Sunday, May 8, 2016

Extraction Flowchart Example

You may have an entire lab dedicated to separating organic compounds using extraction methods, but you will likely be using extraction in numerous labs throughout the year. The lab dedicated to extraction often has the students separating two or three organic compounds from each other by using only extraction techniques. In this post, I'm going to walk through the flowchart for separating three compounds: 3-methylphenol, 2-ethyl-3-methylbenzoic acid, and ethylbenzene.

Let's assume we have these molecules in a single solution of dichloromethane. Our goal is to separate them by extraction.

Eachof these molecules have aromatic rings (hydrophobic) but two of them also have ionizable groups (OH for 3-methylphenol and COOH for 2-ethyl-3-methylbenzoic acid). By using acid/base reactions, you can make these molecules charged/ionized which makes them more polar and therefore able to partition to the aqueous phase.

The following is a flowchart designed to separate these three molecules. In this example, the steps are already predetermined, the task is for the student to identify the structures of the molecules at each step:

In the first step, treating with a weak base (sodium bicarbonate) will convert the COOH to a carboxylate salt (COO- Na+) which is more water-soluble:

RCOOH  +  NaHCO3    <---->    RCOO-  +   Na+    +     H2CO3

By doing so, the benzoic acid molecule would become negatively charged (this makes it very polar), and because of this, it becomes much more soluble in water and will partition to the aqueous phase.

The 3-methylphenol has an OH group, but alcohols typically require strong base to deprotonate the H and become charged. Therefore, treatment with sodium bicarbonate (weak base) will have no effect on 3-methylphenol, and it will remain in the dichloromethane phase (organic layer).

Ethylbenzene has no ionizable groups, so treatment with any base or acid will have no effect on ethylbenzene. 

After the first step, the flowchart would now look like this:

In your aqueous layer, you would have your 2-ethyl-3-methyl benzoic acid molecule, but as the carboxylate ion form. In order to get it back to its carboxylic acid form, you just have to add acid:

RCOO-  +   Na+   +   HCl   <----->    RCOOH  +  NaCl

In your organic layer, you would still have a mixture of 3-methylphenol and ethylbenzene. Since 3-methylphenol has an ionizable OH group, you can treat this with a strong base:

ROH  +  NaOH   <----->   RO-   +   Na+    +   H2O

By doing so, the molecule becomes negatively charged (polar) which makes it able to partition to the aqueous phase. Ethylbenzene is not affected by NaOH, so would remain hydrophobic and remain in the organic layer. 

The flowchart would now look like this:

At this point, we have successfully isolated 2-ethyl-3-methylbenzoic acid as well as ethylbenzene.

In the last step, we just have to acidify the ionic form of 3-methylphenol. Like before, we just need to treat this with acid:

RO-  +  Na+   +   HCl    <------>     ROH    +    NaCl

The flowchart is now complete:

We started out with a mixture of three different organic molecules in a single solution of dichloromethane. There are numerous separation techniques that organic chemists use to isolate their molecules of interest, but extraction is one of the simplest approaches because it is generally quick and only requires a separatory funnel. 

By taking advantage of the ionizable groups of the molecules here, we were easily able to manipulate their water-solubility by treating with acid/bases accordingly. Ultimately, we were successfully able to design a extraction protocol to isolate all three components!

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