Korry has a Ph.D. in organic chemistry and teaches college chemistry courses.
The focus of this lesson will be on learning the purpose, theory, and procedure behind separating organic acids and bases from one another when they are present together as a mixture.
Acid-Base Extraction Background
Mark just arrived for his weekly organic chemistry lab and today his instructor has informed the class that they will be given a mixture of an organic acid, a base, and a neutral compound. Their task for the day will be to separate and isolate the acid, the base, and the neutral from one another and get them in pure form. How can Mark do this? He's beginning to feel nervous and a bit apprehensive about the experiment because he doesn't really know how he's supposed to separate the different components from the mixture.
The experiment Mark and his classmates will be working on is very common in the organic chemistry lab, and is called an acid-base extraction. The idea is to leverage the acid-base properties of the compounds in the mixture to help separate them from one another. Let's come along with Mark and help him work through his project for the day!
Acid-Base Extraction Purpose
Let us get started by briefly discussing the purpose behind an acid-base extraction. A lot of times organic compounds exist as complex mixtures, and the different components of the mixture must be separated from one another. For instance, when natural products are isolated from plant and marine sources, a lot of times one step in the process involves utilizing an acid-base separation technique. It's a great way to separate and group organic compounds together that contain the same functional groups. In this method, acids, bases, and neutral compounds can be separated and pooled together.
Acid-Base Extraction Theory
The idea behind an acid-base extraction is to utilize the acid-base properties of organic compounds and selectively isolate them from one another when they're present in a mixture. In organic chemistry, acids are known as carboxylic acids and contain the -COOH functional group. Bases contain at least one nitrogen atom and are commonly called amines. When the acid-base mixture is treated with aqueous sodium hydroxide (a base), it will react only with the carboxylic acid in the mixture, forming a water-soluble salt which can be separated later. Neutral organic compounds don't have properties of either acids or bases.
If the mixture is treated with aqueous hydrochloric acid, it will react with the amine in the mixture, again forming a water-soluble salt that can be separated later. So really, we are just leveraging each component's ability to react with either sodium hydroxide or hydrochloric acid to allow us to isolate them from one another.
Acid-Base Extraction Procedure
So how does Mark and the class go about conducting an acid-base extraction experiment? If a few simple steps are followed, anyone can do this with confidence!
Step One: Preparation of the Mixture
The first thing we must do is to dissolve the unknown mixture in some sort of organic solvent. We need an organic solvent that is immiscible (won't mix) with water, so we can separate out the different components. Common solvents that are used for this type of procedure include chloroform, diethyl ether, and dichloromethane. The choice of the organic solvent doesn't really matter, providing it will dissolve all of the components in our mixture (the carboxylic acid, the amine, and the neutral compound).
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Once we get our mixture dissolved in the organic solvent, the first thing to do is to extract the amine (base) out of the mixture. This is done by 'washing' the organic mixture with a dilute acid solution, such as 10% hydrochloric acid (HCl). What happens is the amine accepts a hydrogen ion from the HCl and becomes present as a salt, which is water soluble. The salt gets transferred from the organic solvent into the water layer and then we can separate the two layers. Once we have the water layer containing the amine salt, simply adding dilute sodium hydroxide (NaOH) will cause the amine to go back to its neutral form and precipitate (form a solid) which can then be isolated using a filtration funnel. Now we have the amine from our mixture in pure form!
Step Three: Extraction of the Carboxylic Acid
The next thing to do is to extract the carboxylic acid from the mixture. To do this, we treat the organic solvent containing our mixture (now all that's left is the acid and the neutral) to some 10% NaOH. This will cause the acid to form a water-soluble salt and go into the water layer, similar to what happened with the amine. Once we have the acid in the water layer, we simply add HCl back to the solution and the carboxylic acid will precipitate out as a solid. Collect it with a filtration funnel, and there we have our acid.
Step Four: Isolation of the Neutral Compound
We now have the amine (base) and the carboxylic acid. To finish things off, we simply need to evaporate off the organic solvent to isolate our neutral compound we have still present in the organic solution. Since it's the only component left from the mixture, the solid we get will represent our neutral compound in pure form. And there we have it, our acid, base, and neutral all separated from one another!
In this lesson, we learned that the idea behind an acid-base extraction is to utilize the acid-base properties of organic compounds and selectively isolate them from one another when they're present in a mixture. In organic chemistry, acids are known as carboxylic acids and contain the -COOH functional group. Bases contain at least one nitrogen atom, and are commonly called amines.
The steps involved in any acid-base extraction include:
Preparing the mixture by dissolving it in an appropriate organic solvent that won't mix with water
Extracting the amine (base) by washing with dilute hydrochloric acid
Extracting the carboxylic acid by washing with dilute sodium hydroxide
Evaporation of the organic solvent to provide the neutral compound
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