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

Conjugated carbon - carbon double bonds lower the carbonyl stretching frequency. Aromatic - carbonyl conjugation also lowers the frequency.

Step2: Analyze each structure

The first structure is an $\alpha,\beta$-unsaturated ketone conjugated to an aromatic ring. Its carbonyl stretching frequency is lower than a non - conjugated carbonyl, around 1680 - 1640 $cm^{-1}$. The second structure is an $\alpha,\beta$-unsaturated aldehyde, and its carbonyl stretching frequency is around 1690 - 1665 $cm^{-1}$. The third structure is an aromatic ketone. Aromatic ketones have carbonyl stretching frequencies around 1680 - 1640 $cm^{-1}$.

Answer:

none of these