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1 H NMR Spectrum of C 5 H 6

1 H NMR Spectrum of C 5 H 6. 25 °C. 1 H NMR Spectrum of C 5 H 6. -50 °C. 25 °C. [1,5] Sigmatropic Rearrangement. Hydrogen atom rapidly jumps from carbon to carbon Rearrangement is so fast at 25 °C that NMR spectrometer cannot keep up shows averaged spectrum

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1 H NMR Spectrum of C 5 H 6

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  1. 1H NMR Spectrum of C5H6 25 °C

  2. 1H NMR Spectrum of C5H6 -50 °C 25 °C

  3. [1,5] Sigmatropic Rearrangement • Hydrogen atom rapidly jumps from carbon to carbon • Rearrangement is so fast at 25 °C that NMR spectrometer cannot keep up • shows averaged spectrum • Rearrangement is very slow at -50 °C • shows frozen spectrum • Animation • Hydrogen atom remains on same side of cyclopentadienyl ring • Suprafacial process • Why [1,5] rather than [1,2] sigmatropic shift? • Aromatic transition state • p-orbitals on all 5 C atoms • 1s-orbital on H • total of 6 electrons • generalize to acyclic pentadiene

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