1/30/2015 1 Recrystallization / Filtration Purification and Isolation of an Impure Compound by Recrystallization and Vacuum Filtration Vacuum Filtration

1/30/2015 1

Recrystallization / FiltrationRecrystallization / Filtration

Purification and Isolation of an ImpurePurification and Isolation of an Impure

Compound by Recrystallization andCompound by Recrystallization and

Vacuum FiltrationVacuum Filtration References:References:

●Slayden, et. al.,Slayden, et. al., – pp. 31 – 34– pp. 31 – 34

●Pavia Tech 11Pavia Tech 11 – pp. 662 - 680– pp. 662 - 680

●Pavia Tech 6.2-6.3Pavia Tech 6.2-6.3 – pp. 598 - 608– pp. 598 - 608

●Pavia Tech 8Pavia Tech 8 – pp. 630 - 642 – pp. 630 - 642 (Filtration)(Filtration)

●Web Site:Web Site:

http//:mason.gmu.edu/~jschorni/http//:mason.gmu.edu/~jschorni/

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Recrystallization / Filtration Background

Recrystallization

Solid organic compounds produced in the laboratory usually need to be purified

The most common technique involves Recrystallizing the sample from an appropriate Solvent

The Recrystallization process is a relatively slow and selective formation of crystals from a solvent

Precipitation is a rapid and nonselective process; thus not used to purify samples

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Recrystallization / Filtration Recrystallization (Con’t)

The Recrystallization Process

Dissolve sample in a minimal amount of an appropriate solvent

Sample should be insoluble in solvent at room temperature, but soluble at elevated (near boiling point) temperature

If solution is colorized, it is sometimes necessary to add a decolorizing agent, such as activated charcoal (Norite)

Colorized solutions are first filtered through a fluted filter or a column containing alumina or silica gel

Note: Unless otherwise instructed, decolorization will not be done in this experiment

04/18/23

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The Recrystallization Process (Con’t) The hot solution is cooled slowly to allow the

purified crystals to form; leaving the more soluble impurities in solution

After solution and purified crystals have returned to room temperature, place the beaker in water/ice bath

Collect crystals by Vacuum Filtration Rinse the crystals with small portions of cold

solvent Air-Dry the crystals for a week on a pre-weighed

watchglass or weighing tray in your drawer or the instructor’s drawer

Determine the Melting Point Range of the purified sample

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The Appropriate Solvent The solute particles are generally insoluble in

cold solvent, but soluble in hot solvent The solvent (or mixed solvent) should have a

steep solubility vs temperature curve for the solute

The “C” curve is a good solvent

Solute is sparingly soluble at room temperature

Solute is very soluble at elevated temperature

A

B

C

Temperature

So

lub

ilit

y

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The Appropriate Solvent (Con’t)

Solubility of organic compounds is a function of the polarities of both the solvent and the solute:

“Like Dissolves Like”

Polar solvents dissolve polar solutes

Nonpolar solvents dissolve nonpolar solutes

The stability of the solute crystal lattice affects the solubility. The higher the melting point (higher stability), the less soluble the solute

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The Appropriate Solvent (Con’t) The Boiling Point of the solvent must be less

than the Melting Point of the solute If the Boiling Point of the solvent is higher than

the melting point of the solute, the solute will “Melt” instead of “Dissolving” in the solvent at the elevated temperature

Upon cooling, the “Melted” solute will “Oil” out forming an insoluble mass that is not purified

The solvent should not react with the solute Compounds with functional groups that can form

hydrogen bonds (-OH, -NH-, -COOH, -CONH-) will be more soluble in hydroxylic (polar) solvents such as Methanol and Water

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Recrystallization / Filtration Solvents in Decreasing Order of Polarity

H2O - Water

RCOOH - Organic Acids (Acetic Acid) RCONH2 - Amides (N,M-

dimethylformamide) ROH - Alcohols (Methanol, Ethanol) RNH2 - Amines (Triethylamine, Pyridine)

RCOR - Aldehydes, Ketones (Acetone) RCOOR - Esters (Ethyl Acetate) RX - Halides (CH2Cl2 > CHCl3 > CCl4)

ROR - Ethers (Diethylether) ArH - Aromatics (Benzene, Toluene) RH - Alkanes (Hexane, Petroleum

Ether)

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Recrystallization / Filtration Filtration

Two Purposes Separate purified solid from the soluble impurities

in the solution from which it was recrystallized Remove solid impurities remaining after

recrystallizing a sample from a solvent Two Types

Gravity Vacuum

Filter Paper Porosity – Measure of the size of the

particles than can pass through the paper

Retentivity – Opposite of Porosity; measure of the size of particles that can be retained on the filter paper

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Recrystallization / Filtration Gravity Filtration Types

Filter Cones – Folded paper filter inserted into a class funnel with stem extending into a receiving flask

Applicable Volume – > 10 mL

Fluted Filters – Specially folded (many creases) filter paper inserted into a glass funnel with stem extending into a receiving flask


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Recrystallization / Filtration Gravity Filtration Types (Con’t)

Filtering Pipettes – Microscale technique used Pasteur Pipets

A piece of cotton is inserted into the top of the lower constriction

Applicable Volume – < 10ml

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Recrystallization / Filtration Vacuum Filtration – More rapid than gravity filtration

Buchner Funnels – Primarily used to filter large volumes of liquid from solids, such as crystals from the Recrystallization Process


Hirsch Funnels – Similar, but smaller than Buchner Funnel, with sloping sides

Used in Microscale techniques

Applicable Volume – < 10 mL

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Recrystallization / Filtration Experimental Overview

This experiment will consist of 4 parts: Part A

Determination of an appropriate solvent for recrystallizing an impure sample of Fluorene

Part B Recrystallization of Fluorene from the

applicable solvent Part C

Isolation of purified Fluorene by vacuum filtration

Part D Verification of compound identity by melting

point

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Recrystallization / Filtration Part A

Determine the appropriate solvent forrecrystallizing an impure sample of Fluorene

Notes:

Selecting a good solvent for recrystallization requires development of a practiced and consistent technique

There is no perfect procedure that is applicable to all compounds

Make sure the boiling point of the solvent is less than the melting point of the sample compound

The solvents to be evaluated include: Water, Methanol, and Petroleum Ether (a mixture of low-boiling alkanes, 30-60oC)

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Recrystallization / Filtration Procedure – Part A (con’t)

Set up two hot water baths: 65oC 95oC

Place three (3) small test tubes in a test tube rack Place a label with a solvent name on each test tube To each test tube add about 50 mg of Fluorene To one of the test tubes add 0.5 ml of Petroleum Ether Stir the mixture with a microspatula If all (or almost all) of the Fluorene solute dissolves at

room temperature, then the Petroluem Ether is probably not appropriate for recrystallizing Fluorene

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Using the 65oC water bath, continuously insert and remove the test tube in a manner that gently heats the mixture until it just begins to boil – immediately remove the test tube from the water

Note: Be careful, Methanol and Petroleum Ether reach the boiling point quickly

It is essential to minimize the evaporation of the solvent

If the solid does not dissolve, add more solvent dropwise, up to a maximum of 2.0 ml

Continue the heating process

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If the solid dissolves in the Petroleum Ether, allow the mixture to cool in the test rack for a few minutes and then place the test tube in an ice/water bath for about a minute

If the crystals do not come out of solution, scratch the sides of the test tube with a glass stirring rod to induce crystallization

If the crystals still do not come out, then try reheating the solution until about a third of the solvent has evaporated

Repeat the cooling & scratching process again If the crystals still do not come of solution, then

Petroleum Ether is probably not appropriate for recrystallizing Fluorene

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If the crystals come out of solution, then Petroleum Ether is an appropriate solvent for recrystallizing Fluorene

Repeat the above process for the test tube containing the Methanol solvent using the same 65oC water bath

Repeat the above process for the test tube containing the water solvent, but use the 100oC water bath

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Recrystallization / Filtration Procedure – Part B

Recrystallize sample of Fluorene from applicable solvent

Obtain a vial containing about 1.5 g of impure Fluorene

Weigh the vial containing the sample to the nearest 0.001 g

Empty the contents of the vial into a clean 125 ml Erlenmyer flask

Reweigh the empty vial to the nearest 0.001 g

Use the masses of the filled and empty vial to compute the mass of the Fluorene sample

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Recrystallization / Filtration Procedure – Part B (con’t)

Recrystallize Fluorene from Solvent (con’t) Assemble the recrystallization apparatus as

show in the figure below

Use the setup on the left when using water as the solvent

Use the setup on the right when using low-boiling point solvents. Use only enough water in the beaker to allow the flask to sit on the bottom of the beaker

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Recrystallize Fluorene from Solvent (con’t) Place the flask containing the sample on the

hot plate if using water as the solvent or in the 65oC water bath if using Methanol or Petroleum Ether as the solvent

Add enough solvent to just cover the solid in the bottom of the flask

Stir gently (Do not use a Thermometer or stirring rod, just swirl the solution gently)

Heat solution only as long as it takes to dissolve the sample

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Recrystallize Fluorene from Solvent (con’t) If the sample does not completely dissolve after

heating for a minute or so, add additional solvent (0.5 – 1.0 ml) and continue heating

Repeat this process until sample has completely dissolved

Note: Do not add too much or too little solvent

● Allow the mixture to cool slowly to room temperatureNote: the purified crystals will come out of solution as the solubility decreases with decreasing temperature.

Place the cooled mixture in an ice/water bath for a minute or two

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Recrystallize Fluorene from Solvent (con’t)

If the cooled recrystallized mass can not be swirled, there is insufficient liquid solvent to hold the soluble impurities, thus additional solvent must added and the sample recrystallized again

If little or no crystals come out of solution, it will be necessary to evaporate some the solvent

Once a suitable mixture of purified crystals and solvent have been obtained, the crystals will be separated from the solvent using vacuum filtration

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Recrystallization / Filtration Procedure – Part C

Vacuum Filtration Set up Buchner Funnel and Filter Flask for

Vacuum Filtration (Instructor will demonstrate) Connect the rubber tubing to the vacuum flask

and the vacuum apparatus on the lab bench

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Recrystallization / Filtration Procedure – Part C (con’t)

Vacuum Filtration (con’t)

Turn on the water flow

Place a paper filter in the Buckner Funnel top and moisten the filter paper with cold solvent to seal filter on bottom of funnel

Pour the crystal/solution slurry into the funnel

Rinse the flask with cold solvent to transfer all solid particles to funnel

Allow the filtration to continue until no more drops of solution pass into the filter flask

Wash crystals 3 times with 5 mL cold solvent

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Recrystallization / Filtration Procedure – Part C (con’t)

Vacuum Filtration (con’t)

Dry the Sample

Transfer the crystals to a pre-weighed labelled weighing tray

Place the sample in the instructor’s drawer and allow to dry for a week

Determine the Mass of the purified sample

Determine the % recovery

Determine the Melting Point range of the sample

Verify the compound by its melting point

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Recrystallization / Filtration The Report

From the procedure description determine the number of logical procedures

A procedure is a logical collection of steps leading to a new result

If a procedure involves a calculation, the equation must be setup as part of the procedure description and the computation of the results must be presented in the “Results” section including appropriate units and precision

Create neat, readable templates for presenting the results of each procedure. Use aligned columns and tables where applicable

Note: The computation for each computed result must be shown

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Recrystallization / Filtration Report Notes (con’t)

The Summary section is a paragraph presentation of the results obtained in logical order

The “Analysis/Conclusion” section should comment on the following using applicable arguments and selected results:

The amount of sample recovered relative to the impact of the procedure on potential loss of sample

Verification of the purity of the recovered sample in terms of appearance and comparison of the melting point with the literature value

Discuss the reasons for any difference in the melting points between the pure and impure samples

Documents

1/30/2015 1 Recrystallization / Filtration Purification and Isolation of an Impure Compound by Recrystallization and Vacuum Filtration Vacuum Filtration