Purification Techniques CHEM 241-08A

Distillation

Column Chromatography

Crystallizaton

Extraction

How Does it Work?

  1. Distillation apparatus is assembled. (both simple and fractional distillation has the flask, a distillation head and a receiving vial that is kept in an ice water bath and fractional distillation also has a packed distillation column)
  2. Two compounds are heated
  3. Liquid boils and a condensation line can be observed as it moved up the fractioning column
    4.When the vapors reach the thermometer bulb, the temp has a big increase (boiling point of the lower-boiling compound)
  4. After the compound with the lower boiling point is completely distilled, temperature rises again and the boiling point of the higher-boiling compound is found
  5. Vapors produced are cooled using a condenser and collected

Pros and Cons

Overview
Distillation is used in purifying and identifying organic liquid compounds. It is used to separate compounds based on differences in their boiling points. Can also be used to remove a volatile solvent from a nonvolatile product, separate a volatile product from nonvolatile impurities, or to separate two or more volatile products that have sufficently different boiling points. Appropriate to use on liquids

Video

Overview/Technique

Pros and Cons

How does it work:
In extraction, we have two layers, an aqueous layer and an organic layer. The goal of extraction is to separate the two mixtures, this is the reason it is a purification technique. In class for the microscale extraction, we used centrifuge tubes to hold the reaction mixture, we then would separate the layers via a glass pipet and transfer it into a beaker or another centrifuge tube. For macroscale extraction we used a separatory funnel and separated the mixture this way. If you recall, we could then analyze the acidity of the resulting compound (this is shown in the video below after the 3 minute mark). Although drying the mixture is not necessarily the main extraction part, we often would dry or mixture using anhydrous CaCl2 or saturated NaCl2.

Video
Video #1 - Microscale Extraction:
Exceptions to video #1: We typically did not shake/vent the centrifuge tube (or falcon flask as they say in the video)
Video #2 - Macroscale Extraction:
Watch from 1:00 to 3:00 for a quick overview, however the entire video is a good overview of extraction via macroscale.

How it Works

  1. Basic piece used is a glass tube with a stop cock
  2. Sand is poured in the bottom of the column
  3. Slurry is used to pack the column
  4. Column is agitated more and some liquid is drained
  5. After slurry is settled, liquid is drained until level with the solid
  6. layer of sand is then added to the top
  7. Sample is then drained until it is absorbed through the sand layer
  8. more solvent is added above the sand and that is when the mixture begins to separate and the sample is collected in small amounts in different containers

Pros and Cons

Overview
Chromatography is a technique used to separate mixtures of compounds based on their polarities and affinities for a stationary phase and a mobile phase. All forms of chromatography are based on these types of equilibrium by keeping one stage stationary and other mobile. Aprropriate to use on liquids

Overview
Appropriate to use with organic solids
Used to purify a solid organic compound using an ideal solvent

Video: Column Chromatography
Our slurry was made using 100 mg of ferrocene and acetylferrocene, 3-5 drops of dichloromethane, and 300 mg of alumina.
Diagram From Lab Manual:
20201119_132111

How does it work?

Pros and Cons

Video

Watch video on simple distillation until minute mark 5:20.
Exceptions to what we did in lab: 1. We did not use a stir plate, we just used a sand bath for heating.

  1. We did not use any grease to connect the 3-way adapter.
  2. We did not use a large condenser like shown in the video, we recovered product directly from the 3-way adapter.
  3. We typically stirred the mixture in a separate step with a magnetic stir rod.
  4. We used an ice bath as a means of keeping the receiving flask cool.
    Photo From The Lab Manual:
    20201119_121530 (1)

Pros:
Can determine the presence of impurities if boiling point of solution is not same as desired compound
Fractional distillation results in purer distillate
Fractional distillation creates more theoretical plates more efficiently
Simple, cheap, and very common purification method

Cons:
Does not work for compounds with too similar boiling points
Exact boiling point of homogenous mixture could not be determined using this method
Can be dangerous if done as a closed system

Pros:
Allows selective separation of two immiscible compound layers
liquid-liquid extraction is most ubiquitous purification technique. Extraction is fairly easy to do.

Two Different Types:
Simple Distillation: The two liquid compounds can be separated in one vaporization step. Boiling points of the two liquids differ by less than 30 degrees Celsius.
Fractional Distillation: Boiling points of the two liquids differ by more than 30 degrees celsius. This process has the effect of many simple distillations with the use of a distillation packed with metal turnings. The packed column has more surface area than the simple distillation, which allows for it evaporate and reconvenes more often, creating a more pure distillate.


Extraction is used as one of the steps in isolating a product of an organic reaction. It is the "pulling out" of a substance from one liquid by another liquid. The two liquids must be immiscible or insoluble in each other/. There will be two layer with a junction between them. A separatory funnel or centrifuge tube are typically used to carry out extraction. Used when we want to carry out liquid-liquid extraction.

There are 7 steps:

  1. Choose a suitable ideal solvent or solvent pair
  2. Dissolve solute by heating
  3. Decolorize with activated charcoal to remove any colored impurities (this step was skipped in lab)
  4. Quick/rapid/hot filtration to remove activated charcoal and other impurities
  5. Crystallize (let cool undisturbed with an applicator stick) hot saturated solution
  6. Filter crystals using vacuum filtration and air dry
  7. Weigh crystals, calculate percent yield, and obtain melting point using a MelTemp

Ideal Solvent must meet the following criteria:

  1. must dissolve the solute only when heated and not at room temperature
  2. must not dissolve impurities at room temperature or at all
  3. must not chemically react with solute
  4. must not be toxic or flammable, must be volatile and inexpensive

Cons:
There is a chance of impurities in desired product

Pros:
Can be used for large scale purification
Relatively cheap

Cons:
Can not be used ubiquitously because not every compound or molecule is a solid
Not every solid can be crystallized readily
Some may need seed crystals to start recrystallization process
Heating the solvents can be very labor and energy intensive, especially on a large industrial scale
No single solvent will meet all 4 criteria so a solvent pair or water might be used instead
Solvent must be determined through trial and error

Cons:

  1. This is a pretty difficult experiment to carry out, and there are many ways that the results could become less accurate. If too much solvent is added, the results could be accurate. Also, if the solvent is pushed through too quickly, the sample will not be able to reach equilibrium, and if the solvent passes through the column too slow, diffusion could occur. Lastly, air could get trapped in the column if the solvent ever gets lower the solvent goes lower than the sample

Pros:

  1. Useful when trying to find out the components present in liquids or solids
  2. Can be used to determine the identity of an unknown
  3. Used to find the retention factor. The retention factor is a good indicator of whether an unknown compound and a known
    compound are similar, if not identical.

Video:

Exceptions:

  1. We only used 50 mg of solute with minimum solvent.
  2. We did not stir the solution as it was heating or place beaker directly on hot plate. We instead placed test tube in beaker of boiling water.
  3. Our vacuum procedure also used water, and the apparatus was not clamped down
  4. He did not do an unknown test