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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.

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  • $\begingroup$ The meaning's pretty clear (at least when I read it). I would paraphrase it as: "Compounds having opposite configurations do not necessarily cause opposite directions of optical rotation... just as compounds of the same configuration do not necessarily cause the same direction of optical rotation; with mirror images of a given molecule being an obvious exception." Give this a re-read ;) $\endgroup$ – paracetamol Jul 18 '17 at 16:54
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    $\begingroup$ @paracetamol but if two molecules have opposite configurations, doesnt that mean they are mirror images of each other ? $\endgroup$ – Max white Jul 18 '17 at 17:37
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    $\begingroup$ @paracetamol sorry i am very late to reply but i hope that i can still get an answer from you. Anyway, what you are trying to say is that if molecule A has an opposite configuration to molecule B which is of a different compound, they dont necessarily cause opposite effects on plane polarised light. And even if these molecules had same configurations , they wouldnt necessarily cause same effect on plane polarized light. They are just putting a line between the D/L system and +/- system, right ? $\endgroup$ – Max white Aug 29 '17 at 13:10
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    $\begingroup$ So this means that levo signifies one direction of optical rotation (and dextro signifies the opposite direction) , right ? And the D/L system is only about the arrangement of the molecules in space ? $\endgroup$ – Max white Aug 29 '17 at 14:15
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    $\begingroup$ Yeah i got what you said. Thanks a lot man i finally understood this. @paracetamol $\endgroup$ – Max white Aug 29 '17 at 14:22
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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:

Carvone

Limonene

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.

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  • $\begingroup$ Sorry but i dont know what the terms "absolute configuration" and "stereocenter" mean. I have tried looking them up and i will continue trying to understand what they mean after i post this comment. $\endgroup$ – Max white Jul 18 '17 at 21:51
  • $\begingroup$ do you think the guy in the first comment (paracetamol) paraphrased it correctly ? because i thought that "opposite configuration" means mirror image. If so then why wouldn't they necessarily cause opposite directions of optical rotations ? $\endgroup$ – Max white Jul 18 '17 at 21:56
  • $\begingroup$ I am sorry, I didn't know how much you knew about this topic so I assumed you already knew what those terms mean. A stereocenter in organic chemistry is a carbon atom that can originate chirality or is chiral (i.e has 4 different substituents attached to it). If the molecule has only 1 stereocenter (or chiral carbon), it will have 2 optical isomers, one is called R, the other S (a nice introductory video by Khan Academy about this can be found at khanacademy.org/science/organic-chemistry/stereochemistry-topic/… ) $\endgroup$ – Raul Luciano Jul 18 '17 at 22:29
  • $\begingroup$ What i am not understanding about stereocenters is how every website is saying that exchanging two groups connected to the stereocenter leads to the formation of a stereoisomer but i can see the molecule looking the same if we just rotated it. $\endgroup$ – Max white Jul 18 '17 at 22:30
  • $\begingroup$ Chirality is a broad concept that is present not only in organic chemistry, but in everything, from inorganic compounds to everyday structures such as your hands! Something chiral does not have apparent symmetry. $\endgroup$ – Raul Luciano Jul 18 '17 at 22:32

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