Question

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

Explanation:

Step1: Identify the carbonyl type

The given carbonyl is a cyclic ester (lactone).

Step2: Recall carbonyl - stretching frequency rules

Cyclic esters have carbonyl - stretching frequencies around 1745 cm⁻¹ due to ring - strain effects which increase the force constant of the C=O bond.

Answer:

1745