Experiment 4B

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Title: Determining a partition Coefficient for the system of benzoic acid in Methylene Chloride and 10% Sodium Bicarbonate, Experiment 4B, Objective: To determine the partition coefficient of Benzoic Acid (aromatic ketone) in two immiscible solvents. These solvents that will be used in this experiment are dichloromethane (methylene chloride) and 10% sodium bicarbonate solution. Procedure: The procedure described in Mayo was followed on pages 144-145, in addition to the modifications document located on canvas. Modifications made to this experiment includes: 4A  75mg of benzoic acid and 1.0ml each of methylene chloride and water  A 20ml separatory funnel was used instead of a 5ml conical vial.  10% sodium bicarbonate solution was used instead of water.  The solution was transferred to a test tube rather than a 5ml conical vial.  Anhydrous sodium sulfate was used as the drying agent instead of magnesium sulfate. 4B  75mg of benzoic acid and 1.0ml each of methylene chloride and 10% sodium bicarbonate solution was used.  A 5ml conical vial was used as stated in the lab manual.  The methylene chloride layer from the conical vial was transferred using a simple glass pipette instead of a filter pipette.  The solution was then transferred to a test tube rather than a conical vial.  Like 4A, anhydrous sodium sulfate was used and not magnesium sulfate. Reaction Scheme: Chemical structure of Benzoic Acid Balanced equation for 4B

Methylene chloride Data and Results: Expt Benzoic Acid Methylene chloride Water 10% NaHCO 3 solution Mol. Formula 4A C 7 H 6 O 2 CH 2 Cl 2 H 2 O NaHCO3 Amount (mg) 4A 75 Amount (ml) 4A 1.00 1.00 mmoles 4A 0.61 Amount (mg) 4B 75 Amount (ml) 4B 1.00 1.00 mmoles 4B 0.61 Molar mass(g/mol) 122.13 Density (g/ml) 1.33 1.00 mp(°C) 122 bp ( °C) 40 100 (g) 4A 4B Final weight of vial + solid boiling + stone 4.855 4.975 Weight of vial +boiling stone 4.824 4.958 Observations:  Benzoic acid, which is a white solid, dissolved and formed a clear solution.  The top layer (aqueous layer) had a colorless appearance, while the organic or bottom layer appeared cloudy. Calculations: 1) Benzoic acid in CH2Cl2 = (final weight of vial + solid + boiling stone) – (weight of vial +boiling stone). 4.855- 4.824 = 0.031g / 31mg benzoic acid (4A) 4.975- 4.958= 0.017g/ 17mg (4B) 2) Benzoic acid remaining in aqueous layer (either water for 4A or 10% sodium bicarbonate solution for 4B 0.075g -0.031g= 0.044g/ 44 mg (4A) 0.075g – 0.017= 0.058g. 58mg

3) Benzoic acid KpCH2Cl2/H2O for 4A and KpCH2Cl2/aq. NaHCO3 for 4B Note that for Experiment 4B, this calculation does not provide a true partition coefficient. because benzoic acid is chemically modified in the procedure. The value is still useful for illustrating the distribution of benzoic acid and its derivatives between the two phases 4A: 44mg/31mg =1.419 Kp=1.419 4B: 58mg/ 17mg =3.142 Kp= 3.142 Questions : Answer the following questions. Be as complete as possible, and always explain your reasoning. 1. Laboratory glassware comparison for solvent extraction: conical vial vs separatory funnel... a) According to your experience, which piece of laboratory glassware did you find to be more efficient for the solvent extraction procedure, and why? I found the separatory funnel more efficient when performing solvent extraction as it allowed the separation of the two distinguish layers to be done more uniformly, by lowering the risk of cross contamination of the distinguish layers or phases. However, this occurrence is likely to occur when using a conical vial and glass pipette, thus lowering precision and accuracy. b) Assuming ideal separatory funnel performance (complete phase separation and no loss of solvent into the stem during venting), which piece of laboratory glassware do you think. provides a more accurate determination of the partition coefficient, and why? If a separatory funnel is used ideally, is more likely to yield results closer to that of the or accepted value of an experiment, thus more precise measurements can be achieved. 2. Consider the calculated partition coefficients for Experiments 4A and 4B. a) Was there a significant difference in the two Kp values? If so, which one was larger? The Kp value attained (1.419) for 4A was much smaller than the Kp value calculated for 4B (3.412). This difference in the calculated Kp values is significant to some degree.

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b) Provide an explanation for your answer in part (a). Be as complete and specific as possible. Hint: remember what was discussed during the breakup rooms during this lab. The significant difference in Kp values attained for 4A and 4B can be as a result of substituting water for sodium bicarbonate in part B producing a neutralization reaction due to the combination of sodium bicarbonate (base) and benzoic acid which yielded a salt of the conjugate base, sodium benzoate. The sodium benzoate formed is highly polar when compared to the mixture of benzoic acid and water. Therefore, more benzoic acid was being recovered from methylene chloride which accounts for the larger kp value. 3. You have 10 grams of compound A dissolved in 100 mL of solvent X, and want to extract it into solvent Y using 100 mL of solvent Y. You know that the partition coefficient, K, is given by: K=Cy/Cx =2 a) How much compound A remains in X if you extract with 100 mL of solvent Y? ( 10-x /100) = 2 (x/100) ( 10-x /100) = 2 ( x /100) 10-x = 2x 3.333= x b) How much compound A remains in X if you extract with 2 x 50 mL portions of Y?

Works Cited 1. Mayo, D. W.; Pike, R. M.; Forbes, D. C. Microscale Organic Laboratory with Multistep and Multiscale Syntheses, 5th ed.; John Wiley & Sons, Inc., 2011; pp 141-147. 2. Modifications for Expts 4A and 4B: Partition Coefficient Part B and Solvent Extraction I, Canvas document.