Q15P - Organic Chemistry (Mcmurry)

Question

Cyclopentanecarboxylic acid and 4-hydroxycyclohexanone have the 

same formula $\left(C_6{H}_{10}{O}_2\right)$, 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 $\left(-\mathrm{OH}\right)$functional group in cyclopentanecarboxylic acid shows a very

broad absorption between $2500{\mathrm{cm}}^{-1}$and $3300{\mathrm{cm}}^{-1}$in the IR spectrum.

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

The structure of the cyclopentanecarboxylic 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 $\left(-\mathrm{OH}\right)$group of the carboxyl group in 

cyclopentanecarboxylic acid shows a very broad absorption between 

$2500 {\mathrm{cm}}^{-1}$and $3300 {\mathrm{cm}}^{-1}$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.