Experiment 7:

RECRYSTALLIZATION, MELTING POINT, and

HPLC ANALYSIS of ANALGESICS

Objectives To learn the techniques of

recrystallization and melting point analysis.

To purify aspirin and acetaminophen samples isolated by extraction using recrystallization.

To evaluate the purity of samples by melting point and HPLC analysis.

BEFORE COMING TO LAB…

Watch the following videos:

Recrystallizationhttp://www.wonderhowto.com/how-to/video/how-to-illustrate-recrystallization-in-organic-chemistry-271753/

Melting Point Analysishttp://www.wonderhowto.com/how-to/video/how-to-measure-melting-points-in-the-chemistry-lab-259806/view

/

EFFECT OF TEMPERATURE ON SOLUBILITY

Solubility vs Temperature

0

5

10

15

20

25

30

0 20 40 60 80

Temperature (ºC)

So

lub

ilit

y (g

/L)

LOW TEMPS: the substance is much less soluble, however some of the substance will be dissolved.

HIGH TEMPS: the substance is much more soluble.

ACTIVE INGREDIENTS IN GOODY’S POWDERS

OH3C

O

O

O

N

O

HCH3

OH Aspirin

Acetylsalicylic acid

MF: C9H8O4

MW: 180.2 g/mol

mp: 138-140 oC

Hazards: Toxic, Irritant

Solubility in H20:

1g in100mL at 37oC

1g in 300mL 21oC

1g in 400mL 15oC

~1g in 550 mL at 0oC

Acetaminophen

4-acetamidophenol

MF: C8H9NO2

MW: 151.2 g/mol

mp 169-172oC

Hazards: Toxic, Irritant

Solubility in H20

1g in 70mL at 31oC

1g in 150 mL at 21oC

H

PERCENT RECOVERY• Percent recovery: an indication of how much of the active ingredient you were able to recover after the purification method.• HPLC Area %: the percent of the active ingredient present in the sample submitted for analysis.

• The percent recovery of compound B is calculated by:

% R e c o ve ry o f B = M a s s o f B re c o ve re d

M a s s o f B us e d ini tia llyx 1 0 0

OVERVIEW—ASPIRIN Reweigh aspirin + filter papers from last lab.

Subtract out filter paper weights to get ACTUAL RECOVERY OF ASPIRIN.

Prepare 2 melting point capillaries of the EXTRACTION ASPIRIN.

Dissolve aspirin in hot ethanol, then hot water, then few drops of hot ethanol.

Cool to room temp, then in ice for 15 minutes. Suction filter. Prepare HPLC sample and submit for analysis. Secure small filter paper + aspirin in larger filter

paper and submit. No final weight or melting point sample until next

lab!

OVERVIEW—ACETAMINOPHEN

Transfer acetone/acetaminophen solution from vial to beaker and evaporate acetone completely.

Dissolve acetaminophen in hot water (drop-wise).

Cool to room temp, then in ice for 15 minutes.

Suction filter.

Prepare HPLC sample and submit for analysis.

Secure small filter paper + aspirin in larger filter paper and submit.

No final weight or melting point sample until next lab!

Table 7.1: Experimental Results

Aspirin Acetaminophen

* Theoretical recovery after extraction (g)

• Same # as Table 6.1

• Same # as Table 6.1

*Actual recovery after extraction (g)

• Same # as Table 6.1

• Same # as Table 6.1

*% Recovery after extraction• Same # as Table 6.1

• Same # as Table 6.1

Theoretical recovery after recrystallization (g)

Same as… Same as…

Actual recovery after recrystallization (g)

• Obtain weight during next lab

• Obtain weight during next lab

% Recovery after recrystallization

Actual X 100Theoretical

Actual X 100 Theoretical

• % Recovery is ≠ HPLC Area %

OVERVIEW—MELTING POINT

ANALYSIS

Prepare a melting point capillary containing the provided impure aspirin.

Perform a melting point analysis on this along with the EXTRACTION ASPIRIN prepared above.

Begin MelTemp on setting of 4. Back down temp slightly as thermometer approaches 20o

C from the expected melting point (lit value).

Record melting range (Ti-Tf).

EXPERIMENTAL PROCEDURE

(Melting Point)

Dip the open end of a mp capillary tube into the sample to be tested.

Invert the capillary tube and tap it on the bench top to pack the sample into the closed end.

Insert the capillary tube into one of the slots in the heating block of the Mel-Temp melting point apparatus.

EXPERIMENTAL PROCEDURE

(Melting Point)

• Turn the Mel-Temp power on and adjust the temp control to the desired rate of heating.

• In order to obtain an accurate mp, it is necessary to heat SLOWLY (rate of 2-3oC/min).

• SOURCES OF ERROR:• Heating too fast = inaccurate mp (hard to see 1st crystal melt!)

• Too much sample = broadened mp range (takes a long time to melt entire sample!)

Thermometer

Sample slot

Eyepiece

Temp control

Power

Table 7.2: Melting Point Results

Aspirin Acetaminophen

Impure Aspirin preparationThis lab!

Ti-Tf

---

Extraction sample melting range (oC)

This lab!

Ti-Tf

---

Recrystallized sample melting range (oC)

Next lab!

Ti-Tf

Next lab!

Ti-Tf

Literature melting point value (oC)

139-140 169-172

Table 7.3 Recrystallization HPLC

Results

SampleCompound

HPLC retention times (min) and area %

Standards

Aspirin Sample

Chromatogram

Acetaminophen Sample

Chromatogram

Rt (min)

Rt (min)

area % Rt (min)

area %

Aspirin

Acetaminophen•Samples containing a single compound are considered PURE samples, while those containing both compounds are considered MIXED (IMPURE) samples.

• Be sure to attach both sample chromatograms to final lab report!

• ANY HPLC SAMPLE CONTAINING VISIBLE SOLID WILL BE DISCARDED. Student will receive a penalty for not providing a sample.

SAFETY CONCERNS

• Ethanol and reagent acetone are both extremely flammable. Keep away from open flames, and use extreme caution when applying heat!

WASTE MANAGEMENT

Place all liquid waste from recrystallization in container labeled “LIQUID ORGANIC WASTE”.

Used melting point capillaries should be thrown in the broken glass box, NOT the trashcan! Any student leaving the melting point

capillary tubes in the MelTemp apparatus will receive a technique grade penalty!

IN LAB QUESTIONS… (The following questions should be answered in laboratory

notebook.)

The UV detector on the HPLC instrument detects compounds that absorb UV light in the range of 254 nm. Compounds containing multiple bonds and aromatic rings can typically be detected at this wavelength. Which of the following compounds would you expect to be detected and appear in the HPLC chromatogram of your sample?

OH

O

O

O

N

OH

H

O

O

O

OH HO

H

In some instances, a sample will appear to be pure based on HPLC analysis; however the experimental melting point will be lower than expected. Give one possible source of this inconsistency and explain why it occurs.

IN LAB QUESTIONS… (The following questions should be answered in laboratory

notebook.)