28 Nov 2019

I'm currently taking ochem part 2. I need to perform an acid-base extraction to separate out a mixture of solid organic compounds consisting of 50% 2-chlorobenzoic acid; 40% methyl 4-aminobenzoate; 10% 1,4-dibromobenzene. I need to create a procedure to first separate the compounds. I also need to recrystallize each compound after separating the compounds; therefore, I need to know which solvent to use for each compound. I also need to prove that I separated my compounds successfully using TLC (therefore I need to know which solvent to use for TLC). I need to do this as a micro-scale procedure (therefore, I will not be given large quantities of reagants). I'm given the reagants methanol, ethanol, methylene chloride, hexane, 1M NaOH, 6M NaOH, 1M HCl, 6M HCl, Anhydrous sodium sulfate, saturated sodium chloride, acetone, sodium bicarbonate, diethyl ether. I believe we will be given about 2 grams of the solid organic compound mixture.

Note: I want to run this acid-based extraction without using the separation funnel. Can I use a centrifuge tube instead?

I have a rough procedure of how to carry out the acid-base extraction that I've posted below, please look over my rough procedure of the acid-based extraction portion and tell me if the amounts of reagants look okay and if the process is okay. I also have accompanied this with a couple of follow up questions in regards to the acid-based extraction, recrystallization, and running the TLC all accompanied with subheadings.


Put 0.090g of solid organic mixture into a large test tube. Add 0.5 mL of diethyl ether at a time and stir until all dissolves. If needed, slowly add more diethyl ether solvent. Use minimal solvent.

Extracting methyl 4-aminobenzoate

1. Add this mixture in large test tube into a glass centrifuge tube (labeled “working reaction”).

2. Add 1mL 1.0 M HCl to the centrifuge tube. Shake gently and vent. Do this until layers separate. If needed, centrifuge.

Diethyl ether density: 0.713 g/cm3

Water’s density: 1.00g/cm3 (aqueous layer)

Because diethyl ether has a lower density, it will be on the top layer and consist of the 1,4-dibromobenzene and the 2-chlorobenzoic acid. The bottom aqueous layer will consist of the conjugate acid of the organic base, methyl 4-aminobenzoate because H3O+ protonates the amine of the base to produce an ammonium salt, which is ionic and therefore aqueous in solution. (DRAW THIS OUT)

3. Extract out lower aqueous layer (consisting of the conjugate acid i.e. the ammonium salt, of methyl 4-aminobenzoate) into labeled test tube (ammonium salt).

Note: It is okay to extract a little more out from the bottom bleeding into the organic layer; throw that into the waste to ensure that all that is left are the other 2 organic compounds in diethyl ether. Parafilm.

4. Into the test tube with the ammonium salt, add 6.0 M NaOH DROP BY DROP until the solution returns to become BASIC, this has reproduced the organic base, methyl 4-aminobenzoate. Check with pH paper.

5. Collect the crude methyl 4-aminobenzoate precipitate by filtration using Buchner funnel.

6. Recrystallize from HOT WATER.

6. Filter the pure methyl 4-aminobenzoate precipitate by filtration using Buchner funnel.

Extracting 2-chlorobenzoic acid

1. The “working reaction” centrifuge tube now consists of the solvent (diethyl ether), 1,4-dibromobenzene, and 2-chlorobenzoic acid. Add 1 mL of 1.0 M NaOH. Shake and vent. Centrifuge if necessary. Again, as before, the lower layer is aqueous and the top layer is the organic layer (diethyl ether + 1,4-dibromobenzene)

NOTE: B/c NaOH was added, the 2-chlorobenzoic acid was deprotonated to yield a sodium salt, which produced its conjugate base. The salt is ionic and therefore soluble in water.

2. Extract out the bottom layer with Pasteur pipette (contains conjugate base of 2-chlorobenzoic acid) into test tube labeled sodium salt.

Note: It is okay to extract a little more out from the bottom bleeding into organic layer; throw that out into the waste to ensure that all that is left is 1,4-dibromobenzene in diethyl ether. Parafilm.

3. Into the test tube with the sodium salt, add 6.0 M HCl DROP BY DROP until the solution returns to become ACIDIC. This has now reproduced the organic acid, 2-chlorobenzoic acid.

4. Recrystallize with what solvent?

Extracting 1,4-dibromobenzene

1. The “working reaction” centrifuge tube should now only contain diethyl ether, 1,4-dibromobenzene with some potential water. Add sodium sulfate to dry out the remaining water. Extract the solution (leaving behind the sodium sulfate ppt) into a test tube labeled 1,4-dibromobenzene.

2. Place the test tube labeled “1,4-dibromobenzene” in a hot water bath and boil to that of the solvent (diethyl ether) at 40C.

3. Diethyl ether has a low boiling point of 34.6 C whereas 1,4-dibromobenzene has a high boiling point of 220.4C.

4. Therefore, the remaining organic layer in the “working reaction” centrifuge tube now only containing 1,4-dibromobenzene and diethyl ether can be separated by evaporating diethyl ether.

5. Recrystalize with what solvent?

Acid-Based Extraction Questions:

Q1: Is diethyl ether a good solvent for initially dissolving all three solvents for the acid-base extraction? And if so, how much solvent do I need to add? I thought minimal enough to dissolve the organic comopunds?

Q2: Are my rough amounts of reagants used correctly for the acid-base extraction?

Q3: Should I add anhydrous sodium sulfate to my last step of acid-base extraction which consists of the diethyl ether and 1,40-dibromobenzene because there might be some aqueous layer left?

Q4: Can I perform my acid-base extractions with a centrifuge tube instead of a separation funnel?

Q5: Should I start off with more grams of my organic mixture when I begin? Is 0.090 grams too small?

Recrystallization Question

Q6: Which solvents for each isolated crude compound should I use for recrystallization? Do I need to run a test for a suitable solvent? If so, which solvents are good to start with. I was thinking water and methanol.

a) I was thinking of using hot water as the hot solvent for methyl 4-aminobenzoate and for 2-chlorobenzoic acid. Is this correct to do so?

b) Which solvent should I also use to recrystallize 1,4-dibromobenzene?

TLC Question

Q7: Which solvent should I use to run the TLC for my organic compounds. After I know which solvent I will be using, where should I expect each compound to travel relative to the other organic compounds? (i.e. closest to the solvent line, middle of the solvent line, furthest from solvent line).

Miscellaneous Question

Q8: What's the point of telling me the percentages of the compounds in the mixture? Does that affect how much reagant I should be using during the acid-based extraction?

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Jamar Ferry
Jamar FerryLv2
5 Jun 2019

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