Question

The three isomers of dichlorobenzene are commonly named ortho-chlorobenzene, meta-chlorobenzene, and para-chlorobenzene. These three isomers are difficult to distinguish using proton NMR, but they are instantly identifiable usingNMR.

1. Describe how carbon NMR distinguishes these three isomers.
2. Explain why they are difficult to distinguish using proton NMR.

Short answer

1. Considering the symmetry of structures, ortho-xylene would have 3 carbon signals from the ring, meta-xylene would have 4 carbon signals from the ring and para-xylene would have only 2 carbon signals from the ring.
2. Unless the substituent on the benzene ring is electron withdrawing or electron donating, ring protons absorb at roughly the same position as methyl group has no electronic effect on the ring hydrogens, thus, proton NMR will not be able to distinguish between these isomers.

Step-by-step solution

Explanation of part (a):

Considering the symmetry of structures, ortho-xylene would have 3 carbon signals from the ring and total of 4 peaks whereas meta-xylene would have 4 carbon signals from the ring and total of 5 peaks and para-xylene would have only 2 carbon signals from the ring and total of 3 peaks. Thus, these compounds would be instantly identifiable by number of peaks in carbon-NMR.

Explanation of part (b):

The proton-NMR would not be able to distinguish these isomers as there is no powerful electron withdrawing or donating substituent attached to the ring and methyl group has essentially no electronic effect on the ring hydrogens, thus all ring protons will absorb at roughly same chemical shift. Para isomer would give clean singlet and ortho and meta isomers would have only slightly broadened singlets for their protons. Very high field NMR would only be able to distinguish these isomers.