Question

3. Based on your knowledge of melting points, would you expect the melting points of the compounds isolated in this experiment to be generally higher, lower, or the same as the literature values? Explain your answer.

Answer 1

A pure organic compound usually melts over a range of two degrees or less. If the melting point of a pure compound is within a degree of the value found a lab handbook it is presumed to be pure. A sample is impure if it has a melting point range that is lower and/or wider than that the literature value.More impurities increase this effect. In the organic lab, unless something is wrong with the procedure or the equipment, a substance generally cannot be observed to melt at a higher temperature than its melting point! Experimental melting points should be always reported as a range, for example, 42-44°. A given sample is only used once for mp determination.Always use fresh samples for additional trials. Dispose of used capillary tubes in the proper receptacle, not in the wastebasket.

Melting Point Tips Most errors come from heating the sample too fast. A heating rate of 1 to 2 degrees per minute is will give good results.Going faster than five degrees per minute virtually guarantees poor results in most cases. TIME-SAVING TIP: If a compound has a high mp, it can take a long time to reach it at 1 or 2 degrees per minute! Unless you have prior knowledge of the expected melting point, it is advisable to have extra samples prepared ahead of time. Run the first sample at a high rate of heating to get an approximate mp range. Then repeat the procedure but slovw down the rate of heating as you approach the expected melting point. Too much or too little sample can lead to errors.Generally 3-4 mm in the bottom of a capillary tube works well. The sample should be firmly packed in the bottom of the tube to insure efficient heat transfer.

Melting Point Tips Most errors come from heating the sample too fast. A heating rate of 1 to 2 degrees per minute is will give good results.Going faster than five degrees per minute virtually guarantees poor results in most cases. TIME-SAVING TIP: If a compound has a high mp, it can take a long time to reach it at 1 or 2 degrees per minute! Unless you have prior knowledge of the expected melting point, it is advisable to have extra samples prepared ahead of time. Run the first sample at a high rate of heating to get an approximate mp range. Then repeat the procedure but slovw down the rate of heating as you approach the expected melting point. Too much or too little sample can lead to errors.Generally 3-4 mm in the bottom of a capillary tube works well. The sample should be firmly packed in the bottom of the tube to insure efficient heat transfer.

More Melting Point Tips .If you have trouble packing a tube, consider discarding it (proper receptacle) and starting over. Difficult cases are often not worth the struggle and mp capillaries are inexpensive. .When it doubt about a result, check your technique by running a sample of a pure known substance with a similar melting point. Samples that melt below 50 require extra care in order to get good results. Slow heating is critical. Also, small amounts of impurities seem to have a greater effect on these low melting substances. Mixed Melting Points - The identity of two samples that have the same melting point can be determined by taking a mixed melting point. Grind roughly equal portions of the two samples together to mix well and take a melting point of the mixture. If the two substances are identical the melting point should be the same as that of either sample. If the two substances are not identical, then the melting point will be depressed.