Levorotation

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Levororatory (also Laevorotation, Latin: laevus, left)[1] refers to the substance that rotates polarized light in a counterclockwise direction. This rotation, measure in degrees, is said to be negative (-). A clockwise, positive (+), direction of rotation is called dextrorotatory (Latin: dexter, right).

There is no correlation between the (R) and (S) configuration of enantiomers and the direction of rotation of plane-polarized light. However, as well as (R) and (S) notations, (+) and (-) direction of rotation is incorporated into the names of optically active compounds to indicate differences in physical properties of structurally identical molecules.

A standard measure of the degree of rotation is a quantity called the specific rotation [α].

Eqn.JPG

[α] = specific rotation
α = observed rotation
c = concentration of the solution of an enantiomer
l = length of the tube (Polarimeter tube)

The degree of rotation of plane-polarized light depends on the number of chiral molecules that it encounters on its way through the tube of polarimeter (thus, the length of the tube and concentration of the enantiomer). In many cases, it also depends on the temperature and the wavelength of light that is employed.

Levorotation and dextrorotation are the properties of chiral molecules that are enantiomers of each other.

Notes and references

Solomons, T.W. Graham, and Graig B. Fryhle. Organic Chemistry. 8th. Hoboken: John Wiley & Sons, Inc., 2004.
  1. The first word component levo- comes from Latin word for “on the left side”.



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