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:

Carbon - oxygen double - bond (C=O) stretching frequencies are affected by factors such as conjugation and electronegativity of adjacent groups. Aromatic ketones and some conjugated carbonyls have C=O stretching frequencies around 1685 cm⁻¹. The first structure is a simple ketone (acetone - like), the second is an aldehyde, and the third is an acyl chloride. Conjugated carbonyls lower the stretching frequency from the typical 1715 - 1735 cm⁻¹ for simple carbonyls.

Answer:

None of the given simple carbonyls (ketone, aldehyde, acyl - chloride) without conjugation typically have a stretching frequency around 1685 cm⁻¹. So the answer is none of these.