Help understanding this passage on optical activity Other than mirror images, compounds having opposite configurations do not necessarily cause opposite directions of optical rotation any more than do compounds of the same configuration necessarily cause the same direction of optical rotation.


I know what optical rotation is and I know what "molecular configuration" means but I still can't understand what I just wrote above.
I believe what is causing you confusion is the difference between optical activity and absolute configuration.

A molecule with a single stereocenter such as limonene and carvone have two stereoisomers, one rotates polarized light to the left (-) and the other to the right (+). Additionally, each one has its own absolute configuration in their chiral center, which can be R or S. The thing is, R and S configurations are not attached or are, anyhow, synonyms with (+) and (-) configurations. Carvone and limonene are good examples:

 



In carvone, the S-enantiomer rotates polarized light to the left and so it is named S-(+)-carvone. The S-enantiomer in limonene rotates polarized light to the right and so it is named S-(-)-limonene. The two nomenclature systems are different since they specify distinct properties.

I hope this answers your question. If not, I would be glad to help in anyway I can.