245 * Lab #1
Melting Points

One of the more important techniques that you will need to master in organic chemistry is to determine an accurate melting point (range). All melting points should be recorded as a range. That is the temperature at which the chemical begins to melt up to the temperature that the chemical completely melts. There are certain procedures that must be followed to get an accurate melting point. Once you can be satisfied that you have gotten the correct melting point , the rest of the laboratory conclusions can be made.
 A melting point can determine the purity of a chemical. Along with other physical constants it can help determine the identity of a chemical. All of these types of determinations about solid chemicals you are using in future laboratory experiments will be based on your technique for accurately determining a melting point.

The first thing that you should be sure of is the accuracy of the apparatus that you are using to determine the melting point. Thermometers will always need to be calibrated, as will any melting point apparatus you use that does not involve a thermometer but does involve some other type of temperature measuring system.

We have three types of melting point apparatus: The mel-temp that uses a thermometer to determine the temperature that the capillary tube is subjected to; and the two types of IA-series of melting point apparatus use a thermocouple to determine the temperature and it has a digital readout display. The directions for their use are with the apparatus.

All of the M.P. apparatus have slots for three capillary tubes. For efficient use of these, please utilize all 3 slots when doing labs that require multiple melting point determinations. The best way to determine an accurate M.P. is to first do a quick M.P. with the heat source turned high (not too high). This will give you a close approximation of the M.P. Let the apparatus/thermometer cool to below that temperature. Then, quickly get the temperature close to that M.P. Then, adjust the heat source to approach the approximate M.P. slowly. This will give you an accurate M.P. If you try to determine the M.P. quickly and do not follow up with a slower run, you run the risk of getting an inaccurate reading because the temperature may be rising so fast that when the sample finally melts, the temperature you read is behind what the sample capillary is actually exposed to. Therefore your M.P. is too low.

 You should be comfortable with the following terms before you come to class. Write the definitions on a separate sheet of paper if you are unsure of a concept or definition. These will be discussed in lab, but you must still be prepared.

Important terms: Physical constants ; melting point, boiling point, density, refractive index, specific rotation, pure substance , sublimation, melting range, eutectic point (mixtures), phase diagram (binary system), mixture melting point (mixed melting point).

 We will be doing the following procedures during the lab. It is expected that you will have read the chapters/pages listed in the lab syllabus and this lab write-up before you come to lab. Anyone not prepared for the lab will see it reflected in their grade. All work will be done individually unless otherwise noted.

1. To be done by the instructor-

Demonstrate techniques for filling capillary tubes.
The chemical must be in such a fine granular form that it packs uniformly in the capillary tube. If the chemical seems too granular (sizes of the crystals are too big) then take a very small amount and grind it in a clean mortar & pestle. Move this onto a glassine weigh paper and clean the mortar & pestle. Place a little of the chemical onto the open tip of the capillary tube. Now drop it through a plastic piece (2 ft) of tubing set on your table to encourage the chemical to drop to the bottom of the capillary tube. You don't want any air spaces. Dispose of any remaining chemical in the appropriate waste container. Used capillary tubes should be disposed of in the appropriate chemical waste container. This procedure should be able to be performed without coming into contact (in any way) with the chemical. Gloves are available.

The instructor will demonstrate use and care of various melting point equipment. (Mel-Temps, Electrothermal models IA 8100 & IA 9100)
 
 


  IA 9100

Directions for use of these are located with the apparatus in plastic sheaths and at the end of this lab write-up.

To be done by the students:

 Calibration of a thermometer (0-150 deg range). - Use: Benzophenone: 50°C; Naphthalene: 81°C; and either Benzoic acid: 122°C.

 Determine the identity of an unknown compound by use of M.P.'s of mixtures.

 When determining the identity of an unknown by using a mixture you need to know that the resulting M.P. of a mixture (of two different compounds) will be lowered. If the mixture is of the same compound then the M.P. will not change. The unknowns listed below (#1, #2 & #3) have three potential identities. By use of comparing the M.P. of the unknown against the M.P. of the unknown mixed with its two separate possible identities you should be able to determine which compound is the unknown.

The three unknowns are benzoin, cinnamic acid, and urea (all melt at 133oC). Choose one of the three unknowns, and determine the identity of the unknown using the mixed melting point technique. Genuine, pure samples of benzoin, cinnamic acid and urea will be available.

 A) Determine the melting point of Unknown #1, Unknown #2 or Unknown #3.

 B) Do a mixture melting point of the same unknown with each of the other possibilities. (As many as needed)

 C) Identify the Unknown.

 

 

CHEMICALS: 

LOCATION

Benzophenone, Naphthalene, Benzoic acid, 

Hood #5

Urea, Benzoin, Cinnamic acid, Unk, 1,2 &3

 

 

 

SUPPLIES: 

 

melting point apparatus & directions 

C-20, C-21

melting point tubes ( 0.8 mm ) 

Org_prep

tubes (filling m.p. capillaries) 

Org_prep

mortar & pestles, watch glasses

C-23

 

 

Waste bottle for all solids, capillaries and debris from M.P. Lab

Hood #5

WASTE Disposal - G: THERE WILL BE WASTE JARS AVAILABLE FOR THE CAPILLARY TUBES (WITH THE CHEMICAL SAMPLE) AND UNUSED SOLIDS PUT OUT WITH THE LAB. ALL CHEMICALS (including used capillary tubes with these chemicals) MUST BE PUT IN THIS JAR !
DO NOT PUT USED WEIGH BOATS IN THE HW CHEMICAL CONTAINER. CONTAMINATED PAPER TOWELS AND WEIGH BOATS ARE PUT INTO THE CLEAR PLASTIC HW BOX IN THE HOOD. PAPER ENVELOPES W/CHEMICALS SHOULD BE FOLDED MULTIPLE TIMES AND SEALED BEFORE BEING PUT INTO THE SPECIFIED HW CONTAINER.
 
 

Directions for IA 9100 MP Apparatus

 When the approximate MP is known

1. Prepare a Sample.

Up to three samples may be accommodated in the tube guide. To prepare a sample proceed as follows:

Insert the powdered sample in the capillary by dropping the capillary w/powder through the plastic tube to the table top. The quantity should be such that the height does not exceed 1 mm (packed).

Place the sample (s) in the tube guide.

2. Enter the `Set Point'

(This should be 5 oC below expected melt point).
 Proceed as follows (a figure of 107 oC is given as an

 example):.

x 11   Press arrowed UP key eleven times (value 10 oC per  keystroke) to set a temperature of 110 oC.

 +

x 3     Then press the arrowed DOWN key three times (value 1 oC per keystroke) to reduce the `Set Point' to 107 oC.

If an error Is made, press the CLEAR key and start again.

3. GOTO `Set Point'

 GOTOPress Goto key.

The four bar L.E.D.s will illuminate consecutively as the temperature rises.
 
 

To review `Set Point' Press UP arrowed key  and `Set Point'  will be displayed for approximately 1 second.

When the `Set Point' temperature is reached the left hand L.E.D. on the GOTO key will illuminate and three audible bleeps will sound to indicate that the temperature has stabilized.

8. Initiate Ramp

 Press GOTO  to initiate the temperature ramp cf 1.0oC per minute.

The four bar L.E.D.s and the left hand L.E.D. on the GOTO will go out. The right-hand L.E.D. will illuminate.
After a brief delay the oven temperature will commence to rise at 1.0 oC per minute.
 

Fast Ramp Selection
For use when the MP is unknown

1. Fast Ramp Selection

In order to facilitate the determination of unknown melt points, a fast ramp at 10 oC per minute may be selected with or without the pre-selection of a set point temperature. This may be achieved as follows:-

x 4   Press CLEAR 4 times (or once if at set point)

Press arrowed UP key. The display will-show 010.0

Press GOTO. The display will show 10:10 to indicate that `fast ramp' has been selected.

 Press GOTO to initiate fast ramp or CLEAR to cancel if required. The right hand L.E.D. on the GOTO Key will be illuminated and the oven temperature will now rise at 10 deg. C per minute to a maximum temperature of 400 deg.C.

Four melt temperatures may be stored and recalled in the normal manner.