Glucose

Experiment 53: sugars  – 53.2 Part C (acetylation of glucose) Introduction: Carbohydrates, or sugars, are an important part of nature. Monosaccharides, or simple sugars, are polyhydroxyaldehydes or polyhydroxyketones. In a solution, they exist in equilibrium as an open chained or cyclic hemiacetal structures. The cyclic forms exist in alpha and beta anomers (two stereoisomeric configurations). When a cyclic structure forms, a new chiral carbon is also formed which results in two configurations. In the experiment, crude b-d-glucose pentaacetate, a derivative of glucose and acetic anhydride, was produced. Anhydrous sodium acetate was used as the catalyst. The crude product was massed and the melting point was taken. Procedure: 0.9 g (5.00 mmol) of pulverized glucose and 0.5 g (6.09 mmol) of pulverized anhydrous sodium acetate were added to a 25 mL round bottom flask. Then, 5 mL of acetic anhydride and boiling chips were added. The mixture was refluxed with constant stirring stirring for 1.5 hours. The solution was poured into a 250 mL beaker filled with 50 mL of ice cold water. The mixture was stirred continuously for 45 minutes. It was then Buchner filtered and the residue was massed and melting was taken. Table of chemicals: Chemical Glucose Anhydrous sodium acetate Acetic anhydride Water Crude β-D-glucose pentaacetate (product) Mechanism

Quantity 0.904 g 0.5 08 g

MW (g/mol) 180.16 82.03

5 mL 50 mL 1.477 g

102.09 18.01 390.34

MP (actual)

MP (experimental)

135

129.5-131.2

Observations: Glucose was a white, grainy powder. Anhydrous NaOAc was a white powder which had an unpleasant odor. Acetic anhydride was clear and colorless. The mixture initially was a white damp solid at the bottom with a solution layer on top. During refluxing, the solution turned a dark r ed-brown color. After refluxing, the solution was a dark brown solution with dark black chunks. Upon addition to the water, the solution was a turbid, murky br own. After stirring, the solution was somewhat clearer with chunks of dark brown. After filtration, the filtrate was clear with some orange-brown oil-like drops. The residue was a fine brown coco-like powder with large black chunks. Percent Yield:

              )(   )(  )   (                       

Results: The percent yield of the crude a-d-glucose pentaacetate was calculated to be 75.4097% from the experimental product of 1.477 g and the theoretical yield of 1.9586 g. Some sample was lost when the round bottom flask mixture was transferred into the beaker. Also, there was product remaining on the stir bar after filtration, so product was also lost there. Since the product was crude, it contained impurities. Such impurities could have increased the mass of the product, thus lowering the purity of the

final product. This was confirmed by the low melting point. Impurities could be removed by recrystallizing the crude product. The melting point range was determined to be 129.5-131.1 degrees Celsius which is a lower and broader range than the actual melting point of 135 degrees Celsius. This is probably due to t he presence of an impurity, such as water, which would interfere with the homogenous intereactions of a-d-glucose pentaacetate trying to crystrallize out. This is energetically favorable resulting in a lower melting point. The range is broader due to the impurity. A pure compound would have had a sharp point at which the entire compound melted. The product contained both alpha and beta-d-glucose pentaacetate.