Part IV: Techniques and Theory

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Part IV: Techniques and Theory

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Part IV: Techniques and Theory

A. Extraction (See also, Lehman pp. 70-77 and pp. 116-121)

Sample Extraction Procedure for Separation of a Mixture of an Acid, a Base and a Neutral

Compound by Chemically Active Extraction

This procedure is designed to separate a ternary mixture of organic compounds using acid and base

extractions. This example is provided to give you some guidance for both acidic and basic

extraction procedures.

Procedure:

Be sure to record all pH measurements, all weights, mp's and R f values of isolated products in

your Results Section. Be sure compounds are dry before weighing or taking m.p.'s.

1. Dissolution of Sample.

Weigh out, to the nearest thousandth of a gram, approximately 2 g of the mixture. Dissolve

the mixture in ~50 mL of dichloromethane (CH 2 Cl 2 ). If any solid doesn't dissolve, add an

additional 25 mL of CH 2 Cl 2 . If undissolved solid still remains, filter (by gravity) the solution

directly into a separatory funnel. Otherwise, pour the solution into the separatory funnel.

2. Acid Extraction.

Extract the CH 2 Cl 2 solution two times with 25 mL aliquots of 1.0 M HCl. Combine the

aqueous layers from both extractions in a labeled 250 ml beaker and test the pH of the

aqueous solution. It should be as acidic as the original 1.0 M HCl solution. If it is not,

extract the CH 2 Cl 2 solution again with 25 mL of 1.0 M HCl.

3. Basic Extraction.

Extract the CH 2 Cl 2 solution resulting from step 2, two times with 25 mL aliquots of 1.0 M

NaOH. Combine the aqueous layers in a labeled 250 mL beaker and test the pH of the

aqueous solution. It should be as basic as the original 1.0 M NaOH solution. If it is not,

extract the CH 2 Cl 2 solution again with 25 mL of 1.0 M NaOH.

4. Recovery of the Compounds.

CH 2 Cl 2 solution:

Dry the CH 2 Cl 2 solution resulting from step (3.) with anhydrous Na 2 SO 4 . Let this

mixture stand for about 5 min., swirling it frequently. Remove the drying agent by

gravity filtering the solution into a 125 ml Erlenmeyer flask, add a boiling stone and

evaporate the solvent in a hot water bath in a hood. Transfer the residue to a watch glass

and allow it to dry.

Acid Extracts:

While the CH 2 Cl 2 is evaporating, cool the acid solution from step (2.) in an ice bath and

then make the solution basic by slowly adding 6 N NaOH until the solution tests strongly

basic with pH paper. A precipitate should form during the addition of NaOH. Collect the

solid by vacuum filtration and wash the solid on the filter with cold water. Transfer the

compound to a large filter paper and allow it to dry.

Base Extracts:

Cool the basic solution from step (3.) in an ice bath and then make the solution acidic by

slowly adding 6 M HCl and stirring until it tests strongly acidic with pH paper. Collect

the precipitated solid by vacuum filtration and wash the solid on the filter with cold

water. Transfer the compound to a large filter paper and allow it to dry (see section [d.],

below).


V A. Stereoisomers

Drying Procedures:

The compound recovered from the CH 2 Cl 2 solution should be crushed to small particle

size and allowed to dry for a total of about 1 hour. It will dry quickly since it was

isolated from a low-boiling solvent.

The compounds recovered from the acid and base extracts should be allowed to air dry in

your desk until the next period. These compounds were isolated from water and water

evaporates slowly from these polar compounds.

5. Sample Characterization and Submission:

Determine the mass of the compound recovered from the CH 2 Cl 2 solution. Verify the

identity of the recovered compound by TLC analysis vs. authentic samples of the original

components using an appropriate developing solvent system. Finally, determine the mp of

each recovered compound side-by-side with an authentic sample of the compound. Record

your data in your Results Section, place your sample in an appropriately labeled vial (See Lab

Manual Part II. p. 15), and seal it tightly. Submit your sample according to your instructor’s

directions.

Next Lab Period:

After the compounds you recovered from the aqueous solutions have had sufficient time to

dry completely, determine their masses and verify their identity by mp and TLC analysis vs.

authentic samples, label, and submit the samples as with that recovered from CH 2 Cl 2 .

Sample Flow Diagram

Flow Diagram for Separation of the Mixture of an Acid, a Base and a Neutral Compound

described in the procedure on B-1. (See also, Lehman pp. 289-290)

Abbreviations

A-H = acid N = neutral compound

B: = base DCM = dichloromethane

precipitate

B:

-vacuum filter

-wash w/ cold water

-air dry

B:

A-H

A-H

Original Organic Phase

A-H, B:, N, DCM

Extract w/ 1M HCl

aqueous

organic

+

A-H, N

B-H

DCM

+ -

H 3 O , Cl

H 2 O (trace)

-cool

Extract w/ 1M NaOH

-basic w/ 6M NaOH

organic

aqueous

aqueous

N

- +

DCM

H 2 O , Na

+ A , Na

-

H 2 O (trace)

- - H 2 O , OH

Cl , OH

-cool

-acidic w/ 6N HCl

precipitate

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-vacuum filter

-wash w/ cold water

-air dry

aqueous

+

H 2 O , H 3 O

- +

Cl , Na

-dry w/

anhy. Na 2 SO 4

organic

solid

DCM

N Na 2 SO 4 . H 2 O

evaporate solvent

N

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