Question

which of the following carbonyls have a stretching frequency around 1685 cm⁻¹? caution: more than one answer is possible.

Explanation:

Step1: Recall carbonyl stretching frequency rules

Conjugated carbon - yl groups have lower stretching frequencies. A carbon - yl stretching frequency around 1685 cm⁻¹ is characteristic of a conjugated carbon - yl.

Step2: Analyze each structure

The first structure is a simple cyclopentanone (non - conjugated), its carbon - yl stretching frequency is around 1750 cm⁻¹. The second structure is a benzoate (conjugated carbon - yl due to the aromatic ring), its carbon - yl stretching frequency can be around 1685 cm⁻¹. The third structure is a cyclic ester (lactone), and its carbon - yl stretching frequency is typically around 1735 cm⁻¹ for a five - membered lactone and around 1715 cm⁻¹ for a six - membered lactone.

Answer:

The second structure (the benzoate)