Question

The 1h nmr spectrum of a compound which has the molecular formula c8h8 is shown below. Identify the compound.

Answer 1

solution:

That is a one benzene ring.

The really messy multiplet at 7.27ppm pretty much can be interpreted as a monosubstituted ring.

After the ring is accounted for, all that's left is C₂H₃. There's one more double bond equivalent, and only two carbons left, so the only possibility is an alkene.

the alkene protons are the signals at 6.7ppm, 5.6ppm, and 5.2ppm. A benzene ring attached to an alkene will shift the proton geminal to it up about 1.3ppm from the normal 5.25ppm, the cis proton up about 0.36ppm, and have not much effect on the proton trans to it.