Question 3b Reproduced with permission from Green Organic Chemistry-Strategies,

Question

Question 3b Reproduced with permission from Green Organic Chemistry-Strategies, Tools, and Laboratory Experiments by K.M. Doxsee and J.E. Hutchison, Thompson Brooks/Cole, 2004; pp. 228-230. Study the sections in your lab manual regarding: Thin Layer Chromatography (TLC) (Berkman. pp. 82- 84). Recrystallization (including hot filtration & use of decolorizing charcoal) (Berkman. pp. 71-74). and melting point determination (Berkman. 40-43) Prepare your pre-lab notebook entry: flowchart, physical constants, etc. Answer the following questions: Draw figures over viewing the apparatus/procedure for (a) vacuum filtration and (b) hot gravity filtration, two techniques used in this protocol. Briefly explain why the two methods are used in the specific steps of the procedure, contrasting the goals of each method. What is the purpose of the CaCl_2 drying tube? What chemical reaction is it preventing (please supply a mechanism). Draw the product and detailed electron-pushing mechanism for a slightly different reaction: benzene (not ferrocene) being treated with chlorine and FeCl_3 to generate chlorobenzene. Based on the structures of the starting material and product, draw predicted TLCs of: (1) a complete reaction and (2) an incomplete reaction. (We'll pretend the TLC is done at 10:1 H:E, but that is irrelevant for this question).

Explanation / Answer

The reagents used in the reaction are acetic anhydride and polyphosphoric acid . Both of them do react with water/moisture in the atmosphere.

(CH3CO)2O + H2O ----> 2 CH3COOH

H3PO4 is hygroscopic and absorbs water thus the functionof it is modified.

Thus two prevent these two unwanted reactions, CaCl2 drying tube is used.

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