Question

Which one is levorotatory sugar?
A. Sucrose
B. Glucose
C. Fructose
D. None of the above

Answer 1

Hint: Optically active compounds are substances which can rotate planes of plane-polarized light. The rotation can be either clockwise or anti-clockwise or towards left or right. When the plane of the light gets rotated to the right side, it is known as a dextro-rotatory compound, meanwhile changing its plane of rotation to the left side makes it a levorotatory compound.

Complete answer: The main property of fructose is that it changes the rotation of the plane-polarized light towards the left side, or in a counter-clockwise direction, thus, this sugar is also known by the name Levulose due to this property. The main cause of this optical activity is due to the presence of asymmetric carbon atoms in the compound, also known as chiral carbon. Fructose is an isomer of glucose, i.e. a hexose, which means that it is a six carbon-containing sugar molecule, but it exists as a five-member hemiketal ring structure.

Considering the other options:
Sucrose: Being a mixture of glucose and fructose, and is a primarily dextrorotatory compound and rotates the plane of polarized light to the right side.
Glucose: It is a dextrorotatory compound since it changes the direction of the plane of polarized light to the right side, thus it is also known by the name dextrose.
Thus the correct answer is option C.

Note: Optical activity is the ability of an asymmetric or a chiral molecule. It is measured by using a device called a polarimeter. The device consists of a light source, polarizing lens, a sample tube and an analyzing lens. As the light is passed through the sample, it rotates the plane of polarized light. The amount of the light rotated is measured in degrees, the analyzing lens is rotated.