Experiment 1 - Melting Points

Introduction

The melting point of a substance (the temperature at which a substance melts) is a physical property that can be used for its identification.  It is a measure of the amount of kinetic energy (heat) that must be supplied to the particles of the substance in order to overcome the intermolecular forces (such as 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 they generally work with solid molecular compounds that have low melting points (below 300°C) and which can be conveniently measured.  Organic compounds are used in this experiment for the same reasons.

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.  Above that range, the substance exists only as a liquid, and below it only the solid phase is present (no wetness observed).  The extent of this temperature range is a measure of the purity of the substance; that is, impure samples of compounds have lower and broader temperature ranges of melting.  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 85 to 100°C.

An identical or near identical temperature range of melting is not, in itself, proof of the sameness of two organic chemical samples.  There are thousands of solid organic compounds that melt within any relatively short temperature range; overlap of melting points is therefore inevitable. 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 mixing the known and unknown together in relatively equal quantities. A 50:50 mixture will either be a pure sample of the known compound or a highly impure sample of the known compound.  The melting point of the mixture will be identical to that of the known compound in the first instance or lowered and much broadened in the latter.  This identification/confirmation procedure is referred to as the determination of a "mixed" melting point.

The Determination of Melting Points

Melting points will be determined by using one of the DigiMelt units (Figure 1-1 on the next page). The DigiMelt units must always be kept upright. Place a small quantity (1/16 inch in tube) of the solid to be melted in a capillary tube (labeled melting point tubes).  Tap the closed end of the tube on the desk, clean the outside, and use the tamper of the right side of the DigiMelt to compact the solid down to the closed end of the melting point capillary tube. Drop the tube (closed end down) down a section of glass tubing (see TA) to compact the solid in the bottom or closed end of the tube even more. Place the tube loaded with the sample into the sample holder of the DigiMelt with the closed end down. The crystals can be ground up in a clean, dry mortar and pestle if they are too big to fit into the capillary tube.

If the melting point of the sample is unknown or unavailable, a fast run with the DigiMelt set at a ramp of 10 or 20°C per minute to obtain an approximate melting range.  A more precise value can then be obtained by heating the DigiMelt more slowly at a slower ramp (about 2°C/min.) in the vicinity of the known melting temperature.

Figure 1-1:   The DigiMelt apparatus.  Melting point capillary tubes are placed (closed end down) in the slots directly in front of and are viewed through the magnifying lens.  Up to three samples can be viewed at once.  The heating rate of the DigiMelt is adjusted by setting a temperature ramp along with a start and end temperature following the "Quick Start Instructions" on the front of the DigiMelt.  A ramp of 20° C per minute will result in a rapid temperature rise while a ramp of say 2° C per minute will give a slower rise that will more accurately measure the melting range of a solid. 

Record the temperature the crystals begin to melt (crystals will look wet) and the temperature at which the substance becomes a clear liquid. This is the melting range. The DigiMelt provided a digital readout of the temperature equipment are not calibrated and may be off as much as ±3°C.  Consequently, do not expect the melting points obtained with the DigiMelt apparatus to be identical to those listed in the Table shown on page 3. The calibration of the DigiMelt thermometer will be checked using the melting range obtained for pure urea or pure cinnamic acid. Use the same DigiMelt for all your measurements.

The Experiment

Prelab Work: Answer the prelab questions at the end of this write-up on a piece of loose leaf paper (not in lab notebook) after you have read the experiment. A quiz will be given at the start of the period covering the introduction and this experiment in the laboratory manual, any lab lecture material from last week or this week, and the prelab questions.

Laboratory Notebook: Be sure to read the section on the laboratory notebook in the introduction of the laboratory manual. All data, calculations, observations, and conclusions should be recorded directly in the laboratory notebook. Be sure to save the first two pages of the notebook for a table of contents. Results for unknowns are reported by filling a report sheet found at the end of the experiment and giving the sheet to your TA or stockroom (216).

Supplies: DigiMelt, Capillary Tubes (closed end), mortar & pestle, cinnamic acid, urea, and chemicals in the table below. If possible, use the same DigiMelt for all your work.

CAUTION:  All the used chemicals for this experiment should be placed into the bottle marked “Waste Organic Solids.”  Used capillary tubes should be thrown in the broken glass container.  Avoid contact with these chemicals; some are irritants or are toxic.  Wash your hands when finished.

Melting Ranges of a Number of Organic Compounds

Compound	Hazard Code	Description/Uses	Melting Point (^ºC)**
Vanillin	1102	Natural vanilla flavoring	81-82
Dibenzofuran	2111	Minor constituent of coal tar	81-83
Acetamide	3111	Solvent, plasticizer, stabilizer	79-81
Azelaic Acid	0011	Rancidification product of fats	106-107
		containing oleic acids
o-Toluic Acid*	1011	Substituted benzoic acid	103-105
m-Toluic Acid*	1011	Substituted benzoic acid	108-110
Resorcinol	3111	Disinfectant	109-110
Benzoic Acid	1213	Found naturally in berries	122-123
Urea	2122	Used as fertilizer	132-133
Cinnamic Acid	1113	Oxidation product of cinnamon oil	132-133
Acetylsalicylic Acid	2112	Aspirin	135-136
Maleic Acid	2112	Manufacture of resins	137-139
Benzilic Acid	2121	A carboxylic acid	150-153
Adipic Acid	1202	Used to manufacture nylon	152-153
Citric Acid	1123	Sour taste of citrus fruits	153-154
Mannitol	0201	Manufacture of radio condensers	167-170
Tartaric Acid	1111	Used in soft drinks, cream of tartar	168-170
Itaconic Acid	0112	A dicarboxylic acid	166-167
Succinic Acid	0111	Manufacture of dyes, perfumes	187-190
Ascorbic Acid	1111	Vitamin C	190-192
Cholic Acid	2011	Emulsifies fats in the intestine	198-200

*           Note that these two compounds are isomers.

**         These melting points may vary according to supplier.  Be sure to run a "mixed melting" range in identifying unknowns.

Melting Points and Mixed Melting Points of Compounds Having Similar Melting Points

Determine the melting points of pure samples of cinnamic acid and urea as well as a 50-50 mixture of the two, and record the data in your lab notebook.  The three samples can be run simultaneously in the DigiMelt (use temperature range of ~110-140°C). To prepare a 50:50 mixture, mix equal small portions of these compounds (estimate the amount of each, about 0.02-0.06g). Grind the mixture to a fine powder mix in a clean, dry mortar and pestle provided (Wash the mortar and pestle with soap and water, rinse with tap water, distilled water and acetone in hood to clean and dry).

Is the melting point of the mixture different? If it is different explain why. If the melting point of urea differs from the value listed by more than as ±3°C see your TA. You may have to repeat the melting point of urea and succinic acid.

The Unknown

To identify your unknown you must first measure its melting range. Tap a small amount of your unknown into two different capillary tubes.  Just a few crystals are adequate.  You may need to grind some of your unknown into a powder if it is too coarse to fit into the capillary tube.  Find the melting point range of the pure unknown substance by first quickly determining an approximate melting range on a fast ramp (20°C/min from 70-210°C) and then doing a slow, careful melting range with the second capillary tube you prepared (use a ramp of 2°C/min and start about 15°C below the melting range to 10°C above the range). Make sure the DigiMelt is below 70°C before starting the first melting range and 10-20°C below the compound’s melting range before doing a slow careful melting range. Run cinnamic acid again with the unknown if the calibration was off.

Using the melting ranges listed in the table on the previous page determine which possible compounds are within ±8°C of your unknown's melting range.   Make a 50:50 mixture of the unknown with each of these possible compounds and carefully grind each mixture to a uniform powder in a clean, dry mortar and pestle.  Take the melting range of each mixture at the same time using a slow ramp (2°C/min) with a starting temperature 20°C lower that the slow run done with the unknown. Record all of your results on your report sheet.  The mixture where the melting point does not change indicates that the two compounds in the mixture are the same. Clean the mortar and pestle and return to the storage location in the lab. You should now be able to identify your unknown and complete the report sheet for this experiment. Record all results and answers to questions in your laboratory notebook.

Prelab Questions 

1.  Two samples have the exact same melting points.  Are they the same compound? How could you tell for sure?

2.  You have two samples of mannitol. One melted between 168º-169º and the other melted between 161º-168º. Which sample has the greater purity?  Why?

3. Which would be the best way to determine the melting point of a compound? Why?

     a.     Slowly run two very precise melting points.

     b.     Run a very precise melting point and then run a fast one to double check your work.

     c.     Run a quick melting point for an approximate melting range, then a slow precise one.

4.  Risk Assessment: What are the safety hazards and precautions for this experiment?

5.  How much sample needs to be placed in the capillary tube to determine a melting point?

6.  How do you place the capillary tube in the DigiMelt?

7.  Explain how a "mixed" melting point can be used to confirm the identity of a compound.

8. Name three intermolecular forces that hold organic molecules together as solids.

Name______________________________________________ Dana ID_________________

Section Letter____ Locker #___________ Work Station #__________ Date___________

                                              CHM 235L - Melting Points Experiment

                                                    Determination of Melting Points

Experimental:

A. Determination of the melting points of pure cinnamic acid, pure urea, and a 50:50 mix.

Melting Range

Compound                                       Start Finish

            Cinnamic Acid            _____________ ºC_____________ ºC

            Urea                            _____________ ºC_____________ ºC

            50:50 Mixture              _____________ ºC_____________ ºC

B.    Determination of Identity of an Unknown

        Melting Range of Unknown _________________________ ºC

        Mixed melting points (50:50 mix of unknown with compounds of similar melting ranges from table on page 1-2):

Mixture Melting Range

        Unknown   &  ________________________________  ____________ – __________ ºC

        Unknown   &  ________________________________  ____________ – __________ ºC

        Unknown   &  ________________________________  ____________ – __________ ºC

        Unknown   &  ________________________________  ____________ – __________ ºC

Identification of Unknown __________________________________________________

For Unknown #____________

Turn in this completed report sheet to your TA before the end of lab.