Answer to problem 1.

First, look up the From the Tables at the back of the Handbook for Organic Chemistry Lab, or online at Table of Physical Constants and Hazards of Some Organic and Inorganic Compounds.

Naphthalene : A mp of 79-80°C indicates that the compound isolated is only slightly impure, since the range is only 1° and it is very close to the literature value.

Benzophenone : The benzophenone is slightly impure, with a mp range of 2° and the mp depressed by 1-3°. It is probably pure enough for most purposes.

p-Anisic acid : This isolated compound is impure. A mp range of 4° indicates that the compound is less pure than the naphthalene and benzophenone examples given above.

Salicylic acid acetate: This compound is ultra-pure. Good thing, since this is aspirin.

3-Chlorobenzoic acid : This compound is very pure. (Did you have trouble finding this compound in the Handbook? Often compounds are listed in the CRC - and in our Handbook -under the parent compound, in this case benzoic acid.)

Sulfanilamide: This compound is ultra-pure. Note that the literature reports a 1° mp range for this compound.

Ferrocene : This compound is extremely impure, with a 4° mp range over 10° below the literature value. You might even question whether or not the compound is even ferrocene.

Answer to problem 2.

Look up the melting points in a suitable source, such as the CRC, Merck Index, on-line sites, etc. Two values of melting points are listed for each compound below. The first one is from the Cambridge Soft (CS) site, found by going to the Hazard and Physical Data for Compounds page and following the links to sources of information on chemicals used in the lab. The second value was found in a hard copy of the Merck Index (10 th Ed.)

lidocaine : 66-69°C at CS site; 68-69°C in the Merck Index.

novocaine : 155-156°C at CS site;153-156°C (procaine hydrochloride) in the Merck Index.

Answer to problem 3.

Using only methods of melting points, you can prove that you have isolated ibuprofen if you can obtain an authentic sample of ibuprofen. Mix equal amounts of the two compounds together, preferably in a mortar and pestle. Then take the melting point of the mixture. If it is 75-77°C, the compound you isolated is ibuprofen. If the melting point is substantially lowered and has a wide melting point range, the compound is not ibuprofen. This is the technique of "mixed melting points". Even if two different compounds have the same melting point, if they are mixed together the melting point of the mixture will be lowered because the compounds are no longer pure.

Answer to problem 4.

An impurity consisting of 5% total mass will lower the melting point from that of the pure compound, and it will increase the melting point range. There is not a "formula" to predict how much the melting point will be lowered, however, the greater the impurity, the greater the melting point depression. A value of 103-107° would be consistent with this amount of impurity with the pure melting point of 110-111°; values of 100-105°, 97-100°, 102-110° are also good "guesstimates".

Answer to problem 5. 

If a compound changes color upon melting, it is likely that it has undergone a chemical reaction, or decomposed. If the sample has decomposed, it is no longer the same compound and will have a different melting point. Therefore, simply cooling and re-melting the sample will not give you the melting point (or in this case the decomposition point) of the compound you originally intended to analyze. To be safe, start with a fresh sample of the compound.

Answer to problem 6.

If a compound "disappears" on melting, it is likely that it has sublimed , or went directly from the solid to the gas phase. The melting point of such a compound needs to be taken in a sealed capillary tube.

Answer to problem 7.

(The melting points of the two compounds are found in the Tables at the back of Handbook for Organic Chemistry Lab.)

Acetylferrocene melts at 81-83°C. Start with the Fisher-Johns apparatus set at 100 until it reads about 75°, then set it to 30.

p -Anisic acid melts at 184°C. Start with the Fisher-Johns apparatus set at 100 until it reads about 179°, then set it to 75.

Answer to problem 8.

Acetylferrocene melts at 81-83°C. Start with the Mel-Temp apparatus set at 5 until it reads about 75°, then set it to 2.5.

p -Anisic acid melts at 184°C. Start with the Mel-Temp apparatus set at 5 until it reads about 179°, then set it to 4.5.

Answer to problem 9.

In the teaching labs, there are a few reasons which can account for two different people getting a different melting point range for the same sample of the same compound. (Other reasons might be valid too.)

1) There is some variability between melting point apparati. The teaching labs have about 40 such instruments, and occasionally some of them are out of calibration. If you ever suspect that a particular device is out of calibration, you should report it to the Coordinator or ask for some standards so that you can check it yourself.

2) One of the lab partners may have heated the melting point device too rapidly. To obtain an accurate melting point value, the device must be heated slowly near the melting point in order for the thermometer's sensor to be the same temperature as the heating block. If the heating block is heated too rapidly, the thermometer cannot keep up, and reads a value which is lower than the actual temperature of the heating block and of the compound.

3) When observing a melting point, different people can make different judgements as to when a compound has begun to melt. Only one person can look through the lens at a time, so the decision resides solely on that one person's judgement. Sometimes a person will call the melting point as begun when the sample moves a bit, softens, or shrinks, or a droplet outside the bulk of the sample. Some students take a larger mass of crystals when taking a melting point and this can affect the way a compound looks as it melts (smaller amounts give more accurate melting point values). True melting is taken as the first appearance of liquid within the bulk of the sample. Practice taking melting points helps improve your judgement in taking melting points.

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