OPTICAL ACTIVITY

• Since enantiomers are "handed" or "chiral", they can be distinguished by other agents which are chiral. Thus, we can easily tell, in using our right hand to shake hands with another person, whether that person is using his left or right hand. There is a better "fit" of the two right hands than there is of right hand to left hand.
• Chemically this occurs, as noted above, when enantiomers react with another chiral compound. Both the original enantiomer and its reactant distinguish left from right , so then one of the original enantiomers will find a better energetic fit with the chiral compound than will the other.
• One physical property which distinguishes 2 enantiomers is "optical activity". This term refers to the property of chiral compounds (exclusively) of rotating the plane of plane-polarized light to the right (clockwise) or to the left (counterclockwise).
• The two enantiomers have exactly the same ability to rotate this plane, quantitatively, but they rotate it in opposite senses. Thus, if one enantiomer rotates the plane by 10.5 degrees clockwise (considered a positive rotation), the other rotates it by -10.5 degrees (i.e., in the counterclockwise direction).
• Since the exact amount of the rotation of the plane by a given enantiomer depends upon how much of that enentiomer the light encounters as it passes through the solution, the measured rotation is divided by the concentration of the enantiomer and by the path length of the polarimeter cell to give a true measure of the inherent ability of the enantiomer to rotate the plane of polarized light. This number is called the specific rotation. Note that in deriving the specific rotation, the concentration is taken in grams per mL, and the path length in decimeters. The magnitude of the rotation also depends upon the wave length of the plane polarized light, so the a single wave length is usually used, i.e., the sodium D line (529 nm),the line responsible for the yellow color of sodium-vapor lamps.
• A positive (clockwise) rotation is sometimes called dextrorotation and a ngetaive rotation is sometimes called levorotation
• Read more at: