Question

The values of the specific rotation [aplha]_D²⁰_, for the lpa and beta anmers of D-galactopyranose are 150.7 degree and 52.8 degrees, respectively. A mixture that 20% alpha-D galactopyranose is dissolved in water at 20 degrees C.

a. What is its initial specific rotation?

b. After several hours, the specific roatation of ths mixture reaches an equilibrium value of 80.2 degrees. Now what is ts anomeric coposition?

Answer 1

a) specific rotation = Initial specific rotation x mass % of alpha + specific rotation of beta x mass % of beta

                       = ( 150.7 x 20/100) + ( 52.8 x 80/100) = 72.38

b) Let mass % of alpha is X then mass % of beta is 1-X

then 80.2 = ( 150.7X) + ( 1-X) (52.8)

80.2 = 97.9X + 52.8

X = 0.28 , hence alpha anomer is 28 % , beta animer is 72 %