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

Carbon - carbon double - bond stretching frequencies vary based on the nature of the carbonyl compound. A ketone carbonyl typically has a stretching frequency around 1715 cm⁻¹.

Step2: Analyze each compound

• The first compound is a cyclic amide (lactam). Amide carbonyls have stretching frequencies around 1650 - 1690 cm⁻¹ due to resonance with the nitrogen lone - pair.
• The second compound is a cyclic ketone. Cyclic ketones with 5 - or 6 - membered rings have carbonyl stretching frequencies around 1715 cm⁻¹.
• The third option "none of these" is incorrect as the cyclic ketone matches the criteria.
• The fourth compound is a cyclic ester (lactone). Ester carbonyls have stretching frequencies around 1735 - 1750 cm⁻¹.

Answer:

The second structure (the cyclic ketone)