Optical Activity

We said that a enantiomers have the same chemical and physical properties. Well then, how can we tell the difference between enantiomers? What makes them unique (aside from smell and biological activity)?

The answer lies in optical activity. Please read sections 4.8 and 4.9 in the textbook.

Chiral compounds are optically active; they rotate plane-polarized light. Achiral compounds are optically inactive; they do not rotate light. Thus, saying something is optically active or not is the same as saying it is chiral or achiral.

Enantiomers rotate light in equal, but opposite directions. One enantiomer will be the (+) enantiomer and the other will be (-). (Note: There is no connection between +/- to R/S configuration.) When a 50:50 mixture of enantiomers is analyzed in a polarimeter, the specific rotation is 0. They cancel each other out! This type of mixture is known as racemic.

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