Anonymous
Anonymous asked in Science & MathematicsChemistry · 1 decade ago

specific rotation of enantiomers?

What would be a reason that the specific rotation of enantiomers would differ (only by a degree or so) in magnitude. i know normally they are suppose to be equal and opposite, but i need reasoning why it can differ and i cant seem to find a straight forward answer thanks! :)

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  • Anonymous
    1 decade ago
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    You are correct in thinking that the specific rotation of enantiomers should be equal and opposite.

    The answer to your question can be found by looking at the criteria used to define specific rotation:

    “The specific rotation of a chemical compound [α] is defined as the observed angle of optical rotation α when plane-polarized light is passed through a sample with a path length of 1 decimeter and a sample concentration of 1 gram per 1 millilitre. The specific rotation of a pure material is an intrinsic property of that material at a given wavelength and temperature.”

    http://en.wikipedia.org/wiki/Specific_rotation

    You can see that there are a number of factors that can have an effect on your readings: Small variations in the wavelength of light, the temperature, the concentration and path length will all produce minor differences in your readings.

    The optical purity of your samples will have a significant effect on the specific rotation. Natural products can be isolated in an optically pure state. Producing the “un-natural” enantiomer often requires a lengthy synthetic process that inevitably produces some racemisation (optical scrambling) in the product. Separation of the enantiomers with 100% efficiency can sometimes be problematic.

    http://en.wikipedia.org/wiki/Enantiomeric_excess

    Finally, there is a problem associated with spontaneous “natural” racemisation of samples. It is well known that the optical purity of natural amino acids will deteriorate over time.

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    ADDITIONAL NOTES:

    As stated above, the specific rotation of a sample is equal and opposite for most compounds.

    There are examples where this might not be true. Polar compounds (acids, alcohols etc) can form hydrogen bonded dimers or oligomers which, being different species to the monomers, may have different specific rotation. For example, two or more chiral molecules of a particular enantiomer may react to give a new optically active species. If , for steric reasons, the opposite chiral molecules cannot hydrogen bond in the same way, a different new optically active species can result. These hydrogen bonded dimers (or oligomers) will not exhibit equal and opposite rotation.

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