Explore BrainMass

Explore BrainMass

    GC of the diastereoisomers of 3,3,5-trimethylcyclohexanol

    Not what you're looking for? Search our solutions OR ask your own Custom question.

    This content was COPIED from BrainMass.com - View the original, and get the already-completed solution here!

    I need the cis and trans alcohol % from the attached information received by GC. The solvent was methanol. I believe this is done by using the peak area but I'm not sure about how to go about it.

    © BrainMass Inc. brainmass.com December 24, 2021, 11:17 pm ad1c9bdddf
    https://brainmass.com/chemistry/inorganic-chemistry/diastereoisomers-trimethylcyclohexanol-554354

    Attachments

    SOLUTION This solution is FREE courtesy of BrainMass!

    Q. Preparation and properties of the diastereoisomers of 3,3,5-trimethylcyclohexanol
    I need the cis and trans alcohol % from the attached information received by GC. The solvent was methanol. I believe this is done by using the peak area but I'm not sure about how to go about it.

    A. Unfortunately I am unable to open .xlsx files at this moment (.xls files are ok), here is the guideline on how to determine cis-trans content from GC plot:

    Chromatographic techniques separate mixtures of species based on their interactions with
    a stationary phase (e.g. fused silica column) and a mobile phase (e.g. methanol). The column is usually derivatized to control its polarity (ranging from hydrophobic dimethylsiloxane to hydrophilic polyethylene glycol or any of a wide array of specialty coatings). The analyte molecules interact with the surface and exist in dynamic equilibrium between being adsorbed onto the inner wall of the column and dissolved in the carrier liquid. Various components of mixtures will have different equilibria with the stationary phase and will consequently move through the column at different rates. The temperature can be left constant for maximum resolution or varied to minimize analysis time. A detector at the end of the column records the amount of time required for a given compound to elute off of the column. With the proper conditions and column chemistries, very similar compounds can be separated by GC (e.g. isomers of dichlorobenzene, cis- and trans- fatty acids, racemic mixtures, etc.).

    So the amount of peak separation determines the analyte molecules (e.g. cis and trans isomers).

    In this experiment, 3,3,5-trimethylcyclohexanone (1), a racemic ketone with one chiral center, is reduced with sodium borohydride to produce two alcohols, trans2 and cis3.

    Conformational analysis of the two diastereomeric alcohols suggests that the trans alcohol, 2, will tend to keep the OH axial, while the cis isomer, 3, will have an equatorial OH.
    It is well documented that functional groups in an axial orientation are sterically more hindered than their equatorial counterparts. The enhanced accessibility of an equatorial hydroxy group can be seen from the following facts:

    • the boiling point of the cis isomer is higher than the trans isomer
    • the cis isomer runs slower on silica gel than the trans isomer (this is what you are interested in)
    • the cis isomer reacts five times faster than the trans isomer with acetic anhydride in pyridine
    Since the cis and trans compounds are diastereomeric, they produce different IR and 1H NMR spectra, from which the ratio of cis/trans isomers and hence their % can be determined. Read the attached PDF files for details. Alternatively the areas under GC peaks (after normalization using response factors) will give the amounts of isomers directly.

    [Response factor is the ratio between a signal produced by an analyte and the quantity of analyte which produces the signal.
    The response factor can be expressed on a molar, volume or mass basis:

    where A is the signal (e.g. peak area) and the subscript i indicates the sample and the subscript st indicates the standard. The response factor of the standard is assigned an arbitrary factor, for example 1 or 100.]

    Source (PDF attached):
    [1] http://academic.reed.edu/chemistry/alan/201_202/lab_manual/expt_nabh4/background.html
    [2] http://ww2.chemistry.gatech.edu/~kb12/3380/reduct.pdf

    This content was COPIED from BrainMass.com - View the original, and get the already-completed solution here!

    © BrainMass Inc. brainmass.com December 24, 2021, 11:17 pm ad1c9bdddf>
    https://brainmass.com/chemistry/inorganic-chemistry/diastereoisomers-trimethylcyclohexanol-554354

    Attachments

    ADVERTISEMENT