Discuss the differences observed in the IR and NMR spectra of cyclohexanol and cyclohexene that are consistent with dehydration occurring in this experiment.
The correct answer and explanation is :
Differences in IR and NMR Spectra of Cyclohexanol and Cyclohexene Indicating Dehydration
The dehydration of cyclohexanol to cyclohexene results in distinct changes in both IR (Infrared) and NMR (Nuclear Magnetic Resonance) spectra, which confirm the successful conversion of the alcohol to the alkene.
Infrared (IR) Spectroscopy Differences:
- O–H Stretch (Alcohol) Disappearance:
- Cyclohexanol shows a strong, broad O–H stretch around 3200–3600 cm⁻¹ due to hydrogen bonding.
- Cyclohexene lacks this peak, confirming the loss of the hydroxyl (-OH) group.
- C=C Stretch (Alkene) Appearance:
- Cyclohexene exhibits a C=C stretching vibration around 1600–1680 cm⁻¹, which is absent in cyclohexanol.
- C-H Stretching Shifts:
- In cyclohexanol, C-H stretching vibrations appear around 2850–3000 cm⁻¹ (sp³ hybridized C-H).
- In cyclohexene, additional sp² C-H stretching vibrations emerge around 3000–3100 cm⁻¹, confirming alkene formation.
NMR Spectroscopy Differences:
- Loss of the Hydroxyl Proton (¹H NMR):
- Cyclohexanol exhibits a broad singlet around 1-2 ppm due to the O–H proton.
- This peak disappears in cyclohexene, indicating successful dehydration.
- New Alkene Proton Signals:
- Cyclohexene shows two vinylic proton signals around 4.5–6.0 ppm (due to sp² C-H protons), which are absent in cyclohexanol.
- Shifts in Aliphatic Proton Signals:
- In cyclohexanol, protons on sp³ carbons appear in the 1–2 ppm range.
- In cyclohexene, the change in hybridization results in deshielding, causing shifts in the proton signals.
These spectral changes confirm that dehydration of cyclohexanol to cyclohexene has successfully taken place.
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