RECRYSTALIZATION

RECRYSTALIZATION AND SOLVENT CHOICE

1. Chem 338 Selecting a solvent for recrystalization Benjamin Goka

2. Goal In this experiment, you will be given an impure sample of fluorene and your goal is to find a good solvent for crystallizing the impure sample (Technique 11.6). After determining the best solvent, then you carry out crystallization (3A) pg 22. Lastly you determine the melting point of the impure and the pure sample (3C & 3D) pg 32.

3. What is recrystallization? Recrystallization , also called Fractional Crystallization is a common technique used to purify solids. The method of purification is based on the principle that the solubility of most solids increases with increased temperature. It involves dissolving the impure solid in a minimum amount of a hot solvent and allowing the resulting solution to cool slowly to room temperature. During the cooling process, pure (or almost pure) crystals form and are then collected by vacuum filtration.

4. Finding an ideal solvent for crystallization The first step in this process is to find an "ideal” solvent by testing a small amount of the impure solid. The desired compound should be very soluble in the solvent when it is hot (near the boiling), but only slightly soluble at room temperature. Sometimes a solvent mixture (two or more solvents in a specific ratio) is required to achieve this temperature dependent solubility. At the same time, the dissolved impurities (byproducts and unreacted starting material) should be highly soluble in the solvent at BOTH high and low solvent temperatures. The compound must precipitate out of solution at cooler temperatures to allow its separation from the solvent and the dissolved impurities by filtration.

5. In today’s Lab You will determine the ideal solvent for crystallization of impure sample of Fluorene using the procedure from 11.6 (testing solvents for crystallization) on page 676 with the following solvents: Methyl alcohol (CH 3 -OH) Toluene Water (H 2 O)

6. In today’s Lab Once the ideal solvent has been selected you will carry out the crystallization technique using procedure from Experiment 2A on page 11. Finally, you will be determining the melting point of your pure Fluorene . The melting point of pure fluorene is 116 C-117 C. Fluorene has a formula of C 13 H 10 and a molecular weight of ‎166.22 g/ mol