Question

Defend or refute: Unsymmetrical geometric isomers (cis and trans isomers) of alkenes can be distinguished using proton NMR spectroscopy.

Answer 1

Coupling constant, ​J ​value ( ³J_HH​) is used to differentiate cis and trans isomers by proton NMR spectroscopy. Vicinal coupling constants (³J_HH) are larger for trans (usually around 15 Hz) than for cis (usually around 8 Hz) isomers. However in case of unsymmetrical geometric isomers, carbon-carbon double bond bears different pairs of groups instead of vicinal hydrogens as shown below.

Example of unsymmetrical alkene:

As shown above on C2 we have a methyl group and a hydrogen group while C3 has an ethyl and propyl group. In E isomer the larger groups are on opposite side to the double bond while in Z isomer the larger groups are on same side to the double bond  

Since there are no vicinal hydrogens on C2 and C3 this leads to lack of vicinal coupling constant, ³J_HH and this results in lack of differentiation of cis and trans isomers by proton NMR spectroscopy.

So it's refuted that unsymmetrical geometric isomers (cis and trans isomers) of alkenes can be distinguished using proton NMR spectroscopy.