Limonene

Limonene exists in R-(+) and S-(-) formations. Both have a boiling point of 175.5-176 degrees Celsius, both have a molecular weight of 136.2 grams per mol but they differ in their optical rotation. 

R-(+) has a density of about .8402 g/mL and an optical rotation of 125.6 degrees (note the positive number) while the S-(-) has a density of .8407g/mL and an observed optical rotation of -122.1 degrees (note the negative number)

In this lab we'll talk about Isolating the R-(+) formation of Limonene from orange peels. Note the R-(-) can be found in Caraway seeds.

How you do it?
Take the oranges and peel them. It's best to do this right away to prevent the loss of limonene. Try to remove the white pulp from the peel.

In a blender add the peels and 200- 250 mL of water. After blending put in a 500 mL round bottom flask. Add 4 drops of anti foaming agent...or don't if you would like to have a "fun time". With a Claisen adapter prepare for steam distillation with a 50 mL round bottom flask as the receiver. Marking the 50 mL round bottom flask at the 35 mL level will be helpful so pour some water in it, mark it, empty it and voila.

Boil the mix without letting any solid material bump over into the condenser. Collect 35 mL of the distillate. Note if you use a heating mantel a variac will probably be a good idea as to add an element of control. Also it will prevent the mantel from heating up to quickly/getting to hot and in the process you may actually end up burning your orange peels. Burning the orange peels may result in bad results and a load of gunk that accumulates on the bottom of the round bottom flask which take my word will be annoying and nasty to clean up however a bit of acetone, some soap and some time will help you eventually make it clean again. Also the burned peels may affect your observed optical rotation adding an element of impurity to the sample.

Pour the distillate into a separatory funnel (125 mL size should do). Add 5 g sodium chloride and shake. Next add 10 mL dichloromethane via a conical funnel through the top of the separatory funnel. Gently shake the mix and allow for pressure to be release by opening the stop cock every so often. Let the mix sit and the layers to separate. Make sure the stop cock is closed during this process and the stopper is held is place or else by by sample.

Collect the bottom layer (the organic bottom layer and the top is the top aqueous layer) in a flask. Repeat the extraction with 15mL dichloromethane (fresh) every time. Next dry the extracted solution with anhydrous magnesium sulfide for 10 min.
Meanwhile weight and clean an Erlenmeyer flask for later use. Into this Erlenmeyer with the use of a conical funnel fitted with filter paper and pour the solution. This should take out the anhydrous magnesium sulfide.

Then by any means you want be it nitrogen gas, hot water bath or just allowing it to sit allow the dichloromethane to evaporate.

To do polarimetry (link should take you to wikipedia) obtain 10mL of 95% ethanol. Dissolve Limonene in 3 mL of the ethanol via Pasteur pipette. Transfer to a 10mL volumetric flask and do polarimetry on it. Note calculate the polarimeter with the 95% ethanol as the reference solution. The rotation should tell you about entantiomeric excess if applicable.

Questions:

 What is a steam distillation?  Why is it useful in this isolation experiment?

            Steam distillation is a form of distillation often used to distill heterogeneous mixtures. It allows for the adding of pressures of components in the mixture to overcome that of the atmosphere and boil. This typically results in lower temperatures till boiling is reached especially so when compared to a solution that follows Raoults Law.

            Steam distillation is useful in this experiment because it lowers the temperature necessary to give rise to the boiling of limonene. Typically to achieve this point the boiling point would be so high the orange peels would burn and the charring would contaminate the isolated limonene. The H20 and limonene pressures combine give rise to the lowered boiling point. It’s also possible that higher temperatures would result in decomposition of limonene.

 What was the purpose of the extracting of our collected distillate with dichloromethane and salt-water?

            The salt in the salt water allows for more transfer of limonene to the organic layer. The salt is absorbed in the water and saturates it decreasing the total possible amount of limonene that can be absorbed by the water. Similar to a process called "salting out".

            If the dichloromethane was used for extraction the Limonene would have been found in this layer. Because dichloromethane is more dense than water and they do not mix the dichloromethane would have formed a visible layer below the water that could be taken out by opening the stop cock of the separatory funnel and allowing the bottom layer to be collected.

How do I find the specific rotation?

specific rotation = observed rotation                                                                       
                               (density in g/mL) x length of polarimeter tube in decimeters

Or in words its the observed rotation divided by the product of the density (aka concentration) and the length of polarimeter tube in decimeters.

What happens if you shake too vigorously during the extraction with dichloromethane?

The gas could explode due to the build up of pressure inside of the separatory funnel. Also you may mix the layers so well that it won't separate as quickly.


On the other hand what happens if you don't shake vigorously enough?
The layers may not separate and limonene may not separate from the aqueous layer and go into the dichloromethane layer.

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