Objective
The purpose of this laboratory exercise is to introduce the concept of the melting point of an organic compound as a first step in chemically identifying that compound. Additionally, the melting point of a known compound can reveal the purity of that compound.
Grading
You will be assessed on
The pre-lab assignment involves reading and signing both the Equipment Responsibility Form and the Safety Rules.There is an additional Release Form if you are under 18. These sheets must be completed and handed in to your TA at the beginning of lab.
Introduction
Most of the world (at least those areas further from the equator than Texas) are very familiar with melting points. That's because for several months a year these cities have to deal with snow and ice which can reduce the flow of traffic on the roads to a complete stand still. Thus the temperature of 0 °C embodies agony for morning commuters and joy for young school children hoping for a day off of school.
And unsurprisingly all other solids display a characteristic melting point. With a pure substance, a melting point is the quickest and most accessible method for an organic chemist to confirm the identity of a compound (though in all fairness, NMR is the method of choice due to the proof of identity it provides as well as other information it gives).
Melting points can also be used to assess purity. You are all familiar with the concept of freezing point depression. Impurities in the solution prevent the ordering necessary to form a crystal lattice. This is why salt/water solutions will not freeze until sometimes as low as -20 °C. The concepts involved with melting points are similar. In a perfect crystal with no impurities the melting point will occur at one temperature. However when impurities are introduced there is no longer a continuous crystal structure. Instead the solid is made up of different regions, some with more crystal imperfections then others. Not surprisingly the melting point will not be as sharp. This will also cause the melting point to begin earlier then expected. These impurities can even be water (which complicates things here in Houston).
It is a combination of these concepts which will be used in this lab. For the first part of the lab you will examine how adding an impurity will effect the melting point of the compound. In the second part of the lab you will determine the identity of an unknown by applying the knowledge of mixing properties which you gained in the first part of the experiment.
One other useful bit of information. It will quickly become apparent that the melting points you measure will not match the melting points found in literature. You are encouraged, and it is to your advantage, to hypothesize why this is the case and to ask your TA's if your hypothesis is correct.
Pre-Lab Questions
· Find melting point data: For Part I you will be preparing mixtures of two compounds. Before lab you should look up the melting points of these two compounds. Possible sources can be Aldrich (Foundren location: TP202 .A42 2000-2001 REFERENCE-TRADE) or Acros Chemical catalogs. Various online sources are also available and can be used at your discretion. Also look up the three chemicals in part II of the lab. The chemicals for part II are diphenylacetic acid, adipic acid, and diphenylthiourea. Those in part I are cinnamic acid and urea.
Experimental
! WASTE COLLECTION: All capillary tubes are to be disposed of in the SHARPS CONTAINER and NOT in the garbage!!
Part I: The Effect of Purity on Melting Point
In this section you will take two compounds (cinnamic acid and urea) and create mixtures of them to determine what effect this will produce, as well as the extent of this effect depending on the composition of the sample.
Prepare a capillary tube containing urea as well as one containing cinnamic acid. Take the melting point of each and compare it to the "Houston" values (which your TA will have). Repeat if the values are not correct.
Once the two pure melting points have been determined find a partner and together prepare 3 samples: one of 1:9 urea : cinnamic acid, one of 1:1 urea : cinnamic acid, and one of 9:1 urea : cinnamic acid. Take the melting point of each of these mixtures. Plot the melting point vs. the composition of the sample.
Part II: Determination of an Unknown Substance
In this portion of the lab you cannot determine the identity of an unknown by comparing its melting point with the melting point of the three possible compounds. Mixing characteristics will instead be used to determine the identity.
Prepare three capillary tubes in the following manner. Take a piece of weighing paper and mix a small amount of the unknown with a small amount of the sample of diphenylacetic acid. Place this in a capillary tube. Then on a new piece of weighing paper, weigh a small amount of the unknown and mix it with a small amount of adipic acid. Place this in a capillary tube. Repeat this again with the unknown and diphenylthiourea. Place these three capillary tube in the melting point apparatus and record in detail what occurs to each tube and at what temperature. Using this knowledge determine the identity of the unknown.
Post-Lab Questions
No questions; instead the question points will be allotted for the following three things. Be sure to clearly indicate where the following are in your notebook for your TA:
· Plot of melting point versus composition of the mixture (Part I).
· Your explanation as to why your measured melting points do not match those in literature (Part I).
· Identity of the unknown (Part II).