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I have just studied the concept of cyclotron in carbohydrates. My reference book used Glucose as an example and began by saying we can find D-glucose or L-glucose:

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Depending on what enantiomer we have, we will get opposite chair conformations at cyclisation:

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After saying this, my book states that significant differences appear at cyclisation of these enantiomers, and there I just do not follow. The only difference I see is that one is the mirror image of the other. But visually, I see both structures identically equal.

Could someone explain to me what differences I should be observing in both chair conformations, and also, what consequences these have on the molecules' behaviors?

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  • $\begingroup$ They're different. Therefore, they have different chemical properties. $\endgroup$
    – Zhe
    May 22 '18 at 20:46
  • $\begingroup$ They're as visually identical as a left hand and a right hand are visually identical. $\endgroup$
    – Zhe
    May 22 '18 at 20:47
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They are mirror images of one another and therefore their chemical and physical properties are the same, except for the deflection of polarized light, which will be in opposite directions.

Mirror image compounds that are not the same are called enantiomers and it does not matter if they are in the open or cyclic form. They will, however, have different properties if they are in a chiral medium. For instance, if placed in a chiral solvent, they will have different solubilities. Chiral means that the compound has a mirror image that can not be superimposed on the original structure.

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  • $\begingroup$ When you state "their chemical and physical properties are the same", are you suggesting that they behave the same biologically, and that one can substitute L-glucose for D-glucose in one's diet? $\endgroup$
    – Dr. J.
    May 23 '18 at 10:43
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    $\begingroup$ No. Biological environments are chiral, so they can not be replaced. Many biochemical reactions occur only on one of the enantiomers. $\endgroup$
    – Raoul Kessels
    May 23 '18 at 10:47
  • $\begingroup$ Ah ha, makes sense. $\endgroup$
    – Dr. J.
    May 23 '18 at 11:10
  • $\begingroup$ Thank you, I knew about the polarizing of light and completely forgot about it until I read your answer. Is this the only difference when in a non-chiral medium? And when you say that the chiral means the compound's mirror image cannot be superimposed with the original structure -- could you give an example diagram or something? $\endgroup$
    – Bee
    May 24 '18 at 6:46
  • $\begingroup$ @Bee The two structures you have drawn in your question are like that. No matter how you turn them, they will never be the same. Also if you look at your hands, there is now way to get the same disposition, so a left hand will never fit into a right hand glove. In fact, chiral comes from the Greek kheír wich means hand. $\endgroup$
    – Raoul Kessels
    May 24 '18 at 7:23

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