Experiment 4
Acid-Base Extraction of Organic Compounds
Background:
Acid-base extraction is a technique used for purifying acids and/or bases from organic mixtures. This technique takes advantage of differences in the chemical properties of acids, bases, and nonacid/base organic compounds. Acid-base extraction is most commonly performed during the work-up of a chemical reaction. The isolated product is largely free of impurities that are unlike it (neutral, acidic or basic). A major limitation of this technique is that it is not possible to separate chemically similar acids from each other or chemically similar bases from each other.
The fundamental theory behind why this technique works is that salts, which are ionic, tend to be water soluble while neutral organic molecules are generally not. The addition of a strong inorganic acid to an organic mixture consisting of an organic acid and an organic base will result in the acid remaining uncharged, while the base will become protonated. As a result, the base will become positively charged. If the organic acid, such as a carboxylic acid, is sufficiently weak, addition of the stronger acid will suppress the weak acid’s ionization. The weak acid will then be neutrally charged while the base becomes positively charged. The charged base will now be soluble in the aqueous layer, while the neutral acid will remain in the organic layer.
Conversely, the addition of a strong inorganic base to a mixture of an organic acid and organic base will result in the base remaining uncharged, while the acid will be deprotonated, making it negatively charged and more likely to partition into the aqueous phase of a two-phase system. Once again, the self-ionization of a weak base is suppressed by the addition of the stronger inorganic base.
The acid-base extraction procedure can also be used to separate very weak acids from stronger acids and very weak bases from stronger bases as long as the difference of their pKa (or pKb) constants is large enough. For example: Very weak acids with phenolic OH groups like phenol or 2-naphthol (pKa around 10) can be separated from stronger acids like benzoic acid (pKa around 4 – 5) by adjusting the pH of the solution. Usually the pH is adjusted to a value roughly between the pKa (or pKb) constants of the compounds to be separated. Weaker acids like citric acid, phosphoric acid, or diluted sulfuric acid are used for moderately acidic pH values and stronger acids like hydrochloric acid or more concentrated sulfuric acid is used to achieve strongly acidic pH values. Similarly, weaker bases like ammonia or sodium bicarbonate (NaHCO3) are used for moderately basic pH values while stronger bases like potassium carbonate (K2CO3) or sodium hydroxide (NaOH) are used to achieve more strongly alkaline conditions.
Major Limitations:
The procedure only works only for acids and bases with a large difference in solubility between their charged and their uncharged form.
The procedure does not work for:
• Zwitterions with acidic and basic functional groups in the same molecule such as amino acids that tend to be water soluble at most any pH.
• Very hydrophobic acids and/or amines that are not soluble in the aqueous phase in their charged form.
• Very hydrophilic acids and or bases which are miscible or highly soluble in water at most pH such as acetic acid, citric acid, ammonia, methylamine
• Inorganic acids and bases that are insoluble in most organic solvents like sulfuric acid
or phosphoric acid.
General Procedure:
The mixture to be separated is dissolved in the desired solvent such as ethyl acetate or diethyl ether (the solvent MUST be immiscible with water). This solution is then poured into a separatory funnel containing an aqueous solution of the acid or base. The pH of the aqueous phase is then adjusted to ensure the compound of interest is fully ionized. After placing the lid securely on the separatory funnel, it is shaken and the phases are allowed separate. The phase containing the compound of interest is collected. The procedure is then repeated with this phase at the opposite pH range. The order of the steps is not important and the process can be repeated to increase the separation. However, it is often convenient to have the compound dissolved the organic phase after the last step, so that evaporation of the solvent yields the product.
Safety Concerns:
Do not allow any of the chemicals (organic acids and bases or inorganic acids and bases) to come into contact with your skin or eyes.
Safety goggles and gloves must be worn at all times.
Mixing acids and bases generates gas. Be careful when mixing solutions in your separatory funnel, as pressure can build up rapidly.
Vent the separatory funnel often by pointing the spout of the separatory funnel slightly up at the back of the hood and turning the spout 90 degrees. Never point the separatory funnel at anyone.
Procedure Observations
Obtain 30 mL a solution of ethyl acetate (solvent) containing a mixture of dissolved 1.5 g of benzoic acid and 1.5 g of biphenyl, in a 100 mL beaker.
Pour the contents of the beaker into a separatory funnel.
Add ~10 mL of 1 M sodium hydroxide (NaOH) and place the stopper in the funnel.
Take the separatory funnel to the hood, with the stopper firmly secured and the separatory funnel pointing to the back of the hood, gently shake the separatory funnel for approximately 1 min, stopping to vent after about 5 seconds, then vent again every 10 seconds. Note: when venting your separatory funnel ensure the spout is angled up and to the back of the hood. Quickly turn the plastic stopcock 90 degrees, allowing the gas to escape.
Make sure the funnel is closed before returning it to the upright position.
Allow the two layers to separate.
Separate the aqueous layer from the organic layer (which is which?).
Repeat this process by adding another 10 mL of 1M NaOH to the organic layer then separate
Add the 2nd aqueous layer to the first.
Decant the organic layer into a 125 mL Erlenmeyer flask.
Add 1 or 2 spatulas of anhydrous Na2SO4 (note CaCl2 may be used as a substitute) to your flask containing the organic layer and swirl gently, allow the sample to sit for 5 minutes.
Using a funnel and a plug of cotton, filter the Na2SO4 from the solution, collecting the dried organic layer in a clean, pre-weighed 50 mL or 100 mL round bottom flask.
Rinse the Na2SO4 residue with 10 mL of ethyl acetate .Actual Volume:
Appearance:
Top Layer:
Bottom Layer:
Mass of RB flask:
RECOVERY OF ACID AND BASE
Cool the beakers labeled “AQUEOUS ACID” in an ice bath.
Add 5-10 mL of 6 M HCl to the “aqueous acid” tube NOTE: if crystal do not appear you may need to add more of the acid or base. The solution should become cloudy and the products should begin crystallizing.
Allow the beaker to sit on ice for ~ 5 minutes to complete the crystallization.
Isolate the crystals using vacuum filtration using a Buchner or Hirsch funnel. Filter your aqueous layer to collect the precipitated benzoic acid.
Use ~10 mL of cold water to dislodge any remaining crystals in the beaker and pour onto the filter paper.
Collect crystals and transfer crystals to a pre-weighed watch glass.
Dry the crystals in an oven at 90oC. Actual amount:
Appearance:
Mass of Filter paper:
Watch glass Mass:
Evaporate the ethyl acetate from the round bottom flask using a rotovap.
Cool the flask and weigh the flask and contents.
Calculate the percent recovery of biphenyl (assume 1.5 g biphenyl in 30 mL).
Determine the melting point of the biphenyl crystals (remember) Mass of flask and crystals
Mass of crystal (biphenyl):
Mp range:
Weigh the watch glass with the dried crystals.
Calculate the percent recovery of benzoic acid (assume 1.5 g benzoic acid in 30 mL).
Determine the melting point of the benzoic acid crystals
Mass of watch glass and crystals:
Mass of crystals (benzoic acid):
Mp range:
Clean your biphenyl out of the beaker using 1 mL of ethyl acetate. Dispose of the ethyl acetate in the organic waste. Dispose of all filter paper, and crystalline products in the solid waste beaker under the hood. Procedure
Obtain 30 mL a solution of ethyl acetate (solvent) containing a mixture of dissolved 1.5 g of benzoic acid and 1.5 g of biphenyl, in a 100 mL beaker. Pour the contents of the beaker into a separatory funnel. Add ~10 mL of 1 M sodium hydroxide (NaOH) and place the stopper in the funnel. Take the separatory funnel to the hood, with the stopper firmly secured and the separatory funnel pointing to the back of the hood, gently shake the separatory funnel for approximately 1 min, stopping to vent after about 5 seconds, then vent again every 10 seconds. Note: when venting your separatory funnel ensure the spout is angled up and to the back of the hood. Quickly turn the plastic stopcock 90 degrees, allowing the gas to escape. Make sure the funnel is closed before returning it to the upright position. After ~1 min of gentle shaking, allow the two layers to separate. Separate the aqueous layer from the organic layer (which is which?). Repeat this process by adding another 10 mL of 1M NaOH to the organic layer then separate and add the 2nd aqueous layer to the first. Decant the organic layer into a 125 mL Erlenmeyer flask.
Add 1 or 2 spatulas of anhydrous Na2SO4 (note CaCl2 may be used as a substitute) to your flask containing the organic layer and swirl gently, allow the sample to sit for 5 minutes. Using a funnel and a plug of cotton, filter the Na2SO4 from the solution, collecting the dried organic layer in a clean, pre-weighed 50 mL or 100 mL round bottom flask. Rinse the Na2SO4 residue with 10 mL of ethyl acetate .
RECOVERY OF ACID AND BASE
Cool the beakers labeled “AQUEOUS ACID” in an ice bath. Add 5-10 mL of 6 M HCl to the “aqueous acid” tube NOTE: if crystal do not appear you may need to add more of the acid or base. The solution should become cloudy and the products should begin crystallizing. Allow the beaker to sit on ice for ~ 5 minutes to complete the crystallization.
Isolate the crystals using vacuum filtration using a Buchner or Hirsch funnel. Filter your aqueous layer to collect the precipitated benzoic acid. Use ~10 mL of cold water to dislodge any remaining crystals in the beaker and pour onto the filter paper. Collect crystals and transfer crystals to a pre-weighed watch glass. Dry the crystals in an oven at 90oC.
While you are waiting on your crystals to dry, weigh a round bottom flask to the nearest 0.01 g. transfer your organic layer to the flask and evaporate the ethyl acetate from the round bottom flask using a rotovap. Cool the flask and weigh the flask and contents.
Subtract the weight of the flask to obtain your yield of biphenyl.
Weigh the dried product and subtract the weight of the boat to obtain the mass of your benzoic acid.
Determine the melting point for each of your compounds and compare them to the melting point of the pure substances.
Calculate the percent recovery of each compound (assume 1.5 g of starting material).
Pack melting point tubes with your two compounds, and determine the melting point of each. How does the melting point range compare to the published values?
Clean your biphenyl out of the beaker using 1 mL of ethyl acetate. Dispose of the ethyl acetate in the organic waste. Dispose of all filter paper, and crystalline products in the solid waste beaker under the hood.
