Question

A compound of molecular formula C₈H₈Ogives the IR and NMR spectra shown here. Propose a structure and show how it is consistent with the observed absorptions.

Short answer

The formula C₈H₈O has five elements of unsaturation and four are enough for a benzene ring. From the given IR spectrum, it is deduced that, there is neither hydroxyl nor carbonyl group present as there is no strong absorption around 1700 cm^-1so the oxygen must be an ether group.

Step-by-step solution

Structure determination from given IR data:

The formula C₈H₈O has five elements of unsaturation and four are enough for a benzene ring. From the given IR spectrum, it is deduced that, there is neither hydroxyl nor carbonyl group present as there is no strong absorption around 1700 cm^-1so the oxygen must be an ether group.

Structure determination from NMR data:

The proton NMR shows a 5 H signal at chemical shift value 7.2 which indicates a monosubstituted benzene. No peaks in the chemical shift value range 4.5-6.0 indicates the absence of an alkene. Thus, remaining element of unsaturation must be a ring. The three protons are non-equivalent with complex splitting.

So far, we have the following fragments,

These fragments can be assembled in only one manner which is consistent with the data and the methylene hydrogens present are not equivalent, that is, one is cis and other is trans to phenyl and therefore they exhibit different chemical shifts.