Carbon-13, DEPT and Coupling Constants · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.
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Reading carbon spectra and couplings

Science · Chemistry · ages 20-21
Name ______________________   Date ____________

  1. Can carbon peak heights count carbons?

    • Yes, heights follow the carbon count
    • Yes, if the peaks are sharp enough
    • No, heights are not reliable counts

  2. Why does a routine carbon spectrum show singlets?

    • The protons are decoupled
    • Carbon-13 nuclei cannot couple
    • Every carbon holds one proton

  3. What does a DEPT experiment classify?

    • The solvent's deuterium content
    • Each carbon by its attached protons
    • Each proton by its coupling constant

  4. A cis alkene and its trans isomer have H-C=C-H couplings of 8 Hz and 15 Hz. Which value belongs to the trans isomer?

    • The 8 hertz one
    • Neither, coupling ignores geometry
    • The 15 hertz one

  5. A carbon signal is missing in DEPT but present in the carbon spectrum. What is it?

    • A carbon with no attached protons
    • A CH3 carbon, which DEPT never shows
    • A proton with no carbon

  6. A coupling constant reports on geometry, not just on neighbours.

    Circle one:   True   False

  7. In a DEPT-135 spectrum, a signal pointing down belongs to a CH2 carbon.

    Circle one:   True   False

  8. Two isomers differ only at one double bond. Carbon data fits both, but one coupling is far larger. How do you rule one out?

    • The larger coupling means trans
    • Keep both, since carbon data fits both
    • Discard the carbon data and guess
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Answer key

For grown-ups. Fold this page away before handing over the rest.

Reading carbon spectra and couplings W1-mt_9K6GkAkqq1-s1

  1. No, heights are not reliable counts · Heights cannot be trusted for counting in carbon spectra.
  2. The protons are decoupled · Decoupling collapses each carbon to a single line.
  3. Each carbon by its attached protons · DEPT sorts CH3, CH2, CH, and silent quaternary carbons.
  4. The 15 hertz one · The larger coupling argues for trans geometry.
  5. A carbon with no attached protons · Silent in DEPT plus present in carbon means quaternary.
  6. True · Its size in hertz can tell cis from trans.
  7. True · DEPT-135 points CH2 signals down and CH and CH3 signals up, so a downward signal is a CH2 carbon.
  8. The larger coupling means trans · The coupling is the deciding signal when carbon fits both.
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