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When looking at the source of light, the rotation of the plane of polarization may be either to the right (dextrorotatory or dextrorotary — d-rotary, represented by (+), clockwise), or to the left (levorotatory or levorotary — l-rotary, represented by (−), counter-clockwise) depending on which stereoisomer is dominant.
Specific rotation is an intensive property, distinguishing it from the more general phenomenon of optical rotation. As such, the observed rotation (α) of a sample of a compound can be used to quantify the enantiomeric excess of that compound, provided that the specific rotation ([α]) for the enantiopure compound is known.
Clockwise rotation of the light traveling toward the viewer is labeled (+) enantiomer. Its mirror-image is labeled (−). The (+) and (−) isomers have been also termed d-and l-(for dextrorotatory and levorotatory); but, naming with d-and l-is easy to confuse with D - and L - labeling and is therefore discouraged by IUPAC. [13]
The (+) or (−) symbol is used to specify a molecule's optical rotation — the direction in which the polarization of light rotates as it passes through a solution containing the molecule. [12] When a molecule is denoted dextrorotatory, it rotates the plane of polarized light clockwise and can also be denoted as (+). [11]
A right handed rotation is dextrorotary (d); that to the left is levorotary (l). The d- and l-isomers are the same compound but are called enantiomers. An equimolar mixture of the two optical isomers, which is called a racemic mixture, will produce no net rotation of polarized light as it passes through. [19]
In all materials the rotation varies with wavelength. The variation is caused by two quite different phenomena. The first accounts in most cases for the majority of the variation in rotation and should not strictly be termed rotatory dispersion. It depends on the fact that optical activity is actually circular birefringence.
The other refers to Optical rotation, when looking at the source of light, the rotation of the plane of polarization may be either to the right (dextrorotary — d-rotary, represented by (+), clockwise), or to the left (levorotary — l-rotary, represented by (−), counter-clockwise) depending on which stereoisomer is dominant. For instance ...
Although the two enantiomers rotate plane-polarized light in opposite directions, the rotations cancel each other out because they are present in equal amounts of negative (-) counterclockwise (levorotatory) and positive (+) clockwise (dextrorotatory) enantiomers. [6]