A/B Extraction Theory Basics: Isolating Compounds

In summary: Acid-base reactions are not always effective in completely removing impurities, so additional purification methods may be necessary. In summary, acid-base reactions are only the first step in purifying a compound, and the relative strengths of the acid or base and the pKa of the compound play a crucial role in determining which compounds will react and be converted into salts. Additional purification methods, such as recrystallization, may also be necessary to fully isolate the desired compound.
  • #1
Mr_Bojingles
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I understand the basics of acid base reactions but I don't understand how you determine which compounds in a mixture will react to the acid or base and convert into a salt. Does there have to be a large pH difference? For example say the compound I want to isolate has a pH of 11 but two other impurities in there have pH's of 6 and 8. If I was to add an acid with a pH of 2 I know that will convert the compound into a salt but how do I know if its going to convert the 2 impurities into a salt or not.

Is it only compounds with a pH in the range of 4-8 that will remain unchanged by the acid or base? I'm having trouble understanding how A/B extractions can be used effectively to isolate compounds seeing as all chemicals are acids and bases just some are stronger than others.

Also what steps are involved in A/B extractions? Let's say my compound has a pH of 11 and its called lembodine. To begin with I dissolve the mixture into a non polar solvent such as diethyl ether and do a wash with a polar solvent such acetone to lose all the inorganic compounds. Next I add conc. sulfuric acid and convert my compound into lembodine sulfate. Assuming the sulfuric acid has a pH of about 2 how do I know which impurities it will protonate and which it will leave untouched so I can wash out?

Anyhow let's say I add acetone to the acidified mixture so my lembodine sulfate will dissolve in it then I filter out the diethyl ether layer and add more ether to make sure I wash out any organic impurities which were not converted to salts. After this step I'm lost. Everyone says you should rebasify the mixture and do another wash with a polar solvent to further remove polar impurities. I don't understand this. Seeing as I already did a wash with a polar solvent in the beginning what's the point of doing it a second time?

Anyway following everyones advice let's say I add some conc. sodium hydroxide to basify the mixture which will deprotonate my compound and convert it back into lembodine base. After I do another acetone wash let's say I'm left with some impurities which converted back into organic bases with my lembodine. Is there now no way I can remove these basic impurities with acid base extractions alone and should I resort to another method?

Are acid base reactions only really a first step to purifying a compound which should be followed by methods such as recrystallization to insure the compound is properly isolated?
 
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  • #2
Yes, acid-base reactions are only a first step to purifying a compound. In order to determine which compounds in a mixture will react to the acid or base and convert into a salt, you must consider the relative strengths of the acid or base and the pKa of the compound. Generally, if the acid or base is much stronger than the pKa of the compound, then the compound will be protonated or deprotonated as appropriate in an acid-base reaction. The pH difference between the acid or base and the pKa of the compound should be at least 3-4 orders of magnitude. In terms of steps involved in acid-base extractions, you must first dissolve the mixture into a non-polar solvent such as diethyl ether to separate out the inorganic compounds. Then you add a strong acid such as concentrated sulfuric acid to protonate the desired compound and convert it into its salt form. After that, you will need to rebasify the mixture with a strong base such as sodium hydroxide to deprotonate the compound and convert it back into its base form. Then you will add a polar solvent, such as acetone, to wash out the impurities that were not converted to salts. Finally, you may need to do a recrystallization process to completely isolate the compound.
 
  • #3


I can understand your confusion about the A/B extraction theory basics. Let me try to clarify some of your questions.

Firstly, the determination of which compounds will react to the acid or base and convert into a salt depends on their chemical properties. Compounds with functional groups that are basic in nature, such as amines, will react with acids to form salts. Similarly, compounds with acidic functional groups, such as carboxylic acids, will react with bases to form salts. The pH difference is not the only factor, but it does play a role in the strength of the acid or base and the likelihood of a reaction occurring.

In your example, if the compound you want to isolate has a pH of 11 and the impurities have pHs of 6 and 8, it is possible that the acid with a pH of 2 will convert all three compounds into salts. This is because the pH of the acid is significantly lower than the pH of the compound and impurities, making it a stronger acid and more likely to react.

In terms of the range of pH for compounds to remain unchanged by the acid or base, it is not limited to 4-8. It depends on the specific functional groups present in the compounds. Some may remain unchanged at a pH of 4, while others may react at a pH of 8. It is important to consider the chemical properties of each compound in the mixture.

A/B extractions involve several steps, as you mentioned. The first step is to dissolve the mixture in a non-polar solvent and wash with a polar solvent to remove inorganic compounds. Then, an acid or base is added to convert the desired compound into a salt. The mixture is then washed again with a polar solvent to remove any remaining impurities. Rebasifying the mixture and washing with a polar solvent again helps to further remove any polar impurities that may have been missed in the first wash.

After the final wash, the compound is usually converted back to its original form by adding a strong base or acid, depending on the initial conversion. In your example, adding conc. sodium hydroxide will deprotonate the lembodine sulfate and convert it back to lembodine base. This step is important to remove any remaining impurities that may have converted to salts during the initial acid/base reaction.

Acid/base reactions can be an effective method of isolating compounds, but they may not always
 

1. What is the purpose of A/B extraction theory?

The purpose of A/B extraction theory is to isolate specific compounds from a mixture or solution. This is done by taking advantage of the different solubility levels of various compounds in different solvents.

2. How does A/B extraction theory work?

A/B extraction theory works by using two solvents, usually water and an organic solvent such as ether or chloroform. The mixture containing the desired compound is added to the two solvents and shaken to allow the compounds to transfer between the two layers. The compound of interest will dissolve into one of the solvents, while the other compounds remain in the other solvent.

3. What are the key factors that affect A/B extraction?

The key factors that affect A/B extraction include the solvents used, the pH of the solution, and the physical properties of the compounds being extracted. The choice of solvents and their ratio can greatly impact the efficiency of the extraction process, while the pH can affect the solubility of the compounds in the solvents.

4. What are some common techniques used in A/B extraction?

Some common techniques used in A/B extraction include liquid-liquid extraction, solid-phase extraction, and partitioning. Liquid-liquid extraction involves using two immiscible liquids, while solid-phase extraction uses a solid material to extract compounds from a solution. Partitioning involves using a single solvent to extract compounds from a mixture.

5. What are some applications of A/B extraction theory?

A/B extraction theory has many applications in various fields such as chemistry, biochemistry, and pharmaceuticals. It is commonly used to isolate and purify compounds for further analysis or use in experiments. It is also used in the production of drugs, natural products, and essential oils.

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