Question

Question:Reduction of cyclohex-2-enone can yield cyclohexanone, cyclohex-2-enol, or cyclohexanol, depending on the reagent and reaction conditions. How could you use IR spectroscopy to distinguish the three possible products?

Short answer

Answer

Cyclohexanone shows a distinct absorption at around $1700{\mathrm{cm}}^{-1}$for the keto functionality, which is not observed in the case of cyclohex-2-enol or cyclohexanol.

Step-by-step solution

IR spectroscopy

IR spectroscopy is employed to study the presence of functional groups in a compound. The molecules present in a compound tend to absorb specific frequencies of light in the infrared region that corresponds to the vibration frequencies of the bonds, and thus, each functional group shows a specific peak.

Distinguishing the three products formed

The structure of cyclohex-2-enone is given below.

Structure of cyclohex-2-enone

It can be observed that ketone functionality is present along with a double bond. The IR spectra show a peak corresponding to the two functionalities. The absorption occurs toward a lower wavenumber, as the carbonyl group has a partial single-bond character owing to the resonance with the adjacent pi-bond.

When cyclohex-2-enone is subjected to reduction, the following products may form:

1. Cyclohexanone

It is observed that the carbonyl group is present. Therefore, the compound will show a distinct peak corresponding to the -C=O group in the IR spectra. The absorption occurs at around $1700{\mathrm{cm}}^{-1}$.

When compared to cyclohexanone, it is observed that the peak at$1700{\mathrm{cm}}^{-1}$ is no longer showing. This is because cyclohex-2-enol does not contain the same functionality as that of cyclohexanone.

3. Cyclohexanol

Similarly, in this case, the peak at is no longer present. The IR spectra will show a peak for the absorption of alcohol functionality that absorbs at around $3600{\mathrm{cm}}^{-1}$.