Question

which of the following carbonyls have a stretching frequency around 1745 cm⁻¹? caution: more than one answer is possible. none of these

Explanation:

Carbonyl stretching frequencies depend on the nature of the carbon - bonded groups. Aldehydes (first structure) have a stretching frequency around 1720 - 1740 cm⁻¹, ketones (third structure) around 1710 - 1720 cm⁻¹, and acid chlorides (fourth structure) around 1750 - 1800 cm⁻¹. The acid chloride has a stretching frequency around 1745 cm⁻¹.

Answer:

Fourth structure (carbonyl with - Cl group)