Specific Rotation of Chiral Molecules

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Discussion Overview

The discussion revolves around the specific rotation of chiral molecules, specifically (R)-(+)-limonene, and factors that may cause its measured specific rotation to differ from literature values. The scope includes theoretical considerations, experimental methodology, and potential influences of solvents and impurities on optical activity.

Discussion Character

  • Homework-related
  • Exploratory
  • Technical explanation

Main Points Raised

  • One participant questions why the presence of achiral methylene chloride in the sample would affect the specific rotation of (R)-(+)-limonene, suggesting that it should not have an impact.
  • Another participant inquires about the necessary values for calculating specific rotation and whether the solvent could influence those values.
  • A further contribution mentions that ethanol, the solvent used, should not affect the optical activity, but raises the question of other factors, such as methylene chloride, that might have an effect.
  • One participant prompts others to consider the calculation process of specific rotation and how the angle of rotation obtained from a polarimeter is used in that calculation.

Areas of Agreement / Disagreement

Participants express uncertainty regarding the influence of solvents and impurities on specific rotation, with no consensus reached on the effects of methylene chloride or the calculation process.

Contextual Notes

There are unresolved questions about the assumptions underlying the calculation of specific rotation and the potential impact of different solvents and impurities on optical activity.

Cyberchicken
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Homework Statement


Why may the specific rotation of a sample of (R)-(+)-limonene be less than the literature value of (R)-(+)-limonene. Assume that there are so (S) enantiomers present.


Homework Equations





The Attempt at a Solution


There are no (S) enantiomers, but there are some molecules of methylene chloride mixed into the sample that was analyzed. But since these are achiral, I don't understand why they should affect the rotation of the limonene.
 
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When you calculate the specific rotation of a sample, what values do you need to know? Would the presence of solvent affect any of those values?
 
Ygggdrasil said:
When you calculate the specific rotation of a sample, what values do you need to know? Would the presence of solvent affect any of those values?

The solvent used was ethanol, so that should not affect the optical activity. Are there any other factors which would affect it? Like the presence of methylene chloride perhaps? And why would it affect it?
 
Again, think about how you calculate the specific rotation of a sample. You place your sample in the polarimerter and obtain the angle by which your sample rotated the light. How do you calculate the specific rotation from that value?
 

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