, and point deductions.
For a video presentation on the melting point technique, click here.
|©Kathleen Armstrong||March, 2009|