Organic Chemistry II Laboratory (ABCT357)
Although studying fundamental theories of chemistry in the class is important to understand the concept of chemistry, carrying out experiments to corroborate the theories is also important. It is very important for students to get used to experiments in order to speed up their experiments. Expt.1. Acetylation of α-D-glucopyranose
Add slowly 2.5 g (0.014 mol) of powdered D-glucose in small portions (roughly in 7-10 portions and 5 min for each addition) into a preheated solution of ZnCl2 (0.5 g anhydrous ZnCl2 in 12.5 ml acetic anhydride, this will be provided by lab. technician) in a 50 ml pear shape flask (heat in water bath for 10-15 min before addition start). The flask is attached with an air condenser. Swirl the mixture gently during the addition to control the vigorous reaction which ensues. After addition is completed, heat the flask on a water bath for one hour. Cool the content of the flask with cold water, and then pour into ice water (120 ml) and stir vigorously to assist the hydrolysis of unreacted acetic anhydride. After 30 min, the oil which first separates will gradually solidify.* Collect the crude product using Hirsch funnel and record the crude yield of the product. Filter, wash well with cold water and re-crystallize several times from hot methanol/water (3:4) mixture until the mp is constant. Record the melting point. *If solidification is not resulted: Dissolve the oily residue in hot methanol/water (3:4) mixture (heat on steam bath) and then allow it to stand at room temperature for crystals to form. If time is not allowed, label the container for crystallization and hand it in to technician and continue in the next practical class. Question In this experiment, α-D-glucopyranose is obtained. If we use (MeCO)2O/MeCO2Na to react with powdered D-glucose, β-D-glucopyranose will be resulted. Explain. 1
Expt. 2. Cis-1,2,3,6-tetrahydro-4,5-dimethylphthalic anhydride (Diels-alder reaction) The Diels-Alder reaction is an important, synthetically useful reaction in organic chemistry. It is named after Otto Diels and his student Kurt Alder who were awarded the Nobel Prize in chemistry in 1950 in recognition of the importance of their discovery. The Diels-Alder reaction is the cycloaddition of a 1,3-π system (a diene) with a π bond that usually bears an electronwithdrawing group (a dienophile) to produce a six-membered ring. The reaction is concerted (bond breaking and forming occur simultaneously) and therefore results in high stereoselectivity (products with predictable stereochemistry).
In this experiment, you will perform a Diels-Alder reaction using 2,3-dimethyl-1,3-butadiene as the diene and maleic anhydride as the dienophile. These substrates react to form the Diels-Alder product, cis—1,2,3,6-tetrahydro-4,5-dimethylphthalic anhydride. After synthesizing your anhydride, you will need to purify it by recrystallization, and characterize it by obtaining a melting point (the literature value is 78-79 °C) and an IR spectrum.
Add 0.65 g (0.0080 mol) of freshly distilled 2,3-dimethylbuta-1,3-diene (this will be provided by lab. technician) to 0.72 g (0.0074 mol) of finely powdered maleic anhydride contained in a small conical flask (addition should be performed in fume cupboard). Reaction occurs in a few
minutes (indicated by evolution of heat). Allow to stand until the mixture attains room temperature, and then remove the excess maleic anhydride. The excess maleic anhydride in the crude product can be removed by 1.) Extract the excess maleic anhydride with cold water until the aqueous extract no longer gives an acid reaction to Congo red paper. This can be achieved by grinding the product under water using mortar and pestle, or 2.) Dissolve the crude product in a minimum volume of dichloromethane and then wash with double volume of amount of water few times until the aqueous layer no longer gives an acid reaction to Congo red...