How can I draw possible structures for an amide that has a molecular ion with an m / z value of 115?
You know that the structure contains a CON group (42).
So the rest of the molecule has mass = 115 - 42 = 73.
This corresponds to C₆H or C₅H₁₃. Both are impossible.
There must be another mono-isotopic atom in the molecule: Fluorine!
CNOF = 61. So the rest of the molecule has mass = 115 – 61 = 54.
This corresponds to C₄H₆. So the formula for the molecule is C₄H₆ + CNOF = C₅H₆NOF.
The unsaturation value U = 3. The C=O of the amide accounts for one of these.
So the rest of the molecule has a triple bond, two double bonds, a double bond and a ring, or two rings.
There are many possible structures. One of them might be F-CH₂-CH₂-C≡C-CONH₂.