organic chem abstract

RECRYSTALLIZATION OF AN UNKNOWN COMPOUND

OBJECTIVES: to crystallize a pure compound from a contaminated mixture. To observe how different conditions, such as the amount of solvent, the temperature etc. can affect the outcome of the crystallization. To check the purity of the crystals measuring the melting point.

TIME ALLOCATED: ONE LAB PERIOD

Introduction

During a synthetic reaction, the compound produced is rarely pure. There are decomposition products, and side reactions are a common occurrence. Therefore, most compounds must be purified. Recrystallization is one of the most desirable methods for the purification of solid organic compounds. Recrystallization is based upon the observation that samples are more soluble in hot solvents than in cold solvents. There are two main recrystallization techniques:

1) Single-solvent recrystallization. Compound "A" and impurity "B" are dissolved in the smallest amount of hot solvent. After both "A" and "B" are completely dissolved, the solution is allowed to cool. Lowering the temperature decreases the solubility forcing the compound to precipitate. In an ideal situation only compound "A" will precipitate while compound "B" will stay in solution also at low temperature, yielding pure "A". The slower the cooling phase the bigger the crystals formed.

2) Two-solvent Recrystallization. This method is similar to the previous one, but two solvents are used. Compound "A" and Impurity "B" are both soluble in solvent I, then solvent II is slowly added, inducing the precipitation of compound "A". Heating is usually not needed in this method.

For both techniques, the crystals are harvested by filtration, washed with cold solvent to remove insoluble impurities, and allowed to dry.

Impurities. Some samples contain either soluble colored impurities or insoluble ones. Both types may be removed in a single step. The soluble impurities are eliminated by adding activated charcoal to the solution. The impurities are adsorbed on the surface of the activated charcoal and can be filtrated from the solution along with the insoluble impurities.

Characterization of crystals using melting point.

The melting point is defined as the temperature at which the liquid and vapor phases are in equilibrium. The melting point range is reported as the temperature range from the first appearance of liquid to the disappearance of the last crystal. The melting point of a pure compound is one of its intrinsic

physical properties along with other properties such as state of matter, boiling point, refractive index, and density. In determining the melting point of a compound, a melting range must be taken. How sharp that range is indicates the purity of the sample, as the presence of impurity will broaden the melting point range and decrease the initial temperature of the melting point range. The presence of an impurity almost always results in lowering the melting point and broadening the melting point range.

EXPERIMENTAL SECTION

The experiment is divided in two parts:

Part I

You will be given 0.1 g of an unknown impure compound "X".

• Dissolve the unknown "X" in minimum amount of boiling water

• Allow the solution to slowly cool to room temperature

• Cool the solution in ice for 20 minutes

• harvest the crystal

• Let the crystal dry

• Measure the melting point and compare the result with a list of pure compounds to determine the nature of your unknown "X". Calculate the yield. List is provided at the end of the handout.

%A %B

100 0

80 20

60 40

40 60

20 80

0 100

Composition, (mol %)

a

b

ee

m M

E

liquid solution + solid B

liquid solution + solid A

solid A + solid B

liquid solution of A + B

Melting point composition diagram for the binary mixture, A + B. In this diagram, "a" is the melting point of solid A, "b" of solid B, "e" of eutectic mixture E and "m" of the 80% A: 20% B mixture, M.

2

Part II

You will be given other two samples of impure "X" and you will try two of the following recrystallization conditions.

• Dissolve the unknown "X" in 10 mL of hot water, let cool to room temperature and then cool in ice for 20 minutes. Calculate yield.

• Dissolve the unknown "X" in the minimum amount of boiling water, put the flask directly in the ice bath for 20 minutes.

• Dissolve the unknown "X" in ethanol at r.t. Decide if it suitable for re-crystallization. If so, perform the re-crystallization with ethanol let cool to room temperature and put In Ice for 20 minutes..

• Measure the melting points for all samples and record your observations. Calculate the yield.

Record your observations and data for part I and II. These will be part of your lab report.

How to operate the Mel-Temp Instrument

i. Check the temperature reading on the thermometer. Never assume the unit is cold.

ii. Place the loaded melting point tube in one of the three channels of the Mel-Temp.

iii. Turn the unit on. The light should come on and illuminate the sample.

iv. Set the rheostat control to a convenient setting. This setting will depend on the melting point of the sample. Heating curves for the Mel-Temp unit are posted near the Mel-Temps.

v. Determine a rough melting point. (Raise temperature about 10˚Cminute)

vi. Allow the Mel-Temp to cool to at least 15˚C below the rough temperature and put a new sample into the Mel-Temp. Bring the temperature quickly to within 10-15˚C of the rough melting point.

vii. Turn the rheostat down to yield a temperature increase of 2-3˚C/minute.

viii. Record the temperature range from when the crystals first start to melt to the disappearance of the last crystal.

SAFETY

Always use your PPE, be careful handling hot solvents, and make sure that heating blocks are functioning properly and are safe to use. Use fume

hood every time you are using organic compounds, in particular if they are "unknown".

REAL LIFE EXAMPLE OF CRYSTALLIZATION

Recrystallization or crystallization are techniques used, not only to purify synthetic compounds, but also to elucidate the three dimensional structure of many biological active molecules, providing essential information about mechanism of action of life processes. In the picture above (fig.1) you can see the crystal structure of duplex DNA oligonucleotides and two compounds containing Platinum (a) cisplatin, (b) pyriplatin. Both compounds are utilized for cancer treatment.

POST-LAB ASSIGNMENT

• Complete the work sheet with all the entry and data recorded during the lab section

• Write the abstract for the Recrystallization experiment following the template.