INTRODUCTION:partitioning of the compounds to be separated between two distinct phases. unequally distributed
One of the main concerns of experimental organic chemistry is the separation of mixtures and the isolation of compounds in pure form. Crystallization is a useful method for purification and isolation, but is restricted to solids. Other techniques available for separating compounds depend in some way on the By choosing the phases so that the different compounds are between them, fractional separation of the compounds is effected.
Extraction is the general term for recovery of a substance from a mixture by bringing it into contact with a solvent that preferentially dissolves the desired material. The initial mixture may be solid or liquid, and various techniques and apparatus are required for different situations. In synthetic organic chemistry, the reaction product is frequently obtained as a solution or a suspension in water along with inorganic and other organic by-products and reagents. By shaking the aqueous mixture with a water-immiscible organic solvent, the product is transferred to the solvent layer and may be recovered from it by evaporating the solvent.
The extraction of a compound from one liquid phase into another is an equilibrium process governed by the solubilities of the substance in the two solvents. The ratio of solubilities is called distribution coefficient, Kd = C1/C2 , and is an equilibrium constant with a characteristic value for any compound and pair of solvents at a given temperature.
It can also be calculated that doing several, multiple extractions with the same total amount, for example, four 25 mL portions of ether, results in a more efficient extraction of the compounds.
In our lab experiment, we shall be determining a distribution coefficient by measuring the amounts of substance, which have become partitioned between two solvents. The calculations for this will be explained in lab lecture.
To recover the separate compounds, it is possible to evaporate the solvent and obtain the desired substance in relatively pure form. As you perform more experiments, you will utilize this technique frequently. Extraction procedures are used often to isolate spices and natural products from plants or seeds, e.g.: Isolation of leaf pigments, caffeine from coffee to make "decaffeinated coffee", separation of acidic, basic and neutral substances from each other, and many more.
With the data provided below:
Weight of benzoic acid used: 0.5345 g
Volume of solution prepared: 250.0 mL
Volume of NaOH used to titrate initial solution of benzoic acid: 8.94 mL
Volume of benzoic acid used in above titration: 10.00 mL
Volume of benzoic acid in separatory funnel: 50.00 mL
Volume of diethyl ether added for extraction: 10.00 mL
Volume of NaOH used to titrate benzoic acid after extraction: 7.53 mL
Please report and calculate the following with all work shown:
Molarity of the benzoic acid solution
Molarity of the NaOH solution
Mmoles benzoic acid left in the water
Mmoles of benzoic acid extracted into the diethyl ether
Distribution coefficient (diethyl ether/water)
Accepted value for Kd (use 26.0)
% error, high or low
D. Multiple Extractions Place another 50.00 mL sample of the benzoic acid solution in the sepgrgtgry funnel and extract with 5.00 mL of diethyl ether. Drain off the lower layer into a clean Erlenmeyer flask, remove the upper layer, and place the lower aqueous layer back into the flask and then extract this solution again with a second 5.00 mL portion. After the second extraction, remove and titrate the aqueous layer and calculate the amounts of acid in the two diethyl ether layers. Note that this part of the experiment has the disadvantage that very small amounts of diethyl ether are used—do the best you can. After doing both types of extractions, you will want to com pare their efficiencies and make some thoughtful comments about which way are better and why. D. Reactions Perfumed + NaOH _. + 0" OW/ONB H20 0
PROCEDURE In this experiment, a distribution coefficient will be determined and the efficiency of a single and multiple extractions compared. A solution containing a known amount of benzoic acid in water is extracted with diethyl ether. The amount remaining in the aqueous layer (lower layer) is determined by titration with base and the amount of acid in the organic layer is determined by difference. From these two values, a Kd (distribution coefficient) is calculated. Another extraction is then carried out with the same amount of diethyl ether but this time divided into two portions. The amounts of acid extracted in each case are compared. A. Preparation of the Benzoic acid solution. Weigh out accurater 5.00 armies of benzoic acid (approximately 0.6 grams) and place the acid in a 250 mL Erlenmeyer flask. Add about 150 mL of water and heat gently on a hot place with swirling until the acid is dissolved. Pour the cooled solution into a 250 mL volumetric flask and rinse several times with water to transfer all the acid. Adjust the volume to 250.0 mL to give an approximately 0.020 M solution of benzoic acid. Be sure it is well mixed. B. Preparation and Titration of the NaOH solution. Dissolve one pellet of NaOH in 100 mL of water and rinse and fill a buret with this solution. Place a 10.00 mL sample (measured with a volumetric pipet) of the benzoic acid solution in an Erlenmeyer flask and add a drop or two of phenolphthalein indicator. Titrate to a pink end—point. Repeat with a second 10.00 mL portion; average the buret readings, and calculate the molan'ty of the NaOH 0. Distribution Coefficient Place 50.00 mL (accurately measured with a volumetric pipet) of the benzoic acid solution in a separatory funnel, and with a clean and dry volumetric pipet, add 10.00 mL of diethyl ether. Stopper the funnel and shake vigorously for 30 seconds. Remove the stopper, allow the layers to separate, and drain off the lower aqueous layer and save it for titration. After a minute or two, a small additional lower layer can be removed too. Drain the upper layer into a waste beaker. Rinse the funnel with a few mL of water, and add the rinse to the flask. Add a drop or two of phenolphthalein and titrate with the NaOH solution to the same chqr; endpoint used in staudgndiging the base. Calculate the number of meLes of acid in the water, and, by difference, in the diethyl ether layer. From this data, you can calculate the Kd . The correct value is a range of 26 (diethyl etherfwater). Excellent resufts may be obtained with careful work.
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