Today: Your experiences with recrystallization? Mixed-solvent recrystallization MiniQuiz

Table for Part A Solubility of Pure Benzoic Acid Which solvent(s) is/are suitable for recrystallization? Acetone Pet Ether H2OH2O 95% EtOH Rt Hot Warm Cooled Warm

Part B Recrystallization of Impure Benzoic Acid Your sample contained benzoic acid (70%), oxalic acid (30 %) and a trace of a dye. Role of “active charcoal”? Today: Obtain mp. of purified benzoic acid (range!) and of your original impure benzoic acid.

Questions: 1.What do you expect the purities of Crop 1 and Crop 2 to be? Same? Different? What melting point ranges do you expect for Crop 1, Crop 2, original mixture? Explain. 2.What is the “mother liquor”? 3.What compound(s) do you expect to be in the mother liquor after filtering off your first crop of benzoic acid crystals? 4.Is it possible to retrieve 100% of solid sample in a recrystallization? (Assume that you are a very experienced chemist.)

Part C Recrystallization of Naphthalene from a solvent pair Often it is impossible to find one good solvent for recrystallization. Then a solvent pair has to be used: 1)Solvent (#1) should dissolve compound at RT. 2)Solvent (#2) should NOT dissolve compound at RT. 3)Both solvents must be miscible! melting point?

Common Solvent Pairs for Crystallization Methanol-Water Ethanol-Water Acetic acid-Water Acetone-Water Ether-Methanol Ether-Acetone Ether-Petroleum ether Toluene-Ligroin Methylene chloride- Methanol

1)Do solubility tests for Naphthalene: H 2 O, 95% EtOH, Pet Ether, and Acetone Goal is to find a good “solvent pair”: Solvents #1 and #2 Procedure: Part C:

2) Once you have chosen the solvent pair, then use ~ 0.5 g of Naphthalene in 50-mL E-flask. (Record the exact weight to three decimal places g) 3) Dissolve compound in minimum amount of warm solvent #1(“good solvent”), start by using 10 mL of solvent #1. Record additional mL of solvent #1 used. Use 20 drops per 1 mL, when using a disposable glass pipette and a small rubber bulb. Crystals must be just dissolved; do not use a great excess! 4) Solvent #2 is added dropwise to the warm mixture, until mixture just becomes cloudy. Cloudiness indicates precipitation. At this time, add solvent #1, just until the cloudiness disappears. The solution is now saturated. Cool solution to obtain crystals.

Reading Solubility Data Example: Solubility of Benzoic Acid Solubility in water(g/l) at 0 o : 1.7 at 95 o : 68.0

Next time: Molecular Modeling (no Notebook Preparation required, but do preview/prestudy Experiment)