Question

Most sugars are dextrorotatory which means they-
A. Tilt light to right
B. Rotate polarized light to right
C. Tilt light to left
D. Rotate polarized light to left

Answer 1

Hint: the optical rotation of plane-polarized light can be described as dextrorotation or
laevorotation. Depending on the stereoisomer present, the rotation can either be clockwise or
anticlockwise, by this information, we can eliminate options A and C.

Complete step by step answer:
Dextrorotation or laevo rotation describe the optical rotation of plane-polarized
light. Dextrorotation is the clockwise or right-handed rotation while laevorotation refers to
anticlockwise or left-handed rotation of the plane polarized light.
Compound that cause rotation of plane polarized light to right are called dextrorotatory and the
compounds that cause plane polarized light to rotate to the left are called laevorotatory compounds.
Compounds having such properties are considered to have optical activity and consist of chiral
molecules. Enantiomer (geometric mirror image) of dextrorotatory molecule will be laevorotatory, and
vice versa. Enantiomers rotate plane-polarized light at the exact opposite angle in opposite
directions.
A dextrorotary has "(+)-" or "d-" as a prefix while a laevorotatory compound is prefixed with "(−)-" or
"l-". Do not confuse these prefixes with "D-" and "L-" prefixes, which distinguish chiral organic
compounds in biochemistry. Specific rotation is the quantity and the standard measure of the
degree to which a compound is dextrorotatory or levorotatory. Specific rotation of dextrorotatory
compounds have a positive, while it is negative for laevorotatory compounds. The enantiomer pair
have equal but opposite specific rotations.

So, the correct answer is “Option B”.

Note:
The optical rotation of plane polarized light can be determined as dextrorotation or laevorotation. Dextrorotation is the clockwise rotation of the plane polarized light.