Question

which of the following carbonyls have a stretching frequency around 1745 cm⁻¹? caution: more than one answer is possible.

Explanation:

Step1: Recall carbonyl stretching frequencies

Cyclic ketones with smaller ring - size have higher carbonyl stretching frequencies. A five - membered cyclic ketone has a carbonyl stretching frequency around 1745 cm⁻¹.

Step2: Analyze each structure

The first structure is a cyclohexanone (six - membered ring ketone), which has a carbonyl stretching frequency around 1715 cm⁻¹. The second structure is a cyclopentanone (five - membered ring ketone), which has a carbonyl stretching frequency around 1745 cm⁻¹. The third structure is a benzoic acid, and the carbonyl of a carboxylic acid has a stretching frequency around 1700 - 1725 cm⁻¹ in the free form and around 1600 - 1650 cm⁻¹ in dimer form.

Answer:

The second structure (cyclopentanone)