4A Recrystallization - 4A Recrystallization Author: Holly...

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Unformatted text preview: 4A Recrystallization Author: Holly Polk Instructor: Oleksandr Zhurakovskyi Organic Chemistry Lab 243A, Section 010 Date Work Performed: February 19 , 2009 Date Submitted: February 26, 2009 Abstract The objective of the lab is to perform two recrystallizations which simulate commonly encountered situations. In this lab, naphthalene is separated from an impurity with the use of a polar solvent, methanol, and benzophenone is separated from an impurity with a non-polar solvent (petroleum ether). Crude naphthalene had a melting point of 69-81C, and recrystallized naphthalene was 79-82C, while the standard value was 80.26C, and the percent recovery was 30.67%. Crude benzophenone had a melting point of 45-49C, and recrystallized benzophenone was 46-48C, while the standard value was 47.9C, and the percent recovery was 28.11%. Introduction This lab focuses on recrystallization, a physical process based on differences in solubilities of solutes in a solvent. Recrystallization involves dissolution of a solid in a solvent at elevated temperatures and the reformation of the crystals as the solution cools, allowing for impurities to remain in the solution. This technique is a commonly used method for purification of solids, allowing one to achieve one of the highest states of purity. Recrystallization works on the premise that compounds which are less soluble will crystallize first upon cooling. When the crystal forms, the less soluble molecules are the only ones that will form in the crystal lattice structure, which helps to increase the purity of the final product (Padias 33). A molecules polarity influences its physical properties, like melting point, boiling point, and solubility, as well as intermolecular forces (polar molecules usually have higher melting points/boiling points than non-polar molecules). For example, naphthalene, a non- polar molecule, has a boiling point of 218 C, while benzophenone, a polar molecule, has a boiling point of 305.4 C (which is almost a 100 difference). So, naphthalene would be soluble in a non-polar solvent, and benzophenone would be soluble in a polar solvent. An important technique for this lab is to remove deliberately added impurities from target compounds by dissolving. The polarity and amount of solvent is important when dissolving, since just enough solvent should be added to maximize the amount of product isolated. Compounds in this experiment are paired with an impurity that is either more or less polar, allowing for separation based on polarities. So, non-polar naphthalene would be paired with a more soluble impurity and can be separated with the use of a polar solvent like methanol, leaving pure naphthalene in the Hirsch funnel. However, polar benzophenone is paired with a less soluble impurity and can be separated by the use of a non-polar solvent (petroleum ether), leaving pure benzophenone in the Erlenmeyer flask.leaving pure benzophenone in the Erlenmeyer flask....
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4A Recrystallization - 4A Recrystallization Author: Holly...

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