Question

How do you identify the compound with IR spectrum with a molecular formula `C_5H_8O`?

Answer 1

The simple answer is YOU CAN'T.

Explanation:

`C_5H_8O` has 2 degrees of unsaturation. Each degree of unsaturation corresponds to a double bond or a ring. Could you draw ALL the isomers of `C_5H_8O` (there would be a dozen or so), and compare these to the known IR spectra of these isomers? It is possible, but it is much easier to make a few derivatives of your unknown species, and compare these melting points to melting points of the known derivatives.