Experiments 6 & 7: The Separation of an Acid, a Base, and a Neutral Compound

An introduction to liquid-liquid extraction

Liquid-Liquid Extraction• Involves the partitioning of compounds between two

immiscible liquid phases, one of which is aqueous while the other is organic

• You will start with a mixture of the following three compounds dissolved in dichloromethane (CH2Cl2):

4-ChloroanilineBasic

2-Methylbenzoic AcidAcidic

Butyl ButanoateNeutral

Liquid-Liquid Extraction

• 4-Chloroaniline and Butyl Butanoate are listed in the appendix of the lab manual - ensure that you are aware of their safety/handling precautions.

• 2-Methylbenzoic acid is likewise toxic by ingestion or inhalation, and is also irritating to the skin, eyes, and respiratory system

• If you wear gloves (and you should!) be sure not to touch exposed skin with those gloves.

4-ChloroanilineBasic

2-Methylbenzoic AcidAcidic

Butyl ButanoateNeutral

Liquid-Liquid Extraction: Technique

• Dispense 10 mL of the stock solution directly into the separatory funnel and add the aqueous solution

• Compounds can only move between the two liquid phases where they come into contact. Shake the separatory funnel!

• Always hold the stopper in place using one hand. Support the stopcock with the other hand, remembering to frequently vent the separatory funnel.

Liquid-Liquid Extraction: Technique

Return the funnel to its upright position, remove the stopper, and wait for the layers to separate. The lower layer may be drained through the bottom, and the upper layer may be poured out through the top if necessary.

Which layer is which?

EXTRACTION SOLVENT : 5% AQUEOUS HClNH2

Cl

H Cl

NH3Cl

Cl

+

EXTRACT THE AMINE BASE FROM THE ORGANIC LAYERINTO THE AQUEOUS LAYER

UPPER LAYER: AQ HCl

LOWER LAYER: DCM

INSOLUBLE IN DCMSOLUBLE IN WATER

SOLUBLE IN DCMINSOLUBLE IN WATER

CONTAINS 4-CHLOROANILINE.HCl

CONTAINS THE ACID & THE ESTER

Chemically Active Extractions• 5% Aqueous HCl will selectively extract the organic base by

converting it into a water-soluble salt.

Soluble in DCMInsoluble in water

Insoluble in DCMSoluble in water

• After gently shaking the separatory funnel and then allowing the two layers to separate:– Lower layer is dichloromethane (more dense) and contains the

organic acid and the ester– Upper layer is aqueous (less dense) and contains the water-soluble

salt formed from the organic base

EXTRACTION SOLVENT : 3% AQUEOUS NaOH

EXTRACT THE ACID FROM THE ORGANIC LAYERINTO THE AQUEOUS LAYER

UPPER LAYER: AQ NaOH

LOWER LAYER: DCM

SOLUBLE IN DCMINSOLUBLE IN WATER

INSOLUBLE IN DCMSOLUBLE IN WATER

CONTAINS SODIUM2-METHYL BENZOATE

CONTAINS THE ESTER ONLY

Chemically Active Extractions• 3% Aqueous NaOH will selectively extract the organic acid by

converting it into a water-soluble salt.

Soluble in DCMInsoluble in water

Insoluble in DCMSoluble in water

• After gently shaking the separatory funnel and then allowing the two layers to separate:– Lower layer is dichloromethane (more dense) and

contains the ester– Upper layer is aqueous (less dense) and contains the

water-soluble salt formed from the organic acid

CLEANING UP THE ORGANIC LAYER

WASH WITH D.I. WATER ⇒ REMOVE WATER SOLUBLEIMPURITIES FROM THE ORGANIC LAYER

UPPER LAYER: D.I. H2OLOWER LAYER: DCM

DISCARD

CONTAINS THE ESTER ONLY

WASH WITH NaCl SOLUTION ⇒ REMOVE THE BULK OFTHE WATER FROM THE ORGANIC LAYER

UPPER LAYER: AQ NaClLOWER LAYER: DCM

DISCARD

CONTAINS THE ESTER ONLY

Drying the Organic Layer

• Washing the organic (dichloromethane) layer with saturated aqueous NaCl (brine) will remove the majority of the water that had previously been introduced to the organic layer.

• After gently shaking the separatory funnel and allowing the two layers to separate:

– Lower layer is dichloromethane (more dense) and contains the ester

– Upper layer is aqueous (less dense) and contains sodium chloride. This layer will eventually be discarded.

REMOVING THE LAST TRACES OF WATER FROMTHE ORGANIC LAYER

AN ANHYDROUS INORGANICSALT IS USED TO REMOVE THE LAST TRACES OF WATER FROMTHE ORGANIC SOLUTION.

THE SALT BECOMES HYDRATED(I.E. ABSORBS WATER) BUT REMAINS INSOLUBLE IN THE ORGANIC SOLVENT.

THE HYDRATED SALT CAN BEEASILY REMOVED USINGMICRO FILTRATION.

Isolating the Ester - Rotary Evaporation• Rotary evaporation is often used as an alternative to

distillation for the rapid removal of solvent.

• The water bath should be set to about 40 °C, and the house vacuum should be used.

• Rotary evaporation should be complete in approximately 5 minutes. Any residual dichloromethane will evaporate prior to Experiment 7.

Isolating the Organic Base

• Adding 10% NaOH will revert the organic base into its water-insoluble form.

Soluble in water Insoluble in water

• Add NaOH until precipitation occurs, and then add a few milliliters more. Check pH with litmus paper to ensure that pH > 10.

• Collect the organic base by vacuum filtration using the house vacuum as the vacuum source.

Isolating the Organic Acid

• Adding 6M HCl will revert the organic acid into its water-insoluble form.

Soluble in water Insoluble in water

• Add HCl until precipitation occurs, and then add a few milliliters more. Check pH with litmus paper to ensure that pH < 1.

• Collect the organic acid by vacuum filtration using the water aspirator as the vacuum source.

• Throughout this experiment, the organic layer (dichloromethane) is the bottom layer in the separatory funnel as it is more dense than water.

• During the isolation of the acid and base, do not test pH until after a precipitate forms and remains.

Completing Experiment 6

• Submit three vials before leaving:– Crude acid– Crude base– Crude ester

REMEMBER:

• Experiment 7 will focus on the purification of the acid and base, and on the characterization of the acid, base, and neutral using IR spectroscopy and melting point analysis.

• Ensure that you re-weigh the ester before since some small amount of solvent will likely have evaporated over the last week.

Experiment 7

Recrystallization of 4-Chloroaniline

• Set a small portion of crude base aside for melting point analysis.

• Recrystallize from a two-solvent mixture of 2-propanol and water. Do not let the solvent temperature exceed 80 oC to avoid oiling out.

• The compound is an irritant to skin, eyes, and respiratory tract.

Recrystallization of 2-Methylbenzoic Acid

• Set a small portion of crude acid aside for melting point analysis.

• Recrystallize from boiling water in a 125 mL flask. You might expect to use approximately 10-20 mL of water.

• The compound is also an irritant to skin, eyes, and respiratory tract.

Melting Points and IR

• Record 2 melting point ranges:– Pure Acid– Pure Base

• Record the IR spectrum of the ester. Analyze the spectrum by circling and assigning any peaks outside of the fingerprint region.

• Look for any peaks that might indicate impurity (such as -OH or -NH2).

• Identify the peak near 1200 cm-1 that is characteristic of the C-O bond of the ester.

Completing Experiment 7

• Submit recrystallized acid and base, as well as your ester.

• Calculate percent recoveries based on an initial amount of 0.60 g each of acid, base, and neutral.

• Experiments 6 and 7 will be the subject of a quiz during the next lab session.