245 * Lab #7
Extraction
1. Important terms: salting out, acid and base extraction, density.
2. Extraction of an unknown which may contain two or three of the following:
a) Benzoic Acid
b) 2-Naphthol
c) Biphenyl
Extraction is a method of separating chemicals from mixtures dissolved in the same solvent. It is based on the premise that when exposed to a more "Like" substance (in terms of polarity) or introduced to a different solvent that may cause a reversible reaction that enables that chemical to dissolve, then you may be able to remove unwanted impurities or to separate chemicals easily. Benzoic acid is a weak acid (pKa ~4) that will dissolve in weak base (pH~8). 2-naphthol is a very weak acid (pKa~10) that will dissolve in strong base (pH~12).
The type of solvents as well as the order of extraction is very important. You need to pick solvents that will select for only one of the chemicals to be extracted at a time. It will be no use to keep picking solvents that all or some of the chemicals in the mixture would happily dissolve in any appreciable amount. Only one chemical should be selected for. At first, the chemicals above will be all dissolved in dichloromethane ( Carcinogen Hazard, use gloves & ventilation). 1.1 M sodium bicarbonate and 3 M sodium hydroxide will be used to extract two of the chemicals. One will remain in the dichloromethane. The order of the extractions is also very important. What would happen if you extracted with the 3 M sodium hydroxide (~ pH 14) before you used the 1.1 M sodium bicarbonate (~pH 9)?
Once you have your mixture dissolved in the dichloromethane ( Hazard), put it in the separatory funnel. The funnel is fitted with a glass stopper and stopcock. You then add your first extracting solution. In this case it would be the 1.1 M sodium bicarbonate. These solvents must have additional properties than those already indicated. One is that they cannot be miscible. You must have two separate layers. With the stopcock closed and the funnel stoppered, you then gently shake the contents of the funnel to try to mix the two solutions. (When you stop shaking, the two solvents will naturally separate) When the chemicals in the initial solvent come into contact with the extracting solvent, they may be more soluble or readily reacted in the extracting solvent than in the initial solvent. In this case the chemical would be removed from the initial solvent and is now dissolved in the extracting solvent. Sometimes this shaking causes gas or vapor formation. In which case you would turn up the stem of the funnel (holding on to the stopper for dear life) and release the gas (into a nearby hood) by opening the stopcock. When you feel that you have mixed the two solutions sufficiently, you then set the funnel upright onto a support (a wire triangle placed on an iron ring attached to a ring stand). The two solutions will naturally separate. You then open the stopcock to remove the bottom layer. (The stopper must be removed to allow the solutions to drain properly from the separatory funnel.) At this point the dichloromethane will have only two of the chemicals dissolved in it. The 1.1 M sodium bicarbonate will have extracted most of the remaining third chemical. Repeat the extraction of the dichloromethane with the sodium bicarbonate to retrieve any third chemical remaining for that extraction. It is important that you know which layer contains what. After you do the initial extraction with Sodium bicarbonate, you want the dichloromethane solution returned to the separatory funnel. After the second extraction of sodium bicarbonate you need to combine the two sodium bicarbonate solvent extracts. You then perform two extractions with 3 M sodium hydroxide, combining both of those extracts also. You now have three solvents containing one chemical each (if your unknown has all three compounds). You now need to recover these chemicals by some means. The sodium bicarbonate solution (basic) and the sodium hydroxide solution (basic) contain their chemicals as a converted water soluble sodium salt. They only need to be acidified to be recovered. The chemical in the dichloromethane need only to have the dichloromethane removed to be recovered. The dichloromethane will be removed by simple distillation performed with a Rotovap setup. The dichloromethane is then put into a waste bottle.
Obviously, the better extraction will yield the purer compounds. Save the compounds for melting points and for the chromatography lab to determine purity.
You will be given a vial that contains either 2 or 3 of the chemicals listed at the beginning of the lab. You will determine from the extraction which of them that you had in your sample. Follow the directions below.
1. Dissolve ~0.8 gms of the unknown in 30 mL methylene chloride (CH2Cl2). Put into the separatory funnel. If the unknown doesn't dissolve completely, continue anyway.
2. Extract first with the 7 mLs of 1.1 M sodium bicarbonate. Collect the sodium bicarbonate extractant. Extract with a second 7 mLs of sodium bicarbonate. Combine the two extracts. Which of the three chemicals (a, b or c) have you extracted? Which chemical(s) is/are left in the dichloromethane?
3. Extract with the second solvent, 3 M Sodium hydroxide (same directions only using 3 M sodium hydroxide) Which of the three chemicals (a, b or c) have you extracted? Which chemical is left in the dichloromethane?
4. Distill off (simple distillation) the methylene chloride (dichloromethane) using a Rotovap that is setup. Return it to the stock bottle. * Methylene chloride is hazardous ! Do not breathe the vapors or get any on your skin ! * Read spill and disposal methods at the end of this lab. This should give you the non-polar component of the unknown as a solid residue in the pot if it is present in your unknown.
You will reclaim the benzoic acid or the 2-naphthol that you may have by acidifying each extraction using 6M HCl to neutralize the base. You should calculate the approximate volume of 6M HCl needed based on the amounts of each base used to extract the dichloromethane. Add this amount and enough extra to complete precipitation. Cool the mixtures, vacuum filter off each separately and save all three (or both) of the reclaimed components for the chromatography experiment. When dry, melting points should be taken of each component in your unknown. Name the components of your unknown
Save the extracted products for the chromatography experiment!
The methylene chloride will be recycled for use with this lab.
WASTE Spill - F: METHYLENE
CHLORIDE (carcinogen suspect)- If the evaporators are used the CH2Cl2
is collected by the staff. Otherwise, a labeled bottle will be put in Hood #5.
WASTE Spill and Disposal - G:
benzoic acid, 2-naphthol & biphenyl. There will be a labeled collection
bottle put out with the lab.
WASTE Spill and Disposal - E: Any
acetone used to clean glassware. *THERE IS AN ACETONE WASTE JUG IN THE HOOD*
DO NOT PUT USED WEIGH BOATS IN
THE HW CHEMICAL CONTAINER. CONTAMINATED PAPER TOWELS AND WEIGH BOATS ARE PUT
INTO THE CLEAR PLASTIC HW BOX IN THE HOOD. PAPER ENVELOPES W/CHEMICALS SHOULD
BE FOLDED MULTIPLE TIMES AND SEALED BEFORE BEING PUT INTO THE SPECIFIED HW CONTAINER.
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CHEMICALS: |
LOCATION |
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*Recycle Jar for Dichloromethane |
Hood#5 |
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*Waste Jar for solids (benzoic acid, biphenyl, 2-naphthol) |
Hood#5 |
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Dichloromethane (methylene chloride) ( 35 mL) *CARCINOGEN* |
Hood #5 |
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Benzoic Acid(1 gm) |
Hood #5 |
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Hood #5 |
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Hood #5 |
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3 M NaOH (14 mL) |
Hood #5 |
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1.1 M NaHCO3 (14 mL) |
Hood #5 |
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unknowns (poss. benzoic acid, biphenyl, 2-naphthol) |
Hood #5 |
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SUPPLIES: |
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litmus paper |
ChemPrep, C-30 d |
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fluted filter paper |
ChemPrep |
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Vials |
White Supply Cabinet |
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Weigh boats |
C-19 |
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Gloves |
C-26 |
EQUIPMENT : Rotovaps