I?ts specific rotation gradually decreases and reaches a constant value of + 52.7° When α - glucose is dissolved in water specific rotation of pure α - D - glucose is + 112° which gradually decreases and reaches a constant value of + 52.7° on standing. This is due to mutarotation specific rotation is defined as the change in orientation of monochromatic plane - polarized light, per unit distance - concentration product, as the light passes though a sample of a compound in solution S It is the spontaneous change of specific rotation of an optically active compound towards an equilibrium value. Mutarotation can be accounted for by considering the gradual conversion of either forms of glucose (α - D - (+) - glucose and β - D - (+) - glucose) in aqueous solution into other till an equilibrium mixture is formed. This process of conversion of one anomeric form into the other in aqueous form into the other in aqueous solution occurs through the open chain structure. The equilibrium mixture also contains a small amount of open chain form :
α−D−glucose
specificrotation=+112°
⇌
openchainform
specificrotationofequilibrium=+52.7°
⇌
β−D−glucose
specification=+19.2°
Thus the statement which is true is "its specific rotation gradually decreases and reaches a constant value of + 52.7°"