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2013-09-05 03:35:27
CHEM 403 IR Spectroscopy

IR- Hydrocarbons
Show Answers:

  1. C-H stretch (sp3 hybridized) alkanes?
    3000 cm-1 − 2840 cm-1, strong
  2. Methylene (−CH2) bending
    A.  ~1465 cm-1, medium, sharp
  3. A.  methyl (−C−H) bending

    B.  What band will the asymmetric bend typically absorb with (not resolve from)?
    A.  ~1450 cm-1 and ~1375 cm-1, medium, sharp

    B. The methylene bend at ~1475 cm-1
  4. A.  alkane geminal dimethyl group (2 −CH3 bonded to the same carbon)

    B.  alkane tert-butyl group?
    A.  2 strong bands equal in intensity centered around ~1375 cm-1.

    B.  1 sharp/weak band around 1390 cm-1 −1395 cm-1 and 1 sharp/strong band around 1360 cm-1 −1365 cm-1.
  5. A.  Explain how to determine a molecular formula from a mass using the law of 13.

    B.  Explain how to determine the index of hydrogen deficiency once a formula is proposed.
    Mass / 13 = n, r= remainder of mass.

    • CnHn+r
    • Calculate the difference in H from the formula and the calculated CxH([X*2]+2). 

    Add 1 for each group V element and subtract 1 H for each group 7 element.

    .  Divide this by two.  This is the number of pi bonds or rings.
  6. If a compound shows an index of hydrogen deficiency of 1, has only sp3 hydrocarbon absorbtions, and no−CH3 bend at ~1475 cm-1 and the compound, what can be concluded regarding the compound?
    It is a cyclic alkane.
  7. A.  What is a medium/weak sharp band at −720 cm-1 indicative of? 

    B.  What occurs to the position of this band as the number of this type of function groups decrease.
    A.  Rocking band of 4 or more methylene groups in a row ("long chain band").

    • B.  The absorbtion shifts to a higher frequency
    • [all the way to ~ 780 cm-1 for (−CH2)1]
  8. What is the wavenumber, strength, and appearance that a typical sp2 C-H stretch typically occurs at?
    3100-3010 cm-1, med-strong, sharp
  9. A.  What wavenumber range does the out of plane C-H bending occur for alkenes?
    A.  1000cm-1 − 650 cm-1
  10. What type of alkene C-H bending gives rise to:

    A.  two approximately equal strong bands at 990cm-1 and 910cm-1
    B.  1.  One strong band near 700 cm-1
         2.  One medium band near 970cm-1
    C.  One strong band near 890 cm-1
    D.  One medium intensity band near 815 cm-1
    E.  No C-H bending, absence of C=C stretching.
    • A.  Monosubstituted (vinyl) double bonds
    • B.  1.  cis and 2.  trans 1,2 disubstituted double bonds
    • C.  1,1 Disubstituted double bonds
    • D.  Trisubstituted double bonds
    • E.  Tetrasubstituted double bonds
  11. A.  Where does the C=C stretch occur in the IR spectrum?

    B.  What occurs to this absorbtion's intensity and location as the double bond is substituted.
    A.  1660 cm-1 − 1600 cm-1

    B.  Intensity is increased, and C=C is shifted to a lower frequency.
  12. A.  Where is the sp hybridized C-H stretch present in the IR spectra?  When is it present?

    B.  Where is the C≡C strech present in the IR spectra?  What happens when the bond is conjugated?  When is it NOT present?
    A.  ~3200 cm-1, sharp, medium strong.  Terminal alkynes only.

    B.  ~2150 cm-1;  conjugation moves stretch to lower freqencies;  disubstituted or symmetrically substituted give either no or weak absorbtion.
  13. What occurs to an IR absorbance frequency as a bond length gets longer, or weaker?
    The frequency shifts to a lower wavenumber relative to the anticipated wavenumber.
  14. Explain why cyclic compounds with ENDO double bonds have a slightly higher and increasing frequency for the C=C stretch as ring strain decreases?
    This is thought of in vectors.  Streching of the C=C bond is a motion that occurs at 180 degrees of the C=C bond.  When a ring is present, and the C-C bond angle is less than or greater than 90 degrees, there is coupling with the C=C bond which increases the bond frequency. 

    The same is the reason for the cyclopropene being less (45 degree bond angle)
  15. A.  As ring strain increases, what occurs to the C=C stretching with ENDO double bonds (double bond inside a ring)?  What is the exception to this rule?

    B.  Where does cycloheptene and cyclohexane C=C strech absorb?  What does this absorbtion decrease as the ring with the ENDO double bond gets smaller?

    C.  What if the ENDO double bond is at a ring fusion?

    D.  What occurs to this cyclic ENDO double bond as it is conjugated with alkyl groups?
    A.  C=C stretching frequency decreases.  Cyclopropene is the exception. 

    B.  Cycloheptene and cyclohexene C=C at about 1650 cm-1, all others decreased by ~35 cm-1

    C.  It is the equivalent of a ring with one less carbon (i.e. decreases frequency)

    D.  Wavenumber will increase.
  16. What is the effect of conjugated systems and carbonyl groups on the C=C stretch?
    Shifts the C=C stretch to a lower frequency.
  17. What does an aldehyde C-H stretch look like in the IR spectrum?
    2 weak peaks at ~2850 cm-1 and 2750 cm-1
  18. How does an exo (outside of a ring) C=C stretch change with:

    A.  Ring Strain
    B.  Ring Fusion

    C.  Explain why the frequency trends like this for exocyclic double bonds.
    A.  Increases frequency

    B.  Increases frequency

    C.  Smaller rings require the use of more p character to make the small bond angles of the ring.  This removes p character from the sigma bong of the double bond, and gives it more s character, thus giving it more s character.  This strengthens and stiffens the double bond.
  19. A.  Where is the aromatic =C-H (sp2) stretch located?

    B.  Where are the oop =C-H bending absorbtions located for aromatic compounds?  What can they be used to identify? 

    C.  What wavenumber can be used to confirm the bands from question B?  What are they?

    D.  What is the third absorbtion that can be used to identify an aromatic compound?
    A.  3050 to 3010 cm-1

    B.  900 to 690 cm-1, used to identify benzene ring substitution pattern. 

    C.  2000 cm-1 to 1667 cm-1.  They are overtone and combination bands.

    D.  The C=C stretch absorbtion.  2 equally sized strong/sharp absorbtions at ~1600 cm-1 and 1475 cm-1.  May obscure methylene scissoring and assymmetric methyl C-H bending.  Should be more intense though.
  20. Identify where the C-H bends of these aromatic substitutions occur:

    A.  Monosubstituted aromatic
    B.  Ortho (1,2) disubstituted aromatic
    C.  Meta (1,3) disubstituted aromatic
    D.  Para (1,4) disubstituted aromatic
    • A.  690-strong, 750-strong
    • B.  750-strong
    • C. 690-strong, 780-strong, 880-medium-weak
    • D.  800 to 850- strong
  21. What will the overtone/combination bands of the following substituted aromatics look like:

    A.  Monosubstituted aromatic
    B.  Ortho (1,2) disubstituted aromatic
    C.  Meta (1,3) disubstituted aromatic
    D.  Para (1,4) disubstituted aromatic
    • A.  4 weak peaks
    • B.  Variable peaks, 1 pronounced
    • C.  2 peaks and a weaker doublet closer to 1667
    • D.  2 peaks, with the first one being stronger.