Question

which of the following carbonyls have a stretching frequency around 1715 cm⁻¹? caution: more than one answer is possible.

Explanation:

Step1: Recall carbonyl stretching frequencies

Ketones typically have carbon - oxygen double - bond ($C=O$) stretching frequencies around 1715 cm⁻¹. Cyclic ketones also follow this general trend. Esters have $C=O$ stretching frequencies around 1735 cm⁻¹.

Step2: Identify the structures

The first structure is a cyclic ester (lactone), the second structure is a cyclic ketone, and the fourth structure is an ester.

Step3: Determine the answer

The cyclic ketone has a $C = O$ stretching frequency around 1715 cm⁻¹.

Answer:

The second structure (the cyclic ketone)