Experiment one Acetylation of α-D- glucopyranose
1. Reagents
(1)2.5g(0.014mol) powdered D-glucose
(2)0.5g ZnCl2
(3)12.5ml acetic anhydride
(4)Cold water
(5)120ml Ice water
(6)Methanol
2. Physical properties Compounds | M.W. g/mol | M.P.℃ | B.P.℃ | Density g/cm3 | D-glucose | 180.16 | 146 | - | 1.54 | ZnCl2 | 136.315 | 292 | 756 | 2.907 | Acetic anhydride | 102.9 | -73.1 | 139.8 | 1.082 | Methanol | 32.04 | -98~-97 | 64.7 | 0.7918 | α-D- glucopyranose | 390.34 | 109-112 | 452 | 1.3 |
3. Phenomena and data
The reaction in pear shape flask is not vigorously because the amount reflexed solution can even not bring the raw material attached on condenser back to flask. The color of the liquid in reactor is brown, with gentle swirling, glucose at the bottom can plenary react with acetic anhydride. In the ice water, a considerable gelatinous yellow brown solid come from the dark brown solution. The crude product has the weight of 6.92g (crude yield 126.6%). After 4 times recrystallization, the final product is white with light yellow. The most weight product have melting point with 111.4℃, however the M.P. of product after second recrystallization is 93℃
The final product has the weight of 0.35g (yield 6.4%) 4. Discussion (1) Why 4 times of recrystallization is needed
The impurity contains acetic anhydride, acetic acid, Zinc Chloride, beta-D- glucopyranose, not full acetylated product and many unknown impurities. The acetylated products (both full acetylated and partial acetylated) are not soluble; therefore the water wash can only remove that water soluble matter. In the process of recrystallization, little white solid float on solution.