How Specific Rotation Is Measured

A chiral compound rotates the plane of polarization of plane-polarized light. 

A compound that rotates the plane of polarization of plane-polarized light is said to be optically active. In other words, chiral compounds are optically active, and achiral compounds are optically inactive. 

If an optically active compound rotates the plane of polarization clockwise, then the compound is said to be dextrorotatory, which can be indicated in the compound s name by the prefix (-I-). If it rotates the plane of polarization counterclockwise, then it is said to be levorotatory, which can be indicated by (-). Dextro and levo are Latin prefixes for to the right and to the left, respectively. Sometimes lowercase d and I are used instead of (-h) and (-). 

Do not confuse (-h) and (-) with R and S. The (+) and (-) symbols indicate the direction in which an optically active compound rotates the plane of polarization of plane-polarized light, whereas R and S indicate the arrangement of the groups about an asymmetric center. Some compounds with the R configuration are (-I-) and some are (—). Likewise, some compounds with the S configuration are (-I-) and some are (—). 

For example, (S)-lactic acid and (S)-sodium lactate both have an S configuration, but (S)-lactic acid is dextrorotatory whereas (S)-sodium lactate is levorotatory. When we know which direction an optically active compound rotates the plane of polarization, we can incorporate (+) or (-) into its name. 

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