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

Carbonyl stretching frequencies depend on the nature of the group attached to the carbon - carbonyl bond. Acyl chlorides have carbonyl stretching frequencies around 1750 - 1800 cm⁻¹, aldehydes around 1720 - 1740 cm⁻¹, and ketones around 1710 - 1720 cm⁻¹.

Step2: Analyze each structure

The first structure is a ketone, which has a carbonyl stretching frequency around 1710 - 1720 cm⁻¹. The second structure is an aldehyde, with a carbonyl stretching frequency around 1720 - 1740 cm⁻¹. The third structure is an acyl chloride, which has a carbonyl stretching frequency around 1750 - 1800 cm⁻¹.

Answer:

The carbonyl with a stretching frequency around 1745 cm⁻¹ is the aldehyde (the second - shown structure). So the answer is the second - shown structure.