Question

which of the following carbonyls have a stretching frequency around 1685 cm^-1? caution: more than one answer is possible. none of these

Explanation:

Step1: Recall carbonyl - stretching frequency rules

Conjugated carbonyls have lower - stretching frequencies. A carbonyl stretching frequency around 1685 cm⁻¹ is characteristic of a conjugated carbonyl.

Step2: Analyze each structure

The first structure is a cyclic ester (lactone). The carbonyl in a lactone is not conjugated in a way to lower the stretching frequency to 1685 cm⁻¹. The second structure is a cyclopentanone. A simple cyclopentanone carbonyl has a higher stretching frequency than 1685 cm⁻¹. The fourth structure is a cyclic ester (lactone) again. The carbonyl in this lactone is also not conjugated in the right way for a 1685 cm⁻¹ stretching frequency. So the answer is none of these.

Answer:

none of these