Chem 2123Experiment 1Melting Point Determination

1-8-13

Purpose:To learn how to use the melting point instruments by determining the mp of pure compounds and to determine identity of unknown solids using mixed melting points. (Chapter 14 in Mohrig) Also to expose you to other means of identification, such as IR spectroscopy.

Background:

The melting point of a substance is a physical property that can be used for its identification. It is a measure of the amount of heat energy that must be supplied to the particles of a solid to overcome crystal packing forces andintermolecular forces of attraction (Van der Waals, dipole-dipole, and H-bonding) that confine them to the solid state. The determination of melting points is particularly important to organic chemists, since the solids we work with generally havelow melting points (below 300°C), which can be conveniently measured.

Melting points are also used as an indication of purity. Substances melt throughout a temperature range in which both the solid and liquid phases of the substance coexist in a state of equilibrium and as such, melting points are always reported as a range. Above that range, the substance exists only as a liquid, and below it only the solid phase is present (no wetness observed).The size of this temperature range and its closeness to the actual melting point are a measure of the purity of the substance…that is, impure samples of solids have lower and broader temperature melting ranges.For example, if a pure sample of a compound melts from 110 to 111.5°C, the addition of substantial amounts of another compound might result in a new melting point range from 108-112 or even 85 to 100ºC.

A very pure substance will typically have a melting point range of 1 to 3 degrees. Larger ranges usually indicate one of two things: 1. impurities in the sample or 2. poor technique in determination of the melting point. The former can only be corrected by further purification, while the latter requires more practice and patience. The most common technique flaw is heating the sample too fast near the melting point.If the rate of melting lags behind the heating rate, you’ll end up with a broad range.

An identical or near identical temperature melting point range is not, in itself, proof of identity. There are thousands of solid organic compounds that melt within any relatively short temperature range and overlaping of melting points is inevitable. However, if an unknown solid sample is believed to be a certain known compound, it is a relatively simple task to prove or disprove this belief by performing a “mixed” melting point. The unknown and known compounds are mixed in relatively equal quantities. If the two samples are the same compound, a 50:50 mixture will still be a pure sample and the melting point will be unchanged. If they are different compounds, the 50:50 mixture will be very impure and the melting point range will be lowered and broadened.

Requirements:Equipment: Melting point instruments, capillary tubes

Chemicals:

Known Compounds / mp (°C)
trans-Cinnamic acid / 132-133
Urea / 132-133
Possible Unknowns / mp (°C)
Benzophenone / 47-49
Diphenylamine / 52-54
Biphenyl / 68-70
Stearic acid / 68-70
Dibenzoylmethane / 75-79
Napthalene / 79-81
o-Toluic acid / 104-105
m-Toluic acid / 111-113
Benzoic acid / 122-123
exo-Norborneol / 124-126
trans-Cinnamic acid / 133-135
Acetylsalicylic acid / 134-136
Diphenylacetic acid / 146-149
Adipic acid / 151-153
Citric acid / 152-154
Salicylic acid / 158-160
Triphenylmethanol / 161-163
d-Tartaric acid / 171-174
p-Toluic acid / 177-180

General Procedure:

Fisher-Johns Apparatus

Place a coverslip on the heating block and add a few crystals, just enough to see through the magnifying

lens. Add a coverslip to the top and begin heating. If you don’t know the melting point of the solid, perform a rough measurement by setting the controller to 60. Once you have an approximate melting point, repeat the measurement with a setting around 20-30 once you are ~15 degrees from the melting point. If heating becomes too sluggish, adjust the controller gradually.

Stuart 20 Apparatus

Fill a capillary tube with ~1-2 mm of sample and insert the tube in one of the holes on the side of the instrument (there are two…make use of both). For unknown compounds or mixtures, perform a rough measurement by adjusting the set temperature to 200 and pressing “start”. This will heat the sample to 200 °Cat 20 degrees/minute. Once you have an approximate melting point, repeat the measurement by setting the temperature 15 degrees below the rough melting point and pressing “start”. When the yellow “plateau” light comes on, press “start” again to heat the sample at 2 degrees/minute.

Repeat measurements where necessary.RECORD ALL MELTING POINTS AS RANGES!!

Required melting point determinations:

For this experiment you will work with a partner. Alternate between readings, so that you each get a chance to use the instrument.

Part 1 Melting points and mixed melting points of known compounds.

Perform a melting point determination for each of the following

1)Pure urea

2)Pure cinnamic acid

3)50 mol % urea/50 mol % cinnamic acid

Part 2Determination of unknowns using melting point

Obtain an unknown assignment from the TA and make note of the letter you are given.

Perform a mp determination for your unknown, repeat and compare with the list of possible substances given above.

Perform at least one mixed melting point to confirm the identity of the unknown or eliminate other possiblecompounds.

Part 3Determination of unknowns using IR

Use the IR instrument to get a spectrum of your unknown. Compare the spectrum of your unknown to the spectrums produced by the known compounds to help determine the identity of your unknown.