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Acid/Base extraction a comedy in three acts. * 1
    #2377196 - 02/25/04 10:37 PM (14 years, 2 months ago)

Acid/Base Extraction

I thought I?d create this thread as a informational tool behind the mechanisms and principles/theory of acid/base extraction and in an attempt for people to better understand what is going on and perhaps can think outside the box of simply reading a tek.

Pretty much if you?ve taken or will take a college level organich chemistry course with lab you will most likely perform and learn about acid/base extractions. Until then or if you haven?t had the priviledge of the wringer that is organic chemistry lab then this information is for you.

Act I

What is Acid/Base Extraction?

An acid/base extraction is just one of basically three extractions, the other two being a liquid/liquid and a solid/liquid (teabag with water). Acid/base extraction involves carrying out simple acid/base reactions to separate strong organic acids, weak organic acids, neutral organic compounds, and basic organic substances. The general principle is to make insoluble compounds soluble in water through acid/base reactions. This allows one to control which compounds are pulled from the solvent layer and migrate to the aqeuos layer.

Lets consider what is meant by strong and weak.

Acid strength is gauged in a logarithmic scale. It is found by taking the log of a dissociation constant for that compound. Values range into the Negatives and are very strong acids like HF and H2SO4. And go as high and higher than 50, which is not very acidic at all, most straight chain hydrocarbons have pKa?s at or above 50.

Base strength: pKb is found similarly as acid strength but in this case it?s the dissociation constant of the base instead of acid.
  • A strong organic acid in this case has a pKa of about 4 - 5. This is about as strong as vinegar.
  • Weak organic acids have a pKa of ~10 for our purposes. This is about as strong an acid as amphetamines are or book glue (phenol).
  • Neutral compounds are usually left in the solvent and not reacted. It is recovered with evaporation of the solvent. No pKa can be given, but neutral generally implies pH of 7 when in solution. Though this is not a good way to understand it, it?s good enough.
  • Basic compounds generally have high pKa?s or low pKb?s. A basic compound has a pKb of ~9 - 10. Thus a pKb is about as strongly basic at 9 - 10 as an acid is strong at pKa 9 - 10.

So what are my alkaloids?

Alkaloids are by the nature of their name, alkaline. That is basic. So we?re persuing the extraction of basic compounds.

Act II

Mechanisms behind A/B: Reactions
For the sake of simplicity, I will use Benzene derivatives to keep things stream-lined.

Strong Organic Acids:
  • To a strong acid, weak base is added.

Weak base will be in this example NaHCO3

At this point the molecule is now soluble in water because it has become an ionic sodium "salt" so to speak. Thus it has migrated out of the organic solvent and into the aqueous layer.

  • The compound is regenerated by adding strong acid, checking pH until base is neutralized (pH = 7).

It is at this point the organic acid is now insoluble in water again. And will precipitate out. When strong acid is used to regenerate it.

Weak organic acids:

  • To weak organic acid strong base is added

  • To regenerate the weak acid strong acid is added until strong base is neutralized (pH = 7) check pH for neutralization.

Weak organic bases:
  • To organic base strong acid is added

  • To regenerate the base strong base is used


What reagents will I need?

In this reaction we?ll need the chemicals used as examples above.

Listed here they are
Benzoic Acid
t-butylphenol (since phenol is a liquid at room temp and t-butylphenol isn?t)

Dichloromethane, Naptha, Ether. liquid Organic solvents like those.

HCl, muriatic acid

NaOH lye, or sodium hydroxide pellets, use as molar solution 1M. Add water to pellets and not pellets to water to make solution.

NaHCO3 Sodium Hydrogen Carbonate, or Sodiumb Bicarb(onate), baking soda. Use as saturated solution.

So what?s the procedure?

To one container grind up and finely powder material to be extracted

In this case the entire mixture of benzoic acid, phenol, aniline, and a neutral compound we?ll call biphenyl is dissolved in dichloromethane or some other organic solvent. The entire mixture is dissolved in as little solvent as possible but enough so it?s all dissolved.

You?ll need to know your solvent density relative to water to know which layers are which.

1) Since we?re looking to extract the alkaloids we?ll go for those first.

The aniline represents the alkloids since both are basic. Concentrated HCl (say 3M) is added dropwise to the solution, mix solvent and aqeuos layer for several minutes. allow to separate and check pH of aqeous layer, once acid pH is reacted all the base is reacted it can react for now.

2) Separate aqeuos layer (know what the density of your solvents are) into its own container and label it alkaloid.

3) Repeast Step 1 and separate aqeuos layer into container labeled alkaloid. Save and keep.

4) In the aqeuos layer we have anilinium chloride. Since this is our representative of basic alkaloids we won?t move further in the extractions to separate the weak and strong from the neutral compound still left int he organic solvent as we would not need to go this far if we used plant material.
So we won?t.

If you?re interested in separating the other two parts it involves repeating step 1 with the proper reagents and separating.

5) To the aqeuos layer add concentrated HCl to the aqeuos solution dropwise and mix watch for precipitates.

6) Check pH until pH is neutralized (pH = 7) once no longer basic place vessel in ice bath.

7) Precipitates should be filtered, and oily layers that form ontop of the aqeuos layer should be collected also, with vacuum aparatus or through something to collect the precipitate mixture

8) Recrystallize if you wish. (Perhaps I?ll make a thread for good recrystallization techniques.)

Activated charcoal to decolorize solution?

Large grained activated charcoal powder can be used if you find it necessary to decolorize solution, though it?s not neccessary, decolorize solution while alkaloids are still in solution and filter out charcoal after it?s absorbed some color. Then proceed to regenerate the alkaloids.

So now what?

It?s best to start with finely powdering your material as much as possible and use solvent to dissolve in warm solvent and filter off solids and take up A/B extraction at step 1.

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Re: Acid/Base extraction a comedy in three acts. [Re: neuro]
    #2377527 - 02/26/04 12:47 AM (14 years, 2 months ago)

Simply spectacular job Nuero!

That one goes in the archives.  :thumbup:

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Re: Acid/Base extraction a comedy in three acts. [Re: neuro]
    #2379723 - 02/26/04 03:02 PM (14 years, 2 months ago)

Ya! That definetly clears up some Qs!!!


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Registered: 07/14/03
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Re: Acid/Base extraction a comedy in three acts. [Re: Jackal]
    #2379753 - 02/26/04 03:11 PM (14 years, 2 months ago)

Yeah great writeup Nuero!

I've always wanted someone to explain the theories behind the extractions in terms that I could actually understand and now you have done it! Thanks man.

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Re: Acid/Base extraction a comedy in three acts. [Re: Jackal]
    #2379756 - 02/26/04 03:12 PM (14 years, 2 months ago)

I'm going to make a flow chart when i have some time so this whole thing can succintly be tallied up by one single jpg.

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