Chem Lab 2, Synthesis of Asprin.pdf

Chem Lab 2, Synthesis of Asprin.pdf - 2 Synthesis of...

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2. Synthesis of Aspirin This is a two-part laboratory experiment. In part one, you will synthesize (make) the active ingredient in aspirin through a reaction involving a catalyst. The resulting product will then be purified through a method called recrystallization. Prelaboratory Assignment Read the lab guide, and complete the prelab assignment in Chem21. The prelab assign- ment is due at the beginning of lab on the day your lab meets. 2.1 Introduction The salicylate ion is credited with the pain relieving effect in aspirin. The ion is easily formed from salicylic acid and is a chemical relative to salicin, which is found naturally in the bark of the willow tree. Since salicylic acid is internally corrosive and the sodium salicylate salt has an objectionable sweet taste, acetyl salicylic acid was synthesized (by a German chemist working for the Bayer firm) and found to be non-corrosive and palat- able. Later named aspirin, this compound is stable under the acidic conditions found in the stomach but is converted to the salicylate ion under the more basic conditions found in the intestines. Aspirin is usually prepared by reacting salicylic acid with acetic anhydride in the pres- ence of a catalyst. The general class of reaction is called ester formation. In the reaction scheme below, the chemical structures of the compounds in the reaction are shown. The hexagonal, ring-looking structures are called benzene rings and each point of the ring represents a carbon atom. If there is no other bond coming off the structure, you can as- sume there is a hydrogen atom attached to the carbon. The ring in the center means that every other bond is a double bond. 1
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O OH OH + C H 3 O CH 3 O O + CH 3 O H O O OH O C H 3 O salicylic acid C 7 H 6 O 3 acetic anhydride C 4 H 6 O 3 acetylsalicylic acid (aspirin) C 9 H 8 O 4 acetic acid C 2 H 4 O 2 Due to incomplete reaction or side-reactions that often occur, chemical reactions rarely lead to the formation of pure products. Therefore, products must be purified after the reaction is over. Well-packed crystal structures form when many similarly-structured molecules pack together. However, if there are impurities in the collection of molecules, they tend to disrupt the crystal packing process. (See Figure 2.1.)
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