Question

what is the stretching frequency (in cm⁻¹) of the following carbonyl?
none of the above
1775
1745
1685

Explanation:

Step1: Identify the carbon - carbonyl type

The given compound is an $\alpha,\beta$-unsaturated ester.

Step2: Recall the IR stretching frequency range

For $\alpha,\beta$-unsaturated esters, the carbonyl stretching frequency is around $1680 - 1700\ cm^{-1}$ due to the conjugation of the carbonyl group with the double - bond which lowers the force constant and thus the stretching frequency.

Answer:

D. 1685