22_2510_PSet2_Extraction

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Chemistry

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Feb 20, 2024

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CHEM2510 Fall 2022 PSet #2: Due Friday, February 25 at 6pm Your responses must be typed. Numerical answers must be supported with calculations, which may be neatly hand-written and pasted in. You must submit a single pdf to Canvas. Similarly, you may hand- draw structures but illegible structures will not be graded favorably. Given the three compounds below, please answer the following questions: n.b. Notice that cyclohexanamine is an amine, which means that the pKa listed is of the conjugate acid (as indicated by the “b” in parentheses). 1) (6 pts) Please fill in the table below: (“Structure” = “predominant structure”) Compound Structure if pH solution > pK a Structure if pH solution < pK a 4-isopropylphenol 4-isopropylbenzoic acid cyclohexanamine 2) (5 pts) Gordon Ramsey says that 4-isopropylphenol and cyclohexanamine cannot be separated by acid- base extraction because they have the same pK a . Masaharu Morimoto believes that they can be. Who is correct? Please explain your answer. Masaharu Morimoto is correct because when they are put into the acidic environment, their charges will be different. The cyclohexanamine will be positively charged, and thus more soluble in water while 4- isopropylphenol will be neutral charge (which is more soluble in organic solvent). In basic environment, 4-isopropylphenol will have a negative charged while the cyclohexanamine is neutral. The negative

charge is more soluble in aqueous layer/water while neutral is more soluble in organic solvent. Thus, in either environment, although they have the same pKa, one will be charged and the other one will be neutral, and thus we can separate them. Also, the pKa given for cyclohexanamine is of conjugate acid and not its real pKb, thus we cannot use that to compare. 3) (5 pts) Sunny Anderson believes that “acids are acids and bases are bases”. He notices that 4- isopropylphenol and 4-isopropylbenzoic acid are both acids; therefore, he opines that they are inseparable by acid-base liquid-liquid extraction. JJ Johnson looks around the lab supply of solvents and solutions and states that he believes the separation is possible with readily available reagents. Assuming that the neutral forms of the compounds are soluble in the same organic solvents, consider the possibilities presented by the following aqueous solutions and decide who is correct (and explain your response). 1.5 M HCl (pH = 1.60), 1.5 M NaOH (pH = 13.8), 2.5 M NaOH (pH = 13.6), 1.0 M KH 2 PO 4 (pH 8.3). JJ Johnson is correct because 4-isopropylbenzoic acid would be made into a water-soluble compound using KH 2 PO 4 . Sicne this solvent has pH = 8.3 < pKa = 10.3 of 4-isopropylphenol, the compound will stay insoluble since it is not deprotonated and stays neutral so only soluble in organic solvent. Meanwhile, 4- isopropylbenzoic acid will be deprotonated and become conjugated base, which is negatively charged. This will create two different layers so it is separable. 4) (4 pts) For a compound with a partition coefficient of 5, please calculate how many grams of compound will remain in the organic layer given the following parameters: Starting mass of target compound: 7.0 g Volume of organic solvent: 100 mL Volume of aqueous solutions: two separate extractions of 50.0 mL each. (n.b. “Two separate extractions of 50.0 mL each” means: add 50.0 mL of the aqueous solution, perform a full extraction, set the aqueous layer aside, then add a second aliquot of 50.0 mL of the aqueous solution to the remaining organic solvent . It is strongly suggested that you draw pictures for yourself to think this through.)

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