Question

what is the stretching frequency (in cm⁻¹) of the following carbonyl?
1745
1685
1715

Explanation:

Step1: Identify the carbonyl type

The given compound is an $\alpha,\beta -$ unsaturated carboxylic acid. The carbon - oxygen double bond in $\alpha,\beta -$ unsaturated carboxylic acids has a lower stretching frequency compared to saturated carboxylic acids due to resonance and conjugation effects.

Step2: Recall stretching frequency ranges

The stretching frequency of a carbonyl in a saturated carboxylic acid is around 1710 - 1720 $cm^{-1}$. For an $\alpha,\beta -$ unsaturated carboxylic acid, the carbonyl stretching frequency is lower, around 1680 - 1690 $cm^{-1}$ due to the delocalization of the $\pi$ - electrons.

Answer:

1685