Orgo 13.5-13.7

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  1. The variations in the positions of NMR absorptions, arising from electronic shielding and deshielding, are called __.

    Explain it.
    chemical shift

    the difference (in ppm) between the resonance frequency of the proton being observed and tha tof tetramethylsilane
  2. A more accurate method for expressing chemical shifts is to do what?

    The difference?
    determine the value relative to a reference compound added to the sample

    in the magnetic field strength between the resonances of hte samlple protons and the reference protons can be measured very accurately
  3. The most common NMR reference compound is __. Because silicon is __ than carbon, the metyhl groups of TMS are relatively __, and their protons are __. Because they absorb at a higher field strength than nmost hydrogens bonded to carbon or other elements, so most NMR signals appear __ (to the left, __) of the TMS signal.
    • tetramethylsilane
    • less electronegative
    • electron rich
    • well shielded
    • downfield
    • deshielded
  4. For each type of proton, the distance downfield of TMS is the __ of those protons. Newer spectrometers operate at a constant magnetic field, and they measure the chemical shift as a __.
    • the chemical shift
    • the difference in the magnetic field between where the protons in the sample absorb and where those in TMS absorb
  5. For each tpe of proton, th distance downfield of TMS is the __. Newer spectrometers operate at a constant magnetic field, and they measure the chemical shift as a __.
    • chemical shift of those protons
    • frequency difference between teh resonances of the protons in the sample and those in TMS
  6. Chemical shifts are measured in __. The difference (the chemical shift) between the NMR signal of a proton and that of TMS is shown on the __ calibrated in __. 

    A chemical shift in ppm can be calculated by __.
    • ppm
    • horizontal axis
    • frequency units

    dividing the shift measured in hertz by the spectrometer frequency measured in millions of hertz spectrum
  7. chemical shift equation?
    shift downfield from TMS/ total spectrometer frequency
  8. The most common scale of chemical shifts is the __.
    delta scale
  9. The chemical shift of a methyl group in an alkane is about __.

    In general, the effect of an electron-withdrawing substituent __, adn the effects are usually negligible on protons that are separated from teh electronegative group by __ bonds.
    • delta 0.9
    • decreases with increasing distance
    • four or more
  10. Protons that are more distant than the __ are deshieled by a __.

    If more than one electron-withdrawing grouop is present, __.
    • beta protons
    • negligible amount

    the deshielding effects are nearly (but not quite) additive
  11. Double bonds and aromatic rings produce large __ on their __ and __ protons. 

    These deshielding effects result from the same type of __ that normally shields nuclei from the magnetic field.

    In benzene and its derivatives, the aromatic ring of pi bonding electrons acts as a __, and the external magnetic field induces a __. At the center fo the ring the induced field acts to __. These induced field lines curve around, however, and on the edge of the ring the induced field adds to the external field. As a result, the aromatic protons are strongly __, resulting in a large chemical shift.
    • deshielding effects
    • vinyl
    • aromatic protons
    • circulation of electrons
    • conductor
    • ring current
    • oppose the external field
    • deshielded
  12. The pi electrons of an alkene do what?

    Since the pi bond of an alkene deshields the vinyl protons, we might expect an acetylenic hydrogen to be even __. The opposite is true:
    deshield the vinyl protons in the same way that an aromatic ring of electrons deshieds the aromatic protons; but it is not as large

    even more deshielded by the two pi bonds of hte triple bond

    acetylenic hydrogens absorb around dlta 2.5 compared with 5 or 6 for vinyl protons
  13. Aldehyde protons absorb at even __ than vinyl and aromatic protons: between __.

    The chemical shifts of O-H protons in alcohols and NH protons in amines depends on the __. In concentrated solutions, these protons are deshield by __, and they absorb at a relatively low field: about __for an amine NH and about __for an alcohol OH. When the alcohol or amine is diluted with a non-hydrogen bonding solvent such as CCl4, hydrogen bonding becomes __. In dilute solutions, these signals are observed around __.
    • lower fields 
    • delta 9 and delta 10

    • concentration
    • hydrogen bonding
    • delta 3.5 
    • delta 4.5 
    • less important
    • delta 2
  14. Carboxylic acid protons have considerable positive character because??

    They are strongly __ and absorb at chemical shifts __.

    Carboxylic acids frequently exist as __, with moderate rates of proton exchange that broaden the absorption of the acid proton.
    they are bonded to an oxygen next to a carbonyl group

    • deshielded
    • greater than d10

    hydrogen-bonded dimers
  15. In general, the number of __ corresponds to the number of different kinds of protons present in the molecule.
    NMR signals
  16. Protons in identical chemical environments with the same shielding have the __. Such protons are said to be __. This is what is meant whenever we use teh term __ in discussing NMR spec. In methyl tert-butyl ether, the three __ are chemcialyl equivalent and the nine tert0butly protons are chemically equivalent.
    • chemical shift
    • chemically equivalent
    • equivalent
    • methoxy protons
  17. Protons that are not chemically equivalent but happen to absorb at the same chemical shift are said to be __.
    accidentally equivalent
  18. The area under a peak is proprotional to __.
    the number of H contributing to that peak

Card Set Information

Orgo 13.5-13.7
2014-02-13 11:57:56
CHM 202
Organic Chemistry
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