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Size:  195.8 KB HI for this question I have identified that X is an alcohol with the molecular formula C3H8O. The mass spec has an error as the molecular ion peak should be at m/z 60. What I don't get is the absorption with the black arrow next to it is showing. The mark scheme doesn't mention anything about it but it looks significant though.

    Thanks
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    (Original post by coconut64)
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Size:  195.8 KB HI for this question I have identified that X is an alcohol with the molecular formula C3H8O. The mass spec has an error as the molecular ion peak should be at m/z 60. What I don't get is the absorption with the black arrow next to it is showing. The mark scheme doesn't mention anything about it but it looks significant though.

    Thanks
    Any stretches that high in the IR spectrum (more than ~2500cm^-1) are bound to be stretches of X-H bonds.

    The nice broad stretch at the highest frequency is the alcohol O-H stretch. (X=O here)

    The question tells you its a saturated molecule, so what could be causing the other X-H stretch
    X=?
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    (Original post by MexicanKeith)
    Any stretches that high in the IR spectrum (more than ~2500cm^-1) are bound to be stretches of X-H bonds.

    The nice broad stretch at the highest frequency is the alcohol O-H stretch. (X=O here)

    The question tells you its a saturated molecule, so what could be causing the other X-H stretch
    X=?
    This C-H bond doesn't show that X is alkane though, it can only show that there are alkyl groups in the molecule.
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    The reason why the MS doesn't mention it (i.e. mentioning it won't score you marks) is that it is caused by a C-H bond. Not too many organic molecules don't have a C-H bond in them. The exam writer didn't want to let you have a mark for identifying something that was more or less bound to be present.
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    (Original post by coconut64)
    This C-H bond doesn't show that X is alkane though, it can only show that there are alkyl groups in the molecule.
    Sorry I might have caused some confusion by using X as the unknown in my previous comment, I just meant to refer to the atom bonded to the hydrogen, which for this peak is Carbon. The unknown compound (that the question calls x) there fore contains C-H bonds.

    You're right, this peak doesn't show its an alkane. That why it isn't mentioned in the mark scheme. C-H stretches appear in almost every organic IR spectrum, so they aren't useful in determining which compound you have!

    So the peak is due to C-H stretching, but that doesn't give you much useful information for determining structure of the unknown.
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    (Original post by MexicanKeith)
    Sorry I might have caused some confusion by using X as the unknown in my previous comment, I just meant to refer to the atom bonded to the hydrogen, which for this peak is Carbon. The unknown compound (that the question calls x) there fore contains C-H bonds.

    You're right, this peak doesn't show its an alkane. That why it isn't mentioned in the mark scheme. C-H stretches appear in almost every organic IR spectrum, so they aren't useful in determining which compound you have!

    So the peak is due to C-H stretching, but that doesn't give you much useful information for determining structure of the unknown.
    How does the bit in pink work? I thought each absorption in the spec shows you what bond it is. How does this work?!

    Thank you
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    (Original post by coconut64)
    How does the bit in pink work? I thought each absorption in the spec shows you what bond it is. How does this work?!

    Thank you
    sorry if i'm being dim, but which bit in pink?
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    (Original post by MexicanKeith)
    sorry if i'm being dim, but which bit in pink?
    Your previous post. I don't get why the stretching of the bond will be shown I the spec. Thanks
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    (Original post by coconut64)
    Your previous post. I don't get why the stretching of the bond will be shown I the spec. Thanks
    Oh yep sorry I see it now.

    Bond stretching and bending is what IR spectrums show, each adsorbtion in the spectrum corresponds to certain bonds in the molecule absorbing that particular wavelength of radiation.

    The wavelength and shape of the absorption band is characteristic of certain bonds and the region around 2900cm^-1 with a narrow band is characteristic of C-H bonds stretching, a peak around 1700cm^-1 indicates a C=O stretch, a peak around 3200cm^-1 that's broad is due to alcohol protons (the O-H stretch) as some other examples.

    so the spectrum you have has peaks for O-H and C-H, the peaks further right on the spectrum are due to bends in the molecule, and C-C stretches, but they are all close together so you can't get much information from this region (its often called the fingerprint region)
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    (Original post by MexicanKeith)
    Oh yep sorry I see it now.

    Bond stretching and bending is what IR spectrums show, each adsorbtion in the spectrum corresponds to certain bonds in the molecule absorbing that particular wavelength of radiation.

    The wavelength and shape of the absorption band is characteristic of certain bonds and the region around 2900cm^-1 with a narrow band is characteristic of C-H bonds stretching, a peak around 1700cm^-1 indicates a C=O stretch, a peak around 3200cm^-1 that's broad is due to alcohol protons (the O-H stretch) as some other examples.

    so the spectrum you have has peaks for O-H and C-H, the peaks further right on the spectrum are due to bends in the molecule, and C-C stretches, but they are all close together so you can't get much information from this region (its often called the fingerprint region)
    SO the C-H bond refers to the alkyl group since X is saturated?
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    (Original post by coconut64)
    SO the C-H bond refers to the alkyl group since X is saturated?
    In this case yep.

    Do you see why that peak isn't of much interest/use?

    The C-H bonds appear in virtually every organic molecule so that isn't useful information
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    (Original post by MexicanKeith)
    In this case yep.

    Do you see why that peak isn't of much interest/use?

    The C-H bonds appear in virtually every organic molecule so that isn't useful information
    thanks
 
 
 
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