Steam Distillaiton: The isolation of Limonene

Lab Manual Experiment 4

Important Terms

• Terpene = a compound whose carbon skeleton can be divided into two or more units identical with the carbon skeleton of isoprene.

• Isoprene = 5 carbon compound with a specific skeletal structure.

• Limonene =

Overview of the lab

• Peel a large orange (or two) and remove the pith.

• Grind the peels in a in a blender containing distilled water until a uniform slurry is obtained.

• Distill an “essential oil,” called limonene, from the resulting solution.

• Isolate the limonene using the liquid/liquid extraction (like last week)

• Characterize the limonene using gas chromatography.

Intro to Distillation

• Distillation is the process of heating a liquid until it boils, capturing and cooling the resultant hot vapors and then collecting the condensed vapors.

• Technique has been used for thousands of years (they have found evidence that goes back to 3500 BC)

Why are distillations used?

• Identification of a compound’s boiling point– Important physical property of any compound, can

be used to help identify an unknown compound

• Purification of a compound by separating it from a non-volatile or less-volatile material

How do distillations work?• A mixture is heated to the boiling point of the more volatile

compound (lower boiling point)• This compound becomes a vapor and can be collected from

the heating chamber and condensed back into a liquid

More terms• Distilling = technique to separate liquids based on their

boiling points.

• Refluxing = boiling = the boiling of a liquid at its boiling point

• Rounded bottom flask = self-explanatory?!

• Condenser = jacketed glass tube to allow cold water to pass through it and condense the vapors.

• Forerun = the first substance to distill over (often at a lower boiling point and thrown away).

• Distillate = the substance collected from the distillation.

• Pot residue = the substance leftover in the boiling flask.

Steam Distillation

• Special type of distillation used for materials that are sensitive to high temperatures– Many organic compounds (like natural aromatic

compounds) decompose at high sustained temperatures so cannot be separated by normal distillation

• Addition of water vapor into apparatus can allow for lower temperatures– Limonene bp is 176oC but it will boil at a temperature

close to 100oC using a steam distillation.

Boiling point• Temperature at which something boils….yes,

but that’s not all of it– Boiling occurs when the vapor pressure of the liquid

becomes equal to the pressure of its external pressure.

• Vapor Pressure:

• Think of boiling water at high elevation

Vapor pressure of different liquids• The vapor pressure of a liquid is principally determined by

the strength of the intermolecular forces of the liquid.• Liquids in which the intermolecular forces are strong have

relatively low vapor pressures (and high boiling points).

Vapor Pressure and Temperature

• Vapor pressure increases significantly with temperature.

Temperature Dependence of Vapor Pressure

• Vaporization requires input of energy to overcome the intermolecular attractions between molecules.

– Only a small fraction of the molecules in a liquid have enough kinetic energy to escape.

– By increasing the temperature, the fraction of the molecules with “escape energy” or kinetic energy increases.

• The higher the temperature, the faster the rate of evaporation by the high-energy molecules at the surface of the liquid.

Vapor Pressures Increases with Temperature

• As a compound is heated, its vapor pressure increases rapidly

• normal boiling point – temp at which a liquids vapor pressure equals 1 atm.

• Does water have a higher or lower boiling point at higher elevation?

Things to do Before the Distillation

• Add boiling chips to your orange peel slurry. Boiling chips work by providing nucleation sites so the liquid boils smoothly without becoming superheated or bumping.

• Make sure the joints of your distillation apparatus are lubricated and sealed.

• Start flowing water through your condenser. ASK YOUR TA TO CHECK YOUR SETUP BEFORE STARTING THE FLOW OF WATER.

• Wrap your rounded bottom flask and distillation adaptor with aluminum foil to prevent heat loss. This will shorten the time it takes to complete the distillation.

Things to do After Finishing the Distillation

• After distillation, the round-bottom flask must be removed from the heat completely. Just turning the heat off is not enough, the remaining water will be evaporated and the pulp will bake inside the flask, and be nearly impossible to get it out.

• Transfer the distillate into a 125-mL separatory funnel and extract the aqueous mixture twice using dichloromethane.

• Combine the dichloromethane extracts and dry them with sufficient amounts of MgSO4.

• Gravity filer the dichloromethane to remove the MgSO4.

• Remove the dichloromethane using a rotary evaporator

• Analyze the product by GC

Common Organic Solvents for Extractions

  Density (g/mL)

Hexane 0.6594

Diethyl ether 0.7133

Toluene 0.8669

Ethyl acetate 0.9006

Dichloromethane 1.326

Immiscible with water (density 0.9982 g/mL)

  Density (g/mL)Ethanol 0.7892

Acetone 0.7900

Methanol 0.7913

Tetrahydrofuran 0.888

Miscible with water (density 0.9982 g/mL)

organic layer on bottom

organic layer on top

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Vacuum Distillations• Vacuum distillation is distillation at a reduced pressure. • Since the boiling point of a compound is lower at a lower

external pressure, the compound will not have to be heated to as high a temperature in order for it to boil.

• Vacuum distillation is used to distill compounds that have a high boiling point or any compound which might undergo decomposition on heating at atmospheric pressure.

• The vacuum is provided either by a water aspirator or by a mechanical pump.

Rotary Evaporators

• Commonly called “Rotovap”

• Very commonly used• Evaporates low boiling

solvents (like DCM) from solutions at reduced pressures

• Higher boiling solute is left behind in evaporating flask

• Example of a vacuum distillation

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1. Add a bump trap onto the rotavapor. Use a green clip to fix it.2. Attach your round-bottom ed flask to the bump trap. 3. Turn on the rotavap motor and adjust the speed.

4. Close the vent5. Turn on the vacuum6. Put your flask into the warm water bath

7. Solvents will be collected

8. Turn off the vacuum9. Open the vent10. Adjust the speed to 0 and turn off the

rotavap motor

11. Take the round-bottomed flask off the rotavap

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• Another separation method in which molecules either stick to the stationary phase or move with the mobile phase (gas).

• Separation is done entirely in the gas phase.

• Separation is based on boiling point for molecules of similar polarity.

• Usually used for analysis rather than preparative purification.

Gas Chromatography (GC)

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• Another separation method in which molecules either stick to the stationary phase or move with the mobile phase (gas).

• Separation is done entirely in the gas phase.

• Separation is based on boiling point for molecules of similar polarity.

• Usually used for analysis rather than preparative purification.

Gas Chromatography (GC)

Four components:

How does GC work?

• A heated cavity that keeps compounds in the gas phase. • GC column is coated with a stationary phase that

separates the compounds.

4. A detector

• Where the sample is injected with a microsyringe.• Heated, so that the sample is immediately vaporized.

2. A column inside the oven.

1. An injection port.

3. Carrier gas (helium). • Inert; no interaction with other compounds.

• Responds to thermal conductivity or ionization.

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• We will be using a standard non-polar GC column

Common stationary phases are:• Cyanopropylphenyl dimethyl polysiloxane• Carbowax polyethyleneglycol • Biscyanopropyl cyanopropylphenyl polysiloxane• Diphenyl dimethyl polysiloxane

Column: the Heart of the System

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• Visualization Concentration-dependent detector: detect the

concentration of solute

Detector

Factors That Affect GC Separation

• Volatility of the compound (bp) higher bp = longer retention times

• Carrier gas flow rate and temperature lower temps and flow rates = longer retention times

• Length of column longer column = longer retention times

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Varying these parameters canimprove separation

Factors that effect retention time on a standard non-polar GC column

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• Remove dichloromethane with rotary evaporator and dissolve the residue in 0.5 mL of dichloromethane

• Show your diluted sample to your TA who will check for dryness• Stockroom attendant or TA will inject 1 microliter onto the GC• Check that you are happy with the results; you may have to re-

inject another sample.

Injection of Limonene into the GC

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First compound to elute has a lower retention time.

Major component of a mixture should have a larger area under the curve.

A

B

compound with lower boiling point

compound with higher boiling point