Distillation

Solution-Phase Extraction (or Liquid-Liquid Extraction)

Green tea leaves house an abundance of really beneficial organic chemicals: catechins (that have antioxidant, anti-cancer, anti-hyperglycemic effects, etc.), theanine (that promotes relaxation and lowers blood pressure), vitamins, caffeine!, gamma-aminobutyric acid (that lowers blood pressure), and many more. Brewing a cup of tea by adding hot water to the tea leaves is a form of extraction. All the chemicals originally inside the leaves (we’ll call it the solid phase) move into the hot water (the liquid phase), which we end up drinking. This happens because the chemicals in green tea leaves are polar, and polar molecules like to interact with other polar substances like water. This is an example of solid-liquid extraction. Can you think of other examples of extractions that occur in our daily routines?

In the organic chemistry lab, you will frequently need to perform liquid-liquid extractions (aka solution-phase extraction), which involves using two immiscible liquids/solvents to partition the solute(s). We do this to extract chemicals from one liquid phase into another. Two liquids are immiscible when they have very different polarities such as in oil, a non-polar organic compound, and water, a polar inorganic compound. When oil is poured onto water, it remains on top, thereby forming two distinct phases or layers. Vegetable oil has a lower density (ρ = 0.91 g/mL) than water (ρ = 1.0 g/mL). Because oil is lighter and has a lower density, it floats on top while water settles underneath. In this case, the top organic oil layer is referred to as the organic layer and the bottom as the aqueous (water) layer. 

Note that some organic solvents such as dichloromethane (DCM) and chloroform (CHCl₃) have densities greater than water (ρ = 1.33 g/mL for DCM and ρ = 1.49 g/mL for CHCl₃). Therefore, these solvents would settle below an aqueous layer during a liquid-liquid extraction. Always use your knowledge of relative densities to unequivocally determine which layer is which in a liquid-liquid extraction.

Practically speaking, one of the solvents will invariably be water-based. Because the two solvents must be insoluble for liquid-liquid extractions to be successful, the second solvent must not be miscible with water. Some common organic solvents that are immiscible with water and are commonly used include:

How to Perform a Liquid-Liquid Extraction (Generally)

A separatory funnel is used to perform liquid-liquid extractions. 

Tips: Always remember to close the stopcock before pouring in any solutions! Always remember to have something underneath the spout of the separatory funnel in case (when) you accidentally forget to close the stopcock! Failure to do these means you may get to enjoy experiencing a bench-top extraction.

The general steps to a liquid-liquid extraction with a clean separatory funnel are:

1. Pour in the organic (or aqueous) layer that contains the desired organic compound(s) through the top of the separatory funnel.  
    

2. Add the aqueous (or organic) solvent that will be used to extract (or wash) the organic compound(s). This will result in a biphasic mixture (Before extraction, Figure C).  
    

3. Cap, shake, and vent (see procedure for details) carefully!

   1. Molecules traverse between two phases only at the interface. Vigorous shaking maximizes the surface area between the two phases so that the organic compound(s) can move from one phase to the other.  
       

4. Most of the organic compound(s) will end up in the organic layer (after extraction). However, some will be stuck behind in the aqueous layer still.

   1. This is why we always perform successive extractions (usually in triplicate). See below for some basic math to support this (no, you do not need to know the math, it's just to illustrate a point).  
       

5. Collect the aqueous and organic layers into two separate flasks. The extract is the layer that you used to pull the organic compound of interest into. This is usually the organic layer for a neutrally changed organic compound, but it can be the aqueous layer if we are exploiting protonation states and are working with a charged compound (charged organic compounds are usually salts, and salts tend to prefer aqueous layers).

   1. The organic layer is shown at the top in Figure C, but this is not always the case! Be sure to know when the organic layer floats on top or sinks to the bottom.  
       

6. Return the layer that you are extracting from to the separatory funnel & repeat Steps 2–5 two more times using fresh extracting solvent. 

   1. Note, if you are extracting with an organic solvent, then you put the aqueous layer back in and extract with a second portion of fresh organic solvent (and vice versa). The reason for this is that you are successively pulling the target compound(s) out of the old aqueous layer, and into the new organic layers.

   2. The first extraction usually does a good job of getting about 70–90% of the desired compound(s) out if performed properly. The subsequent 2 extractions ensure that everything is extracted. There is usually no need to perform more than three extractions.

Reinforcing the Concept of Successive Extractions

Successive extractions is a concept that confuses a lot of students, but it's ultimately very straightforward. We will reinforce this concept now so you can confidently perform this technique in lab without being confused.

Whenever you do a liquid-liquid extraction you:

• Add one liquid phase to another (you should see two phases), mix, and let them separate out.
   

• Then take both layers out of the separatory funnel.

  • You can either take the bottom layer out of the bottom and into one Erlenmeyer flask, and the top layer can then be taken out the bottom into a different Erlenmeyer flask, OR...

  • You can  take the bottom layer out of the bottom and into one Erlenmeyer flask, and the top layer can then be taken out the top into a different Erlenmeyer flask

  • Some people will say it matters which method you choose, and yes, it sometimes does. However, let's keep it simple in our course. You can choose whichever method is more natural to you. So long as you are doing everything correctly, we are fine either way.
     

• Then place the layer you are extracting from back into the separatory funnel.

  • You should see two immiscible phases again.

  • If only one huge layer forms, you just added organic to organic or aqueous to aqueous! Talk to your TA on how to correct this! The solution is usually simple, but it is somewhat situation-dependent, so we cannot offer a "one-size-fits-all" solution here.

• Then add a new portion of the liquid phase you are pulling the analyte(s) into, in the separatory funnel.

• Repeat the above steps, starting with the "mix and letting them separate out." This is what makes it successive, because you are adding a portion of extracting phase, taking it out, adding another portion, taking it out, adding another portion, taking it out, etc. 

  • Notice that you do not add the extracting phase portion, then immediately add another extracting phase portion before taking the first out. That's just one massive extraction and is not successive extractions.
     

• Once you are done, collect all the organic phases together, and collect all the aqueous phases together separately. DO NOT discard your aqueous phases until the END of your experiment.

Some Math to Support the Rationale Behind Successive Extractions

Let's say your extracting solvent can pull 75% of the compound(s) of interest out of the original layer. That means you left 25% behind.

Okay, so you do the extraction a second time. This means you pull another 75% of the leftover 25% (the math is 0.75 x 25% = 18.75%). In other words, you increased your extracted recovery by pulling out another 18.75% of the total amount of compound(s) of interest. This means you now have 75% + 18.75% = 93.75% extracted in total. That's far more than just 75%.

Let's really up our recovery to max out this process. We perform the extraction a third time on the aqueous layer that now contains only 6.25% of the total compound(s) of interest (the math is 25% - 18.75% = 6.25% ... or ... 100% - 93.75% = 6.25%). Of that 6.25%, we can extract out another 75% of the compound(s) of interest. The math is 0.75 x 6.25% which comes to about 4.69%. Adding that to our grand total, we have now extracted 93.75% + 4.69% = 98.44%. Our significant figures are wildly incorrect here, so let's correct that to two sig-figs.

We have extracted 98% of our total available compound(s) of interest over three successive extractions, compared to one big one. There's a lot more complexity to the math behind all this than what is presented here, and it is not something you particularly need to know for this course. All you need to know is that three extractions get you more material than one, and there is math that supports it.

The Basic Ideas of Liquid-Liquid Extraction

• To extract an organic compound out of water, use an organic solvent that will not mix with water. Upon mixing, the organic compound will find its way from water into the organic solvent, which will be collected.

• To extract a water-soluble compound out of an organic solvent, use water. Water will not be miscible with the organic solvent (usually, there are some exceptions stated below). Upon mixing, the water-soluble compound will find its way from the organic solvent into the aqueous layer, which will be collected.

In either case, the stuff that is collected is called the extract.

• Organic solvents like methanol, ethanol, and acetone are completely miscible with water. Therefore, they cannot be used for liquid-liquid extractions.

There is a process called washing that is related to extraction. Some junior practitioners confuse these terms and mistakenly use them interchangeably, and they actually mean different things. Extraction is when you end up obtaining something desirable by pulling it from an old layer to a new layer. Washing is when you end up getting rid of something undesirable from the layer of interest. For example, consider an organic layer that contains the desired organic chemical as well as some acetic acid that we don’t want. Acetic acid is an organic acid: CH₃COOH. We can “wash” the organic layer with a basic aqueous solution such as 1 M aq. NaOH. Recall what happens when an acid reacts with a suitably strong base: The acetic acid (weak acid) in the organic layer reacts with sodium hydroxide (moderately strong base) to form sodium acetate and water (Eq. 1). Sodium acetate (CH3COO-Na+) is an ionic salt and would much prefer to enter the aqueous layer (1 M NaOH). The newly formed H₂O will also enter the aqueous layer. The 1 M NaOH solution is effectively used to “wash” the organic layer. The end result is a more clean organic layer that is purified of the undesirable acetic acid. In this case, we discard the “wash” at the end of our experiment. Because the acid is being remove by a base, this can also be referred to as an acid-base wash.