Q15P - Organic Chemistry (Mcmurry)

Question

Cyclopentane carboxylic acid and 4-hydroxycyclohexanone have the same formula (${\mathbf{C}}_{\mathbf{6}}{\mathbf{H}}_{\mathbf{10}}{\mathbf{O}}_{\mathbf{2}}$), and both contain an -OH and a C=O group. How could you distinguish between them using IR spectroscopy?

Step-by-Step Solution

Answer

The hydroxyl $\left(-OH\right)$ functional group in cyclopentane carboxylic acid shows a very broad absorption between $2500 c{m}^{-1}$ and $3300 c{m}^{-1}$ in the IR spectrum.

Step 1: Draw the structures for the given IUPAC names.

The structure of the cyclopentane carboxylic acid is drawn as follows:

The structure of the 4-hydroxycyclohexanone is drawn as follows:

Step 2: Distinguish the given compounds based on the functional using IR spectroscopy.

The carbonyl group absorptions are more or less same for both compounds. 

But the hydroxyl $\left(-OH\right)$ group of the carboxyl group in cyclopentane carboxylic acid shows a very broad absorption between $2500 c{m}^{-1}$ and $3300 c{m}^{-1}$ compared to 4-hydroxycyclohexanone. 

Therefore, we could differentiate these compounds with the help of the absorption of the hydroxyl group alone by using IR spectroscopy.