Melting Point

Name: ____________________________ Date: ___________________

MELTING POINT PRE-LAB

1. How much sample should be loaded into a capillary? _________________________

2. What is the ideal heating rate near the melting point? _________________________

3. If the heating rate is too high, will the observed melting point be too high or too low?

Why? _______________________________________________________________

4. What 2 effects do impurities have on melting points?

a. __________________________________________________________________

b. __________________________________________________________________

MELTING POINT (reviewed 20 Jun 13 RCR)

BACKGROUND:

The Melting Point (MP) of an organic solid is a cheap and relatively quick way of obtaining information about the compound. The MP of a substance is that temperature at which a solid and its liquid are in equilibrium. There should be no temperature increase in a substance while it is melting. In practice, the melting of an organic solid does not usually occur at a single temperature. Instead, the material will begin to form liquid at an initial temperature and ultimately become a single liquid phase at a slightly higher temperature. This is the MP range of the compound.

Historically, the MP’s of organic compounds have been used for a variety of applications. These include:

1. Purity. 2. Identification. 3. Molecular weight.4. Determination of a safe drying temperature.

Purity of a compound is indicated by the narrowness of the MP range. The narrower the MP range, the greater the purity of the compound. This is a qualitative analysis. You cannot say how pure a compound is based on its MP. You can only say that it looks pure or impure. Today, quantitative methods of purity determination are used instead. These include chromatographic techniques such as Gas Chromatography (GC) or High Performance Liquid Chromatography (HPLC). Both of these methods can be used to give relative, quantitative values for purity. They can give absolute values of purity if a standard of the compound under study is available.

Identification of a substance can be made by first comparing its MP to a list of known compounds with the same MP. This must then be followed by mixed MP measurements to determine which compound in the list corresponds to the substance under study. This can still lead to several possibilities depending on the molal melting point depression constants (Kf) of both the reference compounds and the substance under study. Today, spectroscopic techniques are used instead. These include Infrared spectroscopy (IR) and Nuclear Magnetic Resonance (NMR). Both of these give direct structural information about compounds. Again, if a standard of the compound under study is available, they can give absolute values of purity. The MP of a hitherto unknown compound should always be included when reporting the compound for the first time in the chemical literature.

Molecular Weight (MW) of a material can be determined by running a mixed MP according to the Rast method. Ordinarily, this is done using D-camphor. However, there are certain constraints that must be met:

1. The compound must be stable at the melting point of D-camphor (179o C).2. The compound must not react or complex with D-camphor.3. A concentration of 0.2 to 0.5 M must be used.

Once these conditions are met, the MW is given by:

MW = (Kf)(w)(1000) / ()(W)

Where: Kf = molal melting point depression constant w = weight of compound = depression in the melting point W = weight of camphor

If any of the 3 conditions are not met, an alternative, “solvent,” to D-camphor must be selected. These can include:

solvent MP Kf

Camphene 49.0o C 31.1 oC/mCyclopentadecanone 65.6 21.3Naphthalene 80.3 6.82,4,6-Tribromophenol 96.0 20.4D-camphor 179 39.7

The larger the Kf, the greater the and the greater the accuracy. Today, Mass Spectroscopy (MS) is used instead. High resolution MS can give accurate MW values to many decimal places meaning that it can even replace elemental analysis.

Determination of a safe drying temperature is about the only thing that the MP of a compound is used for today. Organic crystals can be dried conveniently at 50o C in a vacuum oven under a stream of nitrogen. However, melting of the crystals should be avoided. Thus, the MP of a compound should be above 60o C to avoid melting.

Why should we even bother with MP’s if there are so many better ways of getting information on a compound? Cost !

Method Equipment Cost

MP $ 100 to $ 3,000GC $ 13,000HPLC $ 24,000IR $ 14,000NMR $ 280,000MS

The following pages illustrate different types of MP equipment followed by brief descriptions of each.

Thiele Tube Melting Point Set-Up

Thiele-Dennis Tube Long-Necked Flask

Thiele Tube MP’s rely entirely on operator skill for both heating rate and observation of melting point. A Capillary tube (or tubes) loaded with solid is (are) secured to the termometer with an O-ring or slice of a rubber tubing. Only 2-6 mm of solid should be placed in the bottom of the closed capillary tube. This can be done by:

1. Poking the open end of the capillary into the sample.2. Moving the sample to the bottom of the tube by either:

a. Dropping the tube down a 1.5-foot glass tube several times or byb. Pushing the solid down with a thin wire or by c. Tapping the bottom of the capillary gently on the bench top or byd. Vibrating the capillary by drawing a triangular file along the side.

The solid should be centered on the thermometer bulb and the O-ring should be well above the liquid level. The Thiel tube is heated with a Bunsen burner on the side arm to establish a uniform flow of heat in the oil. This is a form of external Cotrell pump. Observe the solid and the heating rate. Record the temperature at the initial melting when liquid sample first appears. Also record the final temperature when all of the solid has melted. Ideally, the heating rate at the melting point should be about 1-2o C/minute. Also record other observations such as softening, crystal appearance changes, color formation, gas production and disappearance of sample such as by sublimation. A long-necked flask (5-10 mL bulb) can be used instead of the Thiele tube. The heat flow may be less uniform, but the bulb of the flask will act as a magnifying glass to make observation easier. Equipment cost is about $100.

Thomas-Hoover Melting Point Apparatus

The Thomas-Hoover machine is similar to the Thiele tube. However, it is easier to use. The loaded capillary is dropped into one of typically 5 holes in the tube support of the oil bath. This allows for side-by-side comparison of sample melting point to those of standards and mixtures. The variable transformer is set to provide an acceptable heating rate through the immersion heater. The illuminated sample is observed through the attached magnifying glass and temperature is conveniently monitored using the periscope. The temperature control is remarkably good for such a bulky unit.(Video = http://www.youtube.com/watch?v=yiSt_qM1tCc&feature=related) on 9 Aug 11.Used equipment cost is less than $1,000.

MelTemp Melting Point Apparatus

The Mel-Temp machine is a cheaper version of the Thomas-Hoover. A heating block is used instead of an oil bath. Although the sample is illuminated, there is only room for 3 capillaries. Both the Thomas-Hoover and the Mel-Temp take a while to cool between samples. (Video = http://www.youtube.com/watch?v=T3HzRPKj1YE) on 9 Aug 11.Used equipment cost is less than $1,000.

Fisher-Johns Melting Point Apparatus

The Fisher-Johns machine is the easiest to load sample into. Instead of a capillary, the sample is sandwiched between circular, microscope cover slips. Unfortunately, this precludes side-by-side melting point comparisons of standards and/or mixtures. Heat iscontrolled (as above) by a built-in variable transformer. The unit can be cooled quite quickly by placing a cylinder of metal (like an old 500 g balance weight) on the heating block. (Video http://video.google.com/videoplay?docid=5617753349611003526#) on 9 Aug 11, start at 10:50 into video. Used equipment cost is about $500.

Stanford Research OptiMelt Melting Point Apparatus

The OptiMelt is a fully automated unit that has room for 3 capillaries at a time. Pictures are taken by a digital camera during a melt, image data is processed to determine the melting points, and the result is displayed. For completeness, the digital pictures taken during the melt can be played back as a movie. It can operate as a stand-alone unit or it can be linked to a computer by USB. (Video = http://www.thinksrs.com/assets/instr/MPA100/MPA100exp.htm) on 9 Aug 11.Equipment cost is about $2,800.

The Stuart SMP30 is a programmable unit that heats up very quickly to a starting temperature (plateau) selected by the operator. The unit stabilizes at that point for 2 minutes and then signals the operator to start the run at a heating rate that he has previously selected. Equipment cost is about $2,100.

Operation of the Stuart SMP30

The power, on/off (I/O) rocker switch is located on the back of the unit.

On start up, you should see the time in the upper left corner of the screen and the temperature in the upper right corner. There are 2 options listed below the temperature. They are, “NEW MEASUREMENT,” and, “VIEW LAST RESULTS.” Select, “NEW MEASUREMENT,” with the membrane button to the right of that option.

The new screen will ask if you want to use a, “PRESET PROGRAMME?” Select, “NO,” with the membrane button on the right.

The new screen will give you the current plateau (starting temperature) value and ask if you want a, “PLATEAU CHANGE?” Use the membrane buttons on the right to raise or lower this plateau (starting temperature) value. Holding the button down will increase or decrease the speed at which the displayed value changes. When you have reached the desired plateau temperature (starting temperature) setting, push the membrane button below, “NEXT,” located at the bottom left of the screen.

The next screen will give you the current ramp value at the plateau setting and ask if you want a, “RAMP CHANGE?” Use the membrane buttons on the right as before to raise or lower this ramp value. When you have reached the desired ramp rate setting, push the membrane button below, “NEXT,” located at the bottom left of the screen.

The final screen will list the, “PLATEAU,” and, “RAMP,” values that you have entered. Carefully place your capillary loaded with 1-3 mm of sample into one of the 3 holes in the heater block on top of the unit and press the membrane button next to, “START.”

The display will change to, “HEATING/COOLING TO PLATEAU,” and give an estimated time for the instrument to reach that value.

When the plateau (starting temperature) is reached, the screen will change to, “STABILIZING AT PLATEAU 120,” and begin a 120 second countdown.

When the countdown reaches 0, the screen will change to, “AT PLATEAU,” and the unit should make an audible beep if it is programmed to do so.

At this point, the operator should push the, “START RAMP,” membrane button at the right and begin observation for a melting point.

When melting has occurred and the melting range has been recorded, press the membrane button below, “DONE,” in the lower left corner of the screen to stop the heating program and begin the cooling cycle.

Taken from (http://www.stuart-equipment.com/adminimages/smp30(3).pdf) on 25 Jun

13.

Perkin Emer Differential Scanning Calorimeter (DSC)

DSC gives more information than a MP. Glass transition, crystallization, dehydration as well as melting point temperatures can be measured. It measures the temperature of a sample as a function of the amount of heat added. As mentioned above, a sample should not increase in temperature as it melts. However, it should still absorb heat. Simnilarly, when a dissordered solid changes to a more ordered form, heat will be released. This happens when an amorphous solid converts to a glass or a crystalline state. Equipment cost is about $23,000.

Name: ___________________________ Date: ___________________

Partners: ___________________________ ____________________________

LAB DATA SHEETS MELTING POINT

Melting Points of Mixtures

This is a class project. The goal is to run every melting point in duplicate. If the two values are within 10% of each other, the data is deemed good. If not, another measurement must be made by a new individual. This is continued until at least two values meet the 10% criteria. When results do not agree between groups, the bosses decide who will do the definitive check. Five unknowns are to be run.

Work in two groups. Select a boss in each group. The bosses will make melting point assignments to all individuals. Individuals are not allowed to do melting points on the same material as anyone else in their group. Unlike the real world, the bosses are expected to do work, too. That is, the bosses will be doing melting points too.

1. Record the average melting points which are within 10% of each other in the table for pure cinnamic acid, pure urea and the pre-made mixtures of these two.

Compound Start Finish Midpoint Average

Urea _____ _____ _____

Urea _____ _____ _____ _____

Cinnamic Acid _____ _____ _____

Cinnamic Acid _____ _____ _____ _____

25% Urea _____ _____ _____

25% Urea _____ _____ _____ _____

50% Urea _____ _____ _____

50% Urea _____ _____ _____ _____

75% Urea _____ _____ _____

75% Urea _____ _____ _____ _____

2. Graph the midpoints of the above melting points.

Identification of an Unknown

Unknown # MP Compound

____ 50- 53o C bibenzyl

____ 68- 70 biphenyl

____ 76- 80 durene

____ 80- 82 naphthalene

____ 95- 98 glutaric acid

____ 113-115 acetanilide

____ 117-120 4-isopropylbenzoic acid

____ 119-121 mandelic acid

____ 120-122 2-naphtholo

____ 134-138 benzoin

____ 140-142 4-nitro-ortho-anisidine

____ 147-149 cholesterol

____ 158-161 salicylic acid

____ 161-163 benzanilide

____ 182-184 9,10-dimethylanthracene

MP’s from Aldrich Catalog (2005-2006).

Unknown # ___ Melting Point Range _________ Compound = _______________

Unknown # ___ Melting Point Range _________ Compound = _______________

Unknown # ___ Melting Point Range _________ Compound = _______________

Unknown # ___ Melting Point Range _________ Compound = _______________

Unknown # ___ Melting Point Range _________ Compound = _______________

Unknown # ___ Melting Point Range _________ Compound = _______________

Unknown # ___ Melting Point Range _________ Compound = _______________

Unknown # ___ Melting Point Range _________ Compound = _______________

Unknown # ___ Melting Point Range _________ Compound = _______________

Unknown # ___ Melting Point Range _________ Compound = _______________