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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.

paracetamol
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Max white
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  • 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 ;) – paracetamol Jul 18 '17 at 16:54
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    @paracetamol but if two molecules have opposite configurations, doesnt that mean they are mirror images of each other ? – Max white Jul 18 '17 at 17:37
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    The main confusion is with the word "compound." We are not talking about a property that holds true in all compounds. We are saying no relationship of the type described exists when comparing compound A with compound B, unless compound B is the mirror image of compound A (thus implying opposite configurations). This is just a terribly worded statement. – Zhe Jul 18 '17 at 18:50
  • The issue is that knowing the absolute configuration around a chiral centre doesn't help you work out which way the molecule will rotate polarised light. So two different molecules with a similar chiral centre might rotate light the opposite way even though the immediate configuration around the chiral centre is the same. – matt_black Jul 19 '17 at 00:01
  • @Max Example (If you're familiar with Fischer's D-L notation): D-(+)-glucose is dexterotary, while its mirror image, L-(-)-glucose is levorotary. But, D-(-)-fructose, which has the same "conformation" as D-(+)-glucose, is levorotary. The mirror image of D-(-)-fructose is L-(+)-fructose and has the same "conformation" as L-(-)-glucose, but is instead, dexterotary. – paracetamol Jul 19 '17 at 04:40
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    @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 ? – Max white Aug 29 '17 at 13:10
  • @Max Yup, sounds like you got it right ;). The D/L system (No, they don't mean "dextro/levo"...that's another thing) is assigned to a molecule based on its stereochemistry (the relative positioning of its atoms/ groups in space); whereas the +/- system (This one is "dextro/levo") is based on the optical activity of the molecule (which is experimentally obtained). Just because some molecules have, say, the (D) form doesn't mean they're all going to be dextrorotary (+) or levorotary(-)... each molecule polarizes light in whatever way it "wants" to; it can only be experimentally found. – paracetamol Aug 29 '17 at 13:55
  • Once again, I suggest you compare optical activities (+/-) of the D/L forms of glucose and fructose; it might help to have a look at the molecular structures for these 4 molecules). D-glucose and D-fructose have the same configuration (obviously, since they both carry the prefix "D"), but they have opposite optical behavior (D-glucose is dextro, D-fructose is levo). Similarly, L-glucose and L-fructose have the same configuration, but different optical activities (L-glucose is levo, L-fructose is dextro). – paracetamol Aug 29 '17 at 14:03
  • Another useful comparison of optical activities would be between the D/L forms of glucose and glyceraldehyde. D-glucose and D-glyceraldehyde have the same configuration, and they happen to have the same optical behavior (both are dextro). Similarly, L-glucose and L-glyceraldehyde have the same configuration and they happen to have the same optical behavior (both are levo). – paracetamol Aug 29 '17 at 14:07
  • @paracetamol but my book was saying that D/L is the same thing as Dextro/levo, it is only the short form of the words. If not then what is the difference between these two ? – Max white Aug 29 '17 at 14:09
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    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 ? – Max white Aug 29 '17 at 14:15
  • Forgot to mention you @paracetamol – Max white Aug 29 '17 at 14:17
  • @Max Nope. When I was in middle-school, we used "d/l" for "dextro/levo" (not D/L). When you start taking high-school or undergrad Chem, you'll realize that +/- is the preferred notation for "dextro/levo". The D/L system was proposed by Fischer to help him with sugars/sugar-like molecule book-keeping. Gimme, minute....I'll get to what the D/L system's about. – paracetamol Aug 29 '17 at 14:18
  • This might help: [ https://chemistry.stackexchange.com/questions/44260/what-is-the-difference-between-d-and-l-configuration-and-and-%E2%88%92 ] Also, read the comments under the answer to that question. – paracetamol Aug 29 '17 at 14:20
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    Yeah i got what you said. Thanks a lot man i finally understood this. @paracetamol – Max white Aug 29 '17 at 14:22
  • Glad to hear that. @Mods If I get the time (don't think it'll be anytime soon), I'll try to compress all these comments into an answer and post it here. It should make it easier for the OP and the others to refer to later ;) – paracetamol Aug 29 '17 at 14:30
  • Oops, I missed this comment: "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 ?" Yes, that's right. I went and said "Nope" to D/L = dextro/levo. Also, "levo" is specifically LEFT-polarized, while dextro is specifically RIGHT polarized. And obviously, they are opposite. O:) – paracetamol Aug 29 '17 at 14:32

1 Answers1

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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.

Mathew Mahindaratne
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Raul Luciano
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  • 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. – Max white Jul 18 '17 at 21:51
  • 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 ? – Max white Jul 18 '17 at 21:56
  • 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 https://www.khanacademy.org/science/organic-chemistry/stereochemistry-topic/chirality-r-s-system/v/introduction-to-chirality ) – Raul Luciano Jul 18 '17 at 22:29
  • 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. – Max white Jul 18 '17 at 22:30
  • 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. – Raul Luciano Jul 18 '17 at 22:32
  • If you happen to have access to molecular modeling kits (those with sticks and balls used in introductory organic chemistry courses) you can create your own chiral molecule and see for yourself that they are not the same thing just inverted. If you don't, then watch some videos suchs as this one from Khan Academy, they will eventually convince you that those are different compounds :) – Raul Luciano Jul 18 '17 at 22:43
  • @Max white:" other than mirror images" reads as "except for mirror images". Then explanation is as in comment of Zhe. – Alchimista Jul 19 '17 at 00:22