Problem 5

C₄H₈O₂
Rule 2, omit O, gives C₄H₈
4 - 8/2 + 1 = 1 degree of unsaturation.
Look for 1 pi bond or aliphatic ring.

The band at 1743 indicates a carbonyl, probably a saturated aliphatic ester. The bands at 3000-2850 indicate C-H alkane stretches. The bands in the region 1320-1000 could be due to C-O stretch, consistent with an ester.

This is the structure. See if you can assign the peaks on your own.