Infra Red Spectrum - 6

Infrared spectrum 6: cyclohexene from the dehydration of cyclohexanolCyclohexanol, C₆H₁₁OH, can be dehydrated by heating with concentrated phosphoric acid to form cyclohexene, C₆H₁₀. Infrared spectroscopy shows whether the reaction has worked.Spectrum of cyclohexene: key absorptions• A medium to weak absorption at about 1650 cm^-1, caused by the C=C bond. Alkenes absorb at about 1645 to 1670 cm^-1. This absorption is weaker than a C=O absorption, because the C=C bond is much less polar.• An absorption just above 3000 cm^-1 (about 3020 cm^-1), caused by the C-H bonds on the carbon atoms of the C=C bond. Ordinary C-H bonds in the CH₂ groups absorb just below 3000 cm^-1.What is missing• There is no broad O-H absorption at 3200 to 3550 cm^-1, so no cyclohexanol is left in the product.• There is no strong absorption near 1700 cm^-1, so there is no C=O. (Do not confuse the weaker C=C absorption at 1650 cm^-1 with a C=O absorption.)ConclusionComparing the two spectra, the dehydration has worked if the broad O-H absorption of cyclohexanol has disappeared and new absorptions at about 1650 cm^-1 and 3020 cm^-1 have appeared. The C=C absorption can also be used to show that bromine water has reacted with an alkene: after the addition reaction, the absorption at 1650 cm^-1 is gone.