Orgo 12.9-12.12

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Orgo 12.9-12.12
2013-12-29 19:15:13
CHM 202
Organic Chemistry
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  1. Because it has a __, the C=O double bond produces intense __. __ absorb at frequencies around 1700 cm^-1, but the exact frequency depends on the specific __ and hte rest of the molecule. For these reasons, __ is often the best method for detecting and identifying the type of carbonyl group in an unknown compound.
    • infrared stretching absorptions
    • carbonyl groups
    • functional group
    • infrared spectroscopy
  2. The C=O stretchign vibrations of simple __ and __ occur at frequencies around 1710 cm^-1. __ are a little higher, about 1725 cm^-1. These frequencies are higher than those for C=C double bonds because the C=O double bond is __ and __. __ may be so intense that they produce small overtone peaks around 3400 cm^-1, double their __.
    • ketones
    • carboxylic acids
    • aldehydes
    • stronger and stiffer
    • Carbonyl absorptions
    • fundamental frequency
  3. In addition tot he strong C=O stretching absorption, an __shows a characteristic set of two low-frequency C-H stretching frequencies around 2700 and 2800 cm^-1. Neither a __ nor an __ produces absorptions at these positions.
    • aldehyde 
    • ketone nor an acid
  4. A __ produces a characteristic broad O-H absorption in addition to the intense carbonyl stretching absorption. Because of the unusually strong __ in __, the broad O-H stretching frequency is shifted to about 3000 cm^-1 centered on top of the usual C-H absorpiton. This broad O-H absorption gives a characteristic __ to the peaks in the C-H stretching region. Participation of the acid __ in __ frequently results in broadening of the strong __absorption as well.
    • carboxylic acid
    • hydrogen bonding
    • carboxylic acids
    • overinflated shape
    • carbonyl group
    • hydrogen bonding
    • carbonyl
  5. __of a C=C double bond lowers its stretching frequency. This is also true of __. __of the __electrons reduces the __ of the carbonyl double bond, weakening it and lowering the stretching frequency from about 1710 cm^-1 to about 1685 cm^-1 for conjugated __, __, and __.
    • Conjugation 
    • conjugated carbonyl groups
    • Delocalization 
    • pi 
    • electron density
    • ketones, aldehydes, and acids.
  6. The C=C absorption of a __ may not be apparent in the __ because it is so much weaker than the C=O absorption. The presence of the C=C double bond can still be inferred from its __ and the __
    • conjugated carbonyl compound
    • IR spectrum
    • effect on the C-O frequency
    • presence of unsaturated =C-H absorptions above 3000 cm^-1.
  7. The very low frequency of the amide carbonyl might be mistaken for an __.

    Like __, most __ show two spikes in the N-H stretching region, as in the butyraminde spectrum. __ generally show one N-H spike.
    • alkene C=C stretch
    • primary amines
    • primary amides
    • Secondary amides
  8. Some carbonyl groups absorb at frequencies higher than 1725 cm^-1. For ex., simple __ absorb around 1735 cm^-1. These higher frequency absorptions are also seen in strained __. In a small ring, the __ on the carbonyl group forces more __ into the C=O double bond, resulting in a stronger, stiffer bond.
    • carboxylic esters
    • cyclic ketones (in a five membered ring or smaller)
    • angle strain
    • electron density
  9. __ of carbon-nitrogen bonds are similar to those of carbon-carbon bonds, except that carbon-nitrogen bonds are more __ and give __. Carbon-nitrogen single bonds absorb around 1200 cm^-1, in a region close to many C-C and C-O absorptions. tehrefore, the C-N single bond stretch is rarely useful for structure determination.
    • infrared absorptions
    • polar
    • stronger absorptions
  10. __ absorb in the same region as C=C double bonds, around 1660 cm-1; however, the C=N bond gives rise to __ because of its __. The C=N stretch often resembles a __ absorption in intensity. The most readily recognized carbon-nigrogen bond is the __ of a __. The stretching frequency of the __ is close to that of an __, about 2200 cm-1; however, __ generally absorb above 2200 cm-1, while __ absorb below 2200 cm-1. Also, __ are more polar than C***C triple bonds, so nitriles usually produce stronger absorptions than alkynes.
    • carbon-nitrogen double bonds
    • stronger absorptions
    • greater dipole moment
    • carbonyl
    • triple bond
    • nitrile
    • nitrile C***N bond
    • aceetylenic C***C tripe bond
    • nitriles
    • alkynes
    • nitrile triple bonds
  11. C-N bond stretching frequencies
    • C-N 1200 cm-1
    • C=N 1660 cm-1
    • C***N >2200 cm-1

    • for comparison:
    • C***C <2200cm-1
  12. Strengths and Limitations of Infrared Spectroscopy:

    The most useful aspect of infrared spectroscopy is its ability to identify __. IR does not provide much info about the carbon skeleton or the alkyl groups in the compound, however. These aspects of the structure are more easily determined by __.
    • functional groups
    • NMR
  13. Ambiguities often arise in the interpretation of __. __ with other regions of the spectrum usualy enables us to determine which of these __ is present. In some cases, we cannot be entirely certain of the __ without additional info, usually provided by other types of spectroscopy.

    __ can provide conclusive proof that two compounds are either the same or different. The peaks in the fingerprint region depend on complex vibrations involving the entire molecule, and it is highly improbable for any two compounds (except __) to have precisely the same __.
    • IR spectra
    • familiarity
    • functional groups
    • functional gorup
    • infrared spectroscopy
    • infrared spectrum
  14. To summarize: an infrared spectrum is valuable in three ways. What are they
    • 1) it indicates the functional groups in the compound
    • 2) it shows the absence of other functional groups that would give strong absorptions if they were present
    • 3) it can confirm the identity of a compound by comparison with a known sample