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Acid-Base Extraction

 

Required Reading

 

Padias: Extraction - pp. 128-138; Filtration - pp. 26-30; Melting Point pp. 49-53; Calculations

 

pp. 13-16 Introduction*

 

Extraction

 

A widely employed method of separating organic compounds from mixtures in which they are

 

found or produced is that of solvent-solvent extraction. Most reactions of organic compounds

 

require extraction at some stage of product purification. In its simplest form, extraction results

 

from the unequal distribution of a solute between two immiscible solvents. The distribution can

 

be expressed quantitatively in terms of the distribution coefficient K, using the equation shown

 

below for compound A distributed between an organic solvent and water. A org

 

K A aq

 

If the solute is completely soluble in the organic solvent and completely insoluble in water, then

 

K will have a value of infinity. This situation is never actually attained, but any value of K other

 

than 1.0 indicates that the solute is more soluble in one of the two solvents. When choosing a

 

solvent system for an extraction, some general principles should be kept in mind.

 

1. The solvents must be immiscible (e.g. oil and water).

 

2. The solvents must have a favorable distribution coefficient for the component to be

 

separated.

 

3. The solvents must not react chemically with the components of the mixture, except in

 

the cases of acid and base extraction, discussed below.

 

4. The solvent must be readily removed from the solute following extraction. Page 1 of 5 Acid-Base Extraction

 

Organic acids and bases can be separated from each other and from neutral compounds by

 

extraction using aqueous solutions of different pH. Most organic acids (e.g., carboxylic acids)

 

are insoluble or slightly soluble in water, but these compounds are highly soluble in dilute

 

aqueous sodium hydroxide because the organic acid reacts with the base to form ionic

 

compounds (salts), which are soluble in water, as shown in equation 1.

 

RCO2H + NaOH RCO2 - Na+ (water soluble salt) + H2O (eq. 1) Thus, the acid may be selectively removed from a mixture by dissolving the mixture in an

 

organic solvent like dichloromethane (CH2Cl2) and then extracting the solution with dilute

 

sodium hydroxide. The organic acid may be recovered from the aqueous solution by

 

acidification (eq. 2), which causes precipitation, followed by filtration.

 

RCO2- Na+ + HCl RCO2H (s) + NaCl (eq. 2) Likewise, organic bases that are insoluble in water may be separated by extraction with dilute

 

hydrochloric acid. These bases (like amines) are soluble in acid due to the formation of a

 

soluble salt, eq. 3.

 

RNH2 + HCl RNH3 + Cl- (water soluble salt) (eq. 3) After the amine has been removed, it may be recovered from the aqueous solution by

 

treatment with base, eq. 4.

 

RNH3+ Cl- + NaOH RNH2 (s) + NaCl (eq. 4) This experiment will utilize the acid-base properties of different compounds in a mixture,

 

whereby the individual components of the mixture can be separated and isolated through an

 

extraction process.

 

After isolation of the components of the mixture, the components will be characterized by

 

infrared spectroscopy (IR) and melting point (mp). Page 2 of 5 Flow Chart for Separation of Acetanilide, Aspirin and Caffeine

 

Unknoiwn Solution

 

Aspirin, Acetanilide, Caffeine

 

3M HCl Organic Solution I Aqueous Solution I 1. 5% NaOH

 

2. CH2Cl2 1. 5% NaOH

 

2. CH2Cl2 Aqueous Solution IV

 

Organic Solution II Organic Solution

 

IV Aqueous Solution II

 

1. 3M HCl

 

2. CH2Cl2 Aqueous Solution III Organic Solution III Page 3 of 5 Procedure

 

Basic Compound Recovery:

 

To 25 mL of assigned unknown methylene chloride (dichloromethane, CH 2Cl2) solution in a 125

 

mL separatory funnel, add 25 mL of 3M HCl. Stopper, shake and vent the separatory funnel

 

several times to thoroughly mix the contents and allow the funnel to rest in an iron ring to allow

 

the layers to separate. Draw off the lower (organic) layer into an Erlenmeyer flask labeled

 

"organic solution I".

 

To the aqueous extract labeled "aqueous solution I", in the separatory funnel add 65 mL of 5%

 

sodium hydroxide (NaOH) slowly with stirring. The reaction can be considered to be complete

 

when the solution is basic (test with Litmus). Add 15 mL of methylene chloride, stopper and

 

swirl the separatory funnel until gas evolution stops. After gas evolution has ceased, shake and

 

vent the separatory funnel several times to thoroughly mix the contents and allow the funnel to

 

rest in an iron ring to allow the layers to separate. Drain off the lower (organic) layer into an

 

Erlenmeyer flask and label "organic solution IV". Reextract the neutralized aqueous layer in

 

the separatory funnel with 5 mL of methylene chloride and add this methylene chloride extract

 

to the "organic solution IV" labeled Erlenmeyer flask. Dry the combined methylene chloride

 

extracts with a small amount of anhydrous sodium sulfate (Na 2SO4) and gravity filter into a

 

weighed Erlenmeyer flask. Allow the methylene chloride to evaporate on a "hot plate" set to

 

"low" inside the hood. Reweigh the Erlenmeyer flask after the methylene chloride has

 

evaporated and obtain a melting point and infrared spectrum on the recovered basic

 

compound.

 

Neutral Compound Recovery

 

Return the methylene chloride solution labeled "organic solution I" to the cleaned separatory

 

funnel and add 20 mL of 5% NaOH. Stopper and swirl the separatory funnel until gas

 

evolution stops. After gas evolution has ceased, shake and vent the separatory funnel several

 

times to thoroughly mix the contents and allow the funnel to rest in an iron ring to allow the

 

layers to separate. Draw off the lower (organic) layer into an Erlenmeyer flask and label

 

"organic solution II". Reextract the aqueous layer in the separatory funnel with 5 mL of

 

methylene chloride and add this methylene chloride extract to the "organic solution II" labeled

 

Erlenmeyer flask. Dry the methylene chloride extracts with a small amount of anhydrous

 

sodium sulfate (Na2SO4) and gravity filter into a weighed Erlenmeyer flask. Allow the methylene

 

chloride to evaporate on a "hot plate" set to "low" inside the hood. Reweigh the Erlenmeyer

 

flask after the methylene chloride has evaporated and obtain a melting point and infrared

 

spectrum on the recovered neutral compound. Page 4 of 5 Acidic Compound Recovery

 

To the basic aqueous layer (aqueous solution II) in the separatory funnel add 10 mL of 3 M HCl

 

(precipitate should form) and stopper and swirl the separatory funnel until gas evolution stops

 

(test for acidity with Litmus). After gas evolution has ceased add 15 mL of methylene chloride,

 

shake and vent the separatory funnel several times to thoroughly mix the contents and allow

 

the funnel to rest in an iron ring to allow the layers to separate. Draw off the lower (organic)

 

layer into an Erlenmeyer flask and label "organic solution III". Reextract the aqueous layer in

 

the separatory funnel with 5 mL of methylene chloride and add this methylene chloride extract

 

to the "organic solution III" labeled Erlenmeyer flask. Dry the methylene chloride extract with a

 

small amount of anhydrous sodium sulfate (Na 2SO4) and gravity filter into a weighed

 

Erlenmeyer flask. Allow the methylene chloride to evaporate on a "hot plate" set to "low" inside

 

the hood. Reweigh the Erlenmeyer flask after the methylene chloride has evaporated and

 

obtain a melting point and infrared spectrum on the recovered acidic compound.

 

* Introductory information from web.centre.edu/muzyka/organic/lab/24_extraction.htm Page 5 of 5

 







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